Power Topics for Power Supply Users

Advantages of Conduction-Cooled Power Supplies

2012-02-02T10:27:00.000-08:00

Most mid- to high-power supplies use fans to help dissipate the internal heat that is generated as a result of imperfect AC to DC conversion efficiencies. Since fans are electromechanical devices, they reduce the system’s MTBF and add to the required maintenance expenses.

Attached is a photo of a power supply that operated for many years at a postal depot where mail is handled and sorted automatically. As can be seen (after the fan was removed) paper fragments and airborne dust contaminants were pulled into the supply by the fan and eventually caused a blown fuse.

As might be expected, the proper maintenance program for any fan-cooled power supply calls for the periodic inspections of the supply, with the fan removed, and the replacement of the fan with a new one.

A new breed of conduction-cooled power supplies has been developed that do not depend on fans for cooling. Instead, the required cooling is accomplished by conducting the internal heat loads to an external metal structure or enclosure, which act as a large heat sink surface.

The second attached photo shows TDK-Lambda’s new CPFE1000F series, which are conduction-cooled, 1,000 watt AC-DC power supplies. (A 500 watt version is also available.) All heat is conducted to the supply’s aluminum plate, which is designed to easily mount to a metal enclosure or cold plate for cooling. More details and specifications for these power supplies are at this web link: http://www.us.tdk-lambda.com/lp/products/cpfe-series.htm

In some applications, these conduction-cooled devices are mounted to liquid cooled cold plates that are made of metal with internal serpentine channels through which a liquid circulates while removing the unwanted heat. The net result is that the system operates with a substantial reduction in audible noise, reduced maintenance costs (no dust build-up and fan wear-out), and an enhanced MTBF.

Recently, I visited a Television Broadcasting Station that consumes about 100 kilowatts of power. At this location, in separate areas, was a traditional fan-cooled system as well as the latest generation system, which uses conduction-cooled power supplies and RF amplifiers that are cooled via liquid flow cold plates. During the operation of the traditional system with fan cooling, the audible noise was so loud that personnel within 100 feet of the system had to wear hearing protection devices. By comparison, in the other area where the new system with liquid cooling was operating, the noise level was so low (similar to an office environment) that no hearing protection was required.

Simplify Power Supply Decisions via Free Mobile App

2012-01-17T13:49:00.000-08:00

It seems endemic that the final specifications for a power supply to run most new product designs tend to be firmed-up near the final stages of product development. Too often, this occurs when schedules have already slipped. As a result, there tends to be little time allocated to this important task. This happens so frequently that in the trade it has nicknamed ‘the tailpipe syndrome.’

Now, a fast and easy method for the selection of a power supply for your latest new product is as close as your smartphone. TDK-Lambda’s new mobile app works with Android and iPhones to greatly simplify this process. Say ‘goodbye’ to turning catalog pages or running tedious internet searches. The app has been shown to quickly guide the user to a power supply that meets their exact needs. To access this app, go to wbxapp.com/tdk-lambda. After the app loads, just press the AC power plug symbol. This app makes the selection process as easy as 1-2-3:

Step 1. Select Your Application:
LEDs, Medical, General Industrial, Comms, Test & Measure, or Military/COTS

Step 2. Select Your Power Range:
1-25W, 26-50W, 51-150W, 151-600W, or 600W & Higher

Step 3. Select from Available Products:
A number of power supply series that meet your Application and Power Range will be offered. Just click on any of these to view the specs and even detailed datasheets.

Quick Results:
In this example, we needed a Medical power supply with three (3) outputs and a total output power of 600-watts. The NV Series meets these requirements.

More Features:
This unique app includes quick links to YouTube (for power supply demo videos) and to Twitter (for new product info).

Additional information about this handy mobile app and other AC-DC power supplies, power racks, customized solutions, and DC-DC converters, is available at http://www.us.tdk-lambda.com/lp/

What is a Power Supply’s IP Rating?

2011-12-05T13:59:00.000-08:00

The popularity of outdoor electronics has brought the subject of a power supply’s IP rating from almost obscurity to an everyday question. I frequently get asked about it by our sales people now, so I thought it would be nice subject to cover in our blog. In researching this blog article I even discovered something new myself.

IP is the acronym for Ingress Protection and for power supplies the IP Rating Code consists of the letters “IP” and two numbers as defined below.

The first number indicates the power supply’s protection level against the ingress of solid objects or dust.

First Number for Solids or Dust

Level	Size of Object	Type of Object
X	Test not made	Test not made
0	N/A	No protection
1	50mm or larger	Large body surfaces*
2	12.5mm or larger	Fingers
3	2.5mm or larger	Small tools
4	1mm or larger	Screws
5	Dust protected	-
6	Dust tight	-

* Does not include deliberate body part contact

The second number indicates the power supply’s protection against the ingress of water or other liquids.

Second Number for Liquids

Level	Protected against
X	Test not made
0	No protection
1	Water dripping vertically
2	Water dripping at an angle
3	Spray water up to 60° from vertical
4	Splashing water from any angle
5	Low pressure water jets
6	Strong spray jets, heavy seas (ship decks)
7	Temporary immersion (up to 1m)
8	Permanent immersion (deeper than 1m)

Most recently LED power supplies, or drivers as they are often referred to, have ratings of IP66 or higher. Referring to the charts above, an IP66 rating means the unit has ingress protection from Dust and Strong Jet Sprays of Water.

These IP ratings also apply to the end system of course, and many of our customers utilize a NEMA enclosure to make their products meet a higher rating.

TDK-Lambda's IP66 rated LED Driver (ALC/ALV series)

Mounting Precautions for Power Supplies

2011-11-03T10:00:00.000-07:00

Before mounting your power supply, be sure to read its installation manual if you intend to mount it in an orientation other than along the horizontal plane (Fig. A). Many power supplies have restrictions regarding mounting. For example, since heat rises, if you mount some power supplies on a vertical plane (Fig. B, C, & D), the heat from the lower section of the power supply will rise and further heat the upper part of the supply, which may cause over heating problems. Likewise, with some power supplies you are not allowed to mount them upside down (Fig. E) because this traps the heat and restricts the normal convection air cooling around the power supply.

In some cases, vertical mounting of power supplies is permitted as long as you reduce the amount of power that will be drawn from the supply. This is referred to as “Derating” the power supply. Below are the derating curves for the TDK-Lambda’s model LS150-12, a convection cooled 150-Watt, 12V output, AC-DC power supply.

This graph shows the percentage of rated output power on the vertical axis and the operating ambient temperatures on the horizontal axis. Notice when mounting this power supply on the horizontal plane (Fig. A), the power supply is rated at 100% output power from -25°C up to +50°C. However, if you mount this supply on a vertical plane (Fig. B, C, & D), the maximum ambient temperature is reduced to +40°C before the power must be derated.

It is worth mentioning that many low cost competitors do not mention the preferred mounting orientation, and some do not even have an installation manual on their website!

An incorrectly mounted power supply will get too hot resulting in premature electrolytic capacitor degradation, catastrophic semiconductor failure or even a fire due to transformers overheating.

Other general power supply mounting considerations include the following:

Make sure there is adequate space around the power supply to allow air to circulate.
Do not block off vent holes on convection cooled supplies or restrict air inlet or outlet ports on fan cooled supplies.
In the event fans are employed within power supply, a system, or an enclosure make sure the airflow direction for all fans are the same

How to safely power LEDs

2011-10-04T14:33:00.000-07:00

For well over 25 years, LEDs (Light Emitting Diodes) have been used in TV remote controls. These specific LEDs emit invisible light pulses in the infrared (IR) light spectrum. Because the LED can be turned on and off very rapidly, it easily transmits pulses of binary-coded messages to the receiver built into the TV.

In addition, early applications of LEDs included red-segment clocks, calculators and even digital watches that have now been replaced by more modern display technologies, such as LCDs (Liquid Crystal Displays).

Today, white or multi-colored LEDs are rapidly being employed in modern home/street lighting, signage, traffic signals, large screen displays and backlit LCD monitors, etc. In these applications, multiple LEDs are placed in either clusters or connected as strings to provide the required light intensity or light distribution.

LEDs are similar to conventional diodes in that they are designed to conduct current in one direction and when doing so, in most cases, they emit visible light. A basic LED circuit consists of a voltage source, a current limiting resistor and the LED as shown below.

The current limiting resistor (R) is required to maintain the current flowing through the LED at a safe operating level. When conducting current LEDs have an inherent “voltage drop” that can vary from 1.2V to 4.0V, depending upon the model. Referring the circuit diagram, if the LED has a voltage drop of say 2V (Vd) with a safe operating current of 20mA (I), and the voltage source (Vs) is 5VDC, the value of the current limiting resistor can be calculated as follows:

R = (Vs –Vd) ÷ I, therefore, R = (5V – 2V) ÷ 0.02A = 150 ohms

The voltage drop across an LED and its light output will vary with the current flowing through it. Below are curves that show the forward voltage drop (Vd) versus the current (I) flowing through two sample LEDs.

In viewing the white curve above, it’s important to notice that the forward voltage drop across the LED between 3.2V and 3.6V (a 0.4V change), results in a current increase of over five times (from 10mA to 60mA). In this example, if the maximum allowable LED current is 40mA and if 60mA or more current is allowed to flow through it, the LED could be destroyed or its operational life substantially reduced.

As current flows through an LED its forward voltage drop times the current results in wasted power (e.g., 3.3V x 40mA = 132mW). This wasted power, in the form of heat, becomes a real problem when high brightness LEDs is employed in lighting applications. The internal LED heat must be dissipated by either its design, the substrate it’s mounted on, or via added heat sinks. As the internal junction of an LED gets warmer, the current through it at a given voltage increases. If not controlled, this can result in thermal runaway, where the LED self-destructs.

The main point here is that LEDs are “current driven” devices and that this current must be carefully controlled. In the circuit above, the resistor is used to control the current though the LED. However, the resistor also causes a voltage drop which contributes further to wasted power. As a result “constant-current” LED drivers have been developed that maintain the current flowing through the LED (or strings/clusters of multiple LEDs) at a safe level with improved efficiency.

For more information about selecting power supplies and drivers for LEDs, see the article at this web link: http://power-topics.blogspot.com/search/label/LED%20lights

References:

http://en.wikipedia.org/wiki/LED_circuit

http://led.linear1.org/why-do-i-need-a-resistor-with-an-led/

http://us.tdk-lambda.com/lp/products/ledsigns.htm

Using Power Supplies with DC Motors

2011-09-09T09:28:00.000-07:00

There is often confusion regarding the use of external diodes when power supplies are used to power DC motors. Most people know that a diode has to be used, but are unsure where to place them or what their purpose is.

From a power supply concern there are two types of DC motors; a brushed DC motor and a brushless DC motor.

Brushed DC motors

With this type of motor, the magnets are stationary and the coil spins. Electricity is transferred to the spinning coil by the use of “brushes”.

The advantages of this type of motor are low initial cost and easy speed control.

When the power is interrupted, the motor coil will act like an inductor and will try to continue to produce current, effectively becoming an inverted voltage source. This will apply a reverse polarity to the power supply and can cause damage. (Back EMF – Electro-Magnetic Flux)

By using a diode, as shown below, the diode provides a current path for the reverse motor current and will clamp the reverse voltage to a level no greater than the forward voltage drop of the diode. This protects the power supply’s output capacitors and other components from being stressed by the reverse voltage.

Brushless DC motors

Brushless DC motors, often referred to as BDCMs or BLDC motors, have permanent magnets that rotate and the armature is fixed.

Although more expensive, they are more reliable in the long term as there is no brush or commutator wear and position control is more accurate.

When the motor is turned off or reversed, it will act as a generator and produce a high voltage spike. This spike can cause the power supply’s overvoltage protection to trip, shutting down the unit.

By using a diode in series with the output, as shown below, the spike will be blocked from interfering with the power supply.

In both cases a general purpose diode can be used, providing that the voltage and current ratings for the diode are correctly calculated.

LED Drivers for LCD panel backlighting (ALD Series)

2011-08-21T10:50:00.000-07:00

What type of LED driver or power supply do I need?

2011-08-02T14:30:00.000-07:00

Conventional AC-DC power supplies and DC-DC converters provide an output that is regulated to provide a “constant-voltage.” However, LEDs work most efficiently and safest with a “constant-current” drive. As a result, many new devices have been developed to provide this type of LED drive. LED power sources that provide a “constant-current” output have typically been referred to as LED drivers. In the past, AC-DC power supplies that provided a regulated “constant-voltage” to LEDs were referred to as LED power supplies. Today, the terms “LED driver” and “LED Power Supply” are used interchangeably. The important thing to keep in mind is whether the output of the power device provides a “constant-voltage” or a “constant-current.”

When do I need a “constant-voltage” LED driver?
Most commercially available LED “light modules” are constructed by connecting a number of LEDs in series or parallel to form cluster or string configurations. In cases where these light modules include a “constant-current” driver as part of the assembly, an external “constant-voltage” driver or power supply is required. Some LED circuits control the current flowing through the LED with a simple resistor. This is another case where a constant-voltage power source is required. Other examples where external “constant-voltage” supplies have been employed include backlit ad signs, traffic information signs and large screen high definition LED displays, such as those described in this article: http://www.ledsmagazine.com/products/20877. Constant-voltage drivers come in many different forms. They can look like a conventional power supply or they can be enclosed for moisture/environmental protection.

When do I need a “constant-current” LED driver?
In cases where a manufactured cluster or string of LEDs does not include an internal “constant-current” driver, an external LED driver or power supply that provides a “constant-current” is required. Constant current LED drivers are available in many different package configurations, ranging from integrated circuits to enclosed moisture-proof packages, depending on the application and the required output power.

Series and Parallel LED Configurations
Depending on the application, LEDs can be connected in series and/or parallel configurations. Obviously, when LEDs are connected in series the forward voltage drop of each LED in the string are additive. For example, if you put 15 LEDs in series and each one has a voltage drop of 3V (at its nominal current), you need to provide a voltage source of 45V (15 x 3V = 45V) to drive the required current. This is why “constant-current” drivers always include in their specs the output voltage range that it is capable of providing to overcome the LED voltage drops. In order to limit the drive voltage to reasonable levels, multiple strings of series-connected LEDs can be placed in parallel and driven by multi-output constant-current drivers.

Below is an excerpt from the datasheet for TDK-Lambda’s ALD6 series of LED drivers. As you can see from the diagram, this driver contains up to 6 independent “constant-current” LED drivers. The 38V output corresponds to combined forward voltage drop of 10 typical white LEDs connected in series. For high-current applications, up to 300mA is available to power one series-string of high brightness LEDs. For applications where the LEDs require up to 50mA, this device can power up to 6 strings of LEDs via its multi-output drivers. These drivers are ideal for LCD display backlighting and general LED lighting applications.

Click to enlarge

How is LED dimming accomplished?
The light output of LEDs can be controlled by varying the amount of current flowing through the LED (within defined limits) or by turning the LED on and off via pulse width modulation (PWM). LED drivers like the ALD6 series have the capability of providing “dimming” by both of these popular methods.

The drawing above shows the two methods of light dimming that are included in the ALD6 LED driver. It is permitted to use a combination of both of these methods simultaneously.

The “Rbr” is an external variable 10kohm resistor input. By varying this potentiometer from 1k to 10kohms, an analog dimming control is achieved. In this case, the maximum LED brightness occurs when the pot is set to 10k ohms. This same input can operate with variable analog voltage ranging from 1.6 to 3.8-volts. In some applications this input can be connected to a temperature sensing device which could reduce the current flow through the LEDs as the temperature rises, thus providing a means for temperature compensation.
The “Vpwm” is a “Pulse Width Modulation” input that controls the LED brightness by varying the duty-cycle of the input signal from 1% to 100%. Typical PWM frequencies can range from 180 to 270 Hz.

More information about LED drivers/supplies can be found at these web links:
http://us.tdk-lambda.com/lp/products/ledsigns.htm
http://us.tdk-lambda.com/lp/products/ald-series.htm
http://power-topics.blogspot.com/search/label/LED%20lights

What does a BF rating on a power supply mean?

2011-07-06T13:15:00.000-07:00

TDK-Lambda recently launched the EFE-M series, a medically BF rated power supply. It immediately sparked the question from my colleagues – “What is a BF rating?” To answer this question we need to start with the term “Applied Part.”

IEC 60601-1 is the international medical electric safety standard that uses the term “Applied Part” to refer to a part of a medical device which may come in physical contact with the patient during its normal operation.

Applied Parts fall into three classifications according to the nature of the medical device and the type of contact. Each classification must have a different protection level against electrical shock.

Type CF (“Cardiac Floating”) is the most stringent classification, and is used for applied parts that may come in direct contact with the heart, such as dialysis machines.

Type BF (“Body Floating”) is less stringent than Type CF, and is generally used for applied parts that have conductive contact with the patient, or having medium or long term contact with the patient. Examples of this type of equipment are blood pressure monitors, incubators and ultrasound equipment.

Type B (“Body”) is the least stringent classification, and is used for applied parts that are normally not conductive and can be immediately released from the patient. Examples of that would be LED operating lighting, medical lasers, MRI body scanners, hospital beds and phototherapy equipment.

Type B applied parts may be connected to earth ground, but Type BF & CF are separated from earth – hence the term “floating”.

Power supply Isolation Voltages vary according to the type rating.

Type	Input to Output Isolation	Input to Ground Isolation	Output to Ground Isolation
B rated	4000VAC	1500VAC	500VAC
BF/CF rated	4000VAC	1500VAC	1500VAC

Please note: power supplies are not medical devices or applied parts, and the outputs of power supplies should never be connected directly to a patient.

Many medical devices contain medical-rated power supplies. However, only the part of these “medical devices” that may come in contact with a patient during normal operation is classified as an “Applied Part.”

Power Supply Losses and the Impact of Rising Efficiencies

2011-06-07T09:45:00.000-07:00

When comparing two power supply efficiency specifications, for example, one with a 90% efficiency and the other with a 94% efficiency, there is a tendency to think “there’s only a 4% difference.”

However, reviewing the wasted power (as heat) between the two supplies reveals a more dramatic difference.

As a reminder, the formula for the efficiency of a power supply is:

(Output Power ÷ Input Power) x 100 = Efficiency (%)

And the wasted or lost power within a power supply, due to its inefficiencies, is calculated as follows:

(Output Power ÷ Efficiency) - Output Power = Wasted Power (Watts)

A. Let’s see what the wasted power or losses would be within a 400W power supply that is 90% efficient:

(400W ÷ 0.90) - 400W = 44.4W (wasted power)

B. Now, let’s compare the same 400W power supply if it’s 94% efficient:

(400W ÷ 0.94) - 400W = 25.5W (wasted power)

The above calculations (A & B) demonstrate that a 90% efficient power dissipates or wastes an additional 19W internally compared to a 94% efficient unit (44.4W – 25.5W = 18.9W). Imagine that this extra 19-watts is a large power resistor within the power supply, radiating heat and negatively affecting thermal management, component derating, and the resultant MTBF and actual field life for the power supply. The payback for employing high-efficiency power supplies now becomes readily apparent.

Chart 1 below shows the internal power losses (wasted power) versus efficiency for the 400W power supplies described above and for efficiencies between those mentioned. From this chart you can see that if a company claims (exaggerates) a 94% efficiency rating, but in reality only achieves 92% they have to ensure that their internal components can operate correctly with an extra 9.3W of heat dissipation (34.8W – 25.5W = 9.3W). And, as mentioned previously the actual field life of the supply will be compromised.

Chart 1

Chart 2 below shows the percentage Energy Savings of a 94% efficient unit compared to a baseline of a 90% efficient unit. By improving the Efficiency by just 4%, it results in nearly a 43% energy savings! The math from above calculations: [1 - (25.5W ÷ 44.4W)] = 42.5%!

Chart 2

The takeaway is, purchase power supplies from a reputable power supply company that employs conservative component deratings and states realistic efficiency ratings.

Class 2 or Class II power supplies?

2011-05-23T11:59:00.000-07:00

One question I am frequently asked is: “The customer is looking for a Class two power supply; what can you offer him?”

My response is always “Class 2” or “Class II (with Roman numerals)”, or both? The pause on the end of the phone signifies an explanation is in order.

Class 2 is a classification referring to the NEC – National Electric Code. To avoid potential cable overheating due to excessive currents and electric shock, the output of the power supply is limited to 60VDC or 100VA, (100W when used with an AC-DC power supply). You will often see 24V output DIN rail power supplies or LED drivers rated at 91W rather than 100W because if the power supply is overloaded, any tolerance in the over current protection has to be accounted for.

Often these products will be certified to UL1310 and will list this in the datasheet. An example of this is TDK-Lambda’s DSP series. You can see from the model selector list on page 2 of the DSP datasheet that output currents of 4.2A or greater are not approved to UL1310.

Class II (with Roman numerals) refers to power supplies with either a double or reinforced insulation barrier between the input and the output. Class II supplies do not rely on an earth connection to protect against shock hazard. Many cell phone chargers and laptop power supplies are Class II. TDK-Lambda’s DSP series also are Class II, having just a Line and Neutral AC input without a ground connection.

A Class II power supply rating label will show this symbol:

One advantage of Class II is better surge protection between input and ground and usually a lower earth leakage current.

For more information about leakage current, please see another article about power supply leakage current testing.

Where’s the CSA logo on my power supply?

2011-04-01T10:13:00.000-07:00

As I sat here at the end of March pondering what my April blog article was going to be about, I had an email from one of our sales people. Her customer had purchased one of our SWS series of power supplies based on our data sheet, and could not see the CSA certification mark on the product label, just the CE, TUV (the triangle) and the UL (recognized) marks.

Looking at our data sheet though, it clearly claimed the product had CSA 60950.

(Click to enlarge) You can see the safety approvals

This prompted the email to me!

In 2003, UL & CSA drafted a bi-national agreement to recognize each other’s testing and certifications. UL can now cross certify to CSA 60950 and likewise CSA can certify to UL 60950. This avoids manufacturers from having to pay and maintain two separate certifications.

If the product was certified by UL for both countries it would have this mark (often referred to as “cUL”.

If the product was certified by CSA for both countries it would have this mark

As the SWS power supply has the “cUL” mark, it is certified to CSA 60950 (or to be fully correct CSA C22.2 No. 60950-1-07) and that is stated on the UL test report.

How does the AC Fail signal work in a power supply?

2011-03-02T09:07:00.000-08:00

I was recently talking to one of our Design Engineers about my blog and he suggested a clarification of the operation of the AC Fail signal would be a good topic. He stated that he was often asked “at what input voltage does the AC Fail signal operate?”

A power supply’s AC Fail signal is used to provide a warning to the user that the AC input power has either been lost, or is dropping in voltage to a point that the power supply will soon no longer be able to regulate or provide power.

Customers using such a signal will then have a short period of time (typically 5 to 10ms) in which to store any data or start an orderly shutdown of their system.

Internal to the power supply, the AC Fail circuit is usually a simple circuit comparing a reference with the voltage of a primary side housekeeping supply. In the event that voltage drops, drive is removed from an opto-coupler and the user provided with an AC Fail signal state change.

Before the widespread use of Power Factor Correction (PFC), the AC Fail did indeed operate at a set input voltage. I remember as a Test Technician reducing the input voltage with a variable transformer (variac) to check the function.

On those non PFC power supplies, the AC input is peak rectified as shown below. The main switching converter operates off that unregulated high voltage buss, the value of which is a direct function of the AC input voltage – between 120 and 375VDC.

Click to enlarge

On power supplies with PFC though, that high voltage buss is regulated using a boost circuit – to around 360VDC. Now any change to the AC input voltage (within the normal operating range) is not reflected in a change in the DC buss; hence a different test method must be used.

Use a storage oscilloscope to monitor the output voltage, the AC Fail signal and if desired, an isolation transformer to display the AC input voltage.

Turn off the input voltage and measure the time between the AC Fail signal going low and the output voltage starting to drop. This is the amount of warning time you will have.

Unlike with a non PFC power supply, this warning time will not be related to input voltage.

Inrush Currents & External Fusing on Power Supplies

2011-02-07T10:12:00.000-08:00

Most power supplies have some form of an internal inrush current limiting circuit. This avoids a large current being drawn when AC is first applied, causing a circuit breaker to trip or an external fuse to blow.

The power supply inrush circuit usually consists of a thermistor in series with the AC line. This thermistor has a high resistance when cold, but once the power supply has turned on; its self heating effect drops the resistance to reduce losses (increasing the power supply efficiency).

See my previous post at http://power-topics.blogspot.com/2010/04/cold-temperature-start-up-of-low-cost.html

A typical inrush current plot for 115VAC input looks like this:

Click to enlarge

You can see the AC is applied at the peak of the AC input voltage to measure worse case conditions. The peak inrush current is 25.65A for a period of 2-3ms with what we call a cold start, in that the inrush thermistor is initially at room temperature (and in a high resistance state).

If we were to expand the time scale, on top of that peak would be a larger spike of current with a pulse width of less than 200μs, generated by the “X capacitors” charging up. X capacitors are fitted across the input to reduce electrical high frequency noise from exiting the power supply. As this is a low energy spike, most power supply manufacturers exclude it from the inrush current specification. The energy drawn is so small it will not trip a circuit breaker, or blow a fuse.

Many customers are confused with the external fuse rating suggestion found in the installation manual. They see from the power supply datasheet that the inrush current is say 30A, but then read from the application note that the recommended external fuse is only 4A (which corresponds to a steady state input current draw by the power supply of around 2A).

This prompts a call to our technical support group saying that they believe there is an error in the application note.

Most of those application notes specify the use of a time delay, or "slo-blo" fuse.

Looking at Littlefuse®'s datasheet for such a 4A fuse we can see from the graph that the average time for the fuse to open varies with the length of time the current passes through the fuse.

Click to enlarge

Going back to the power supply evaluation data, one can see that the inrush current is a maximum of 25.65A and that the time for that pulse is say 3ms (worst case). From the above graph, even at 10ms (0.01s) the current would have to be some 70A for the 4A fuse to blow, giving an adequate design margin.

If a fast acting fuse (type F) had been chosen, the pulse current for the fuse to open would be approximately 30A, which is why we recommend that slo-blo (type T) fuses be used.

Power Supply Efficiency – How Important is it?

2010-12-15T08:43:00.000-08:00

ENERGY STAR®’s decision to “sunset” programs for EPSs (External Power Supplies), and applications using them, has raised the question “Does this mean that power supply efficiency is no longer a concern?”

In a mainly cost driven power supply market, product marketing is often challenged during a new product business plan review, regarding the demand for high efficiency on the product specification.

The biggest requestor of high efficiency power supplies are the manufacturers of large data centers, primarily because of the huge amount of electricity they consume. In 2007 the EPA estimated that the national annual electricity cost for servers and data centers could be a staggering $7.4 billion in 2011.

Power supply manufacturers have been told by data center producers that they are willing to pay (a little) more for higher efficiency products, because the operators are aware that the ROI is quite short. TDK-Lambda also is participating in the evaluation and use of DC power systems.

DC power systems for data centers use up to 15% less power by removing one or more conversion stage by supplying ~380VDC to the servers rather than high voltage AC.

Current UPS Backed-up System

DC Bus System

The US government has had the 80 Plus program in place for servers and computers for a number of years with a variety of efficiency levels ranging from a basic level to “platinum”.

In consumer electronics where although the power draw is quite small (particularly in standby), there are 100s of millions of devices deployed (laptop power supplies, phone & camera chargers & LCD TVs to mention a few) and the overall energy usage is considerable. The main drivers for efficiency improvement are the state and utility companies, because of the cost and time to build new power stations. The average consumer is not that concerned as it will not dramatically affect their electricity bill.

As an industrial power supply manufacturer, TDK-Lambda is being asked by some customers for higher efficiency power supplies. These customers are selling to end users like hospitals or large retailers who are going "green". Where the purchaser is not being mandated to reduce energy consumption or is driven by price, the efficiency “sell” usually fails.

TDK-Lambda's R&D is very focused on developing higher efficiency power supplies, driven by what we see as a future market and TDK's initiative on environmental consciousness. Current high efficiency products include the new EFE series of power supplies that feature 90% efficiency using digital control. http://us.tdk-lambda.com/lp/products/efe-series.htm.

TDK-Lambda also supplies two of the major automotive manufacturers the DC-DC converters for hybrid electric vehicles and is developing new products for a host of upcoming electric cars. Efficiency standards of automobiles is something that the purchaser understands, primarily because of the cost of running a vehicle is significant.

Using the Inhibit or Enable function on Power Supplies

2010-11-24T09:35:00.000-08:00

The inhibit or enable function allows the user to electronically turn on or off the output voltage of a power supply without having to interrupt the input AC or DC voltage with a relay or switch. This is useful during initial set up of the system, during maintenance or for saving energy during periods of non operation.

An easy way of remembering the difference between the two types is that “Inhibit” requires that the user has to do an action to turn off the output voltage, where as “Enable”, the user has to do an action to turn on the output voltage.

It is standard with most DC-DC converters for example, to utilize an enable type function that requires the remote on / off pin to be connected to the negative input (primary side) to activate the output voltage. This is often referred to as “negative logic”. First time users of DC-DC converters often forget to pull that pin low and call Tech Support to complain about a non functioning power supply.

AC-DC power supplies usually have a remote on / off referenced to the secondary side for safety reasons. An “Inhibit” type function, requiring an external voltage, is usually more popular on simple power supplies because once the output voltage is turned off, any auxiliary voltages driving the secondary control circuit is also turned off.

One way power supply designers overcome this is to have an integrated independent “stand-by” auxiliary output like the one used on pc power supplies. This is also used to power the secondary control circuit and allow a closed contact to the 0V terminal to enable the output.

If the system uses several power supplies, the remote on /off can be used to sequence the voltages. I know of thermal printer applications where if the 5V supply driving the control circuit goes faulty, they require the 24V motor drive be inhibited to avoid embarrassing amounts of paper shooting out!

Where several different voltages are used to drive processors, sequencing the output voltages is often critical to avoid damaging those devices. The 3.3V output is rarely allowed to be applied before the 5V is present.

The industry standard I²C based PMBus is now gaining popularity. Power supplies such as TDK-Lambda’s HFE series can be remotely turned on or off using the PMBus software, either as a group or individually for load shedding to save energy.

Damaging Power Supplies with Repetitive Peak Current Draws

2010-10-28T14:43:00.000-07:00

There are many devices that require peak currents when first turned on including print heads, motors, disk drives and pumps.

Many users often do not measure the actual peak current and rely on an empirical method whereby they try a power supply in the application to see if it will work. If the power supply cannot provide enough peak current, capacitors are added to the output. Those capacitors will act as temporary energy storage, enough to deliver load current for a few hundred micro seconds. If that works then the power system solution is deemed as working, and the Engineer moves on to the next phase of their project.

Many power supplies have the ability to supply high peak currents, even though the datasheet does not mention it. In fact some of the cheapest power supplies on the market can deliver very large currents for a short period of time because the output current limit is very crude, and is primarily there to protect the unit against a short circuit on the output.

In discussion with TDK-Lambda Engineering, I learned that this can lead to field failures. Let me explain further.

Below is a schematic of a forward converter, the power FET is shown as a switch for simplicity. That “switch” operates at a rate usually in the hundreds of kHz, energy is transferred from the secondary side to the output rectifiers and then is smoothed by the output LC filter.

When a pulsed (peak) load is applied to the power supply in excess of its rated current, the energy is first drawn from the output capacitor. This can add to the capacitor ripple current, raising the temperature and reducing the component’s life. Heat, as I explained in earlier blogs, dries out the capacitor’s electrolyte.

When the energy stored in the capacitor starts to deplete, the power supply will then try to continue to provide the peak current from the main switching circuit. This in turn leads to repetitive surge currents in both the output diodes which is then reflected by the transformer to the power FET. Often this peak current exceeds the maximum rating of the semiconductors leading to latent and erratic field failures.

Additional heating in the transformer, inductors and printed circuit board traces is also experienced because, although the average power drawn from the power supply is less than the continuous rating, we are dealing with the formula I2R and the peak current is now squared.

TDK-Lambda recommends using a power supply that has a specified peak power rating like our HWS-P series or working closely with the power supply manufacturer to determine if the product is suitable for the application

How does the new UL Mark affect me?

2010-09-30T14:09:00.000-07:00

UL recently announced their first Transcontinental Mark which denotes product compliance with European EN safety standards as well as the CSA/UL/US Mark for the North American markets. Basically, UL will be providing an “international safety certification”. And, this may affect European safety houses like TUV and others. Here is what the new international UL mark it looks like:

Although, at first glance, this mark looks like a “UL Listed” power supply certification with its encircled UL mark (as used for External Power Supplies), UL-EU confirmed that it also applies to stand-alone component power supplies like those installed in all electronic equipment. Prior to this new mark, the standard “component power supply” UL mark for the US and Canada looked like this:

Some power supply manufacturers have been reviewing use of UL to display the existing C-UL-US mark along with the self-declared CE mark. The CE mark would be backed up with an IEC60950-1 CB report as documented proof of compliance.

It will be interesting to see how the European test houses (e.g., TUV) react and if CSA responds with their own version of this new UL mark.

Power Supply Leakage Current Testing to IEC60990

2010-08-02T13:22:00.000-07:00

A customer recently asked me why we specify leakage current on a Class II power supply, when a Class II power supply has no ground terminal. A good question, but first some background.

As part of the testing for IEC60950, power supply manufacturers measure leakage current to the IEC60990 standard.

To be more accurate, the terms "Touch Current" and "Protective Conductor Current" replace the term "Leakage Current".

Protective Conductor Current (PCC)

Is the current that flows through the protective conductor; commonly referred to as the ground connection.

As a note, the withstand voltage and insulation resistance tests measure the current flowing through the insulation of the unit under test.

Touch Current (TC)
Is the current that flows when a human body touches the equipment, simulated by a body impedance network.

The switches are used to simulate a line, neutral or ground fault, referred to as a single fault condition (S.F.C.). Usually there is a polarity reversal switch to reverse the line and neutral connections to the power supply.

So back to the original customer question, if a Class II power supply is used, there will be current that flows through a human body upon touching conductive parts in a system (like a USB port or a conductive product case). That measured current is usually listed on the power supply datasheet.

Here is an excerpt from a CB report showing the test, input voltage, frequency and the measured touch current. Note half the tests conducted were with the simulated human body touching the “output connector” or pins of the power supply.

Enclosure leakage current (normal conditions, normal polarity)	264 V~	63 Hz	5,3 µA
Enclosure leakage current (normal conditions, reverse polarity)	264 V~	63 Hz	4,1 µA
Enclosure leakage current measured on output connector (normal conditions, normal polarity)	264 V~	63 Hz	89,0 µA
Enclosure leakage current measured on output connector (normal conditions, reverse polarity)	264 V~	63 Hz	87,0 µA
Enclosure leakage current (single fault conditions, neutral open, normal polarity)	264 V~	63 Hz	4,1 µA
Enclosure leakage current (single fault conditions, neutral open, reverse polarity)	264 V~	63 Hz	6,0 µA
Enclosure leakage current measured on output connector (single fault conditions, neutral open, normal polarity)	264 V~	63 Hz	3,0 µA
Enclosure leakage current measured on output connector (single fault conditions, neutral open, reverse polarity)	264 V~	63 Hz	129,0 µA

The ammeter used is a specialized meter; do not use a regular hand-held multi-meter!

For more details, including the limits of the measured currents, please consult a professional safety engineer.

Get the product brochure from TDK-Lambda Americas for product descriptions.

Did my power supply fail or just wear out?

2010-07-06T08:52:00.000-07:00

A true power supply failure is a rare occurrence provided the following occurs:

The manufacturer has taken the appropriate steps on component evaluation, component derating and has utilized sound design techniques.
The user is operating the product in accordance with the manufacturer’s instructions.

TDK-Lambda, for example, puts many components through a myriad of stress tests including voltage testing under extreme humidity and atmospheric pressures, beyond the manufacturer’s specified maximum ratings. Only upon passing those tests is the component supplier added to the approved vendor’s list.

Monitoring primary switching currents at high line and high ambient temperatures during transient loading can reveal how much design margin the power supply has. Below you can see that our competitor’s power transformer is inadequately sized and is drawing a huge increase in current as it saturates.

So, when I hear “My power supply failed after just three years in the field” from potential customers, I review their application and often have to deliver the bad news. Their power supply has just worn out.

Recently a manufacturer of semiconductor fabrication equipment called me. They were using a one-year warranty power supply and were running it at the full rated power level.

I asked the Engineer how long was their equipment typically operated in the field. “Our equipment is usually running 24/7 (24 hours a day, 7 days a week)” was the reply and “our customers expect to use the machine for at least ten years.”

In a power supply, the most frequent wear-out component is the electrolytic capacitors. Capacitor life has improved greatly with reasonably priced electrolytics, now typically rated for 10,000 hours at 105°C. Even then, that is only 1.14 years when used 24 hours a day in a very harsh environment.

The solution is quite simple, choose a suitable grade of power supply and apply sufficient deratings. I always ask about the application, the expected field life, the cost impact of their customer’s equipment being out of service, and the cost impact of having to service the equipment. If the application is in a remote application or would require a service person to drive or fly out to the location, paying an extra $50 or even $100 more for an rugged, industrial grade power supply will be more economical than paying for a $500 field service call, perhaps 3 years times.

I often equate power supply life to that of buying a cheap set of brake pads for one’s car. Yes, it costs less initially, but you will probably be back for another set of pads (and spending time in the repair shop) in less than half the time that a higher quality set of pads would last.

Common safety standards used with power supplies

2010-05-13T15:24:00.000-07:00

A topic that comes up very regularly is what do the various safety standards pertain to. Here is a brief list of commonly used standards, the equipment that uses them, typical products and their purpose.

Base Standard	Country (ies)	Equipment Type	Generic Product Types	Purpose
CCC	China	Many	Many	Safety & Quality Mark
CE	EU	Voluntary declaration based on miscellaneous standards	Many	Indicate conformity to standards
CSA 60950	CSA	Information Technology	Office machines, data & telecom networks, IT, Kiosks	Protect against fire, electric shock, injury
CSA60065	CSA	Audio, Video and Similar	Video, audio, projectors	Protect against fire, electric shock, injury
CSA60601	CSA	Medical Electrical	Surgical, monitoring, hospital equipment	Safety
CSA61010-1	CSA	Measurement, Control and Laboratory	Meters, Oscilloscopes	Protect against fire, electric shock, injury
EN50178	EU	Power	Power generation, power installations	Safety
EN55011	EU	Industrial, scientific, and medical r-f	Monitoring equipment, automation controls, measuring	Limits and measurement of radio disturbance
EN55015	EU	Lighting	Lighting, LED lighting, street lighting	Limits and measurement of radio disturbance
EN55022	EU	Information Technology	Office machines, data & telecom networks, IT, Kiosks	Emission Limits (radiated and conducted)
EN60065	EU	Audio, Video and Similar	Video, audio, projectors	Protect against fire, electric shock, injury
EN60601	EU	Medical Electrical	Surgical, monitoring, hospital equipment	Safety
EN60950-1	EU	Information Technology	Office machines, data & telecom networks, IT, Kiosks	Protect against fire, electric shock, injury
EN61010-1	EU	Measurement, Control and Laboratory	Meters, Oscilloscopes	Protect against fire, electric shock, injury
Factory Mutual	US	Use in hazardous locations	Oil refinery, petrochemical	Protect against explosion
FCC Part 15	US	Many	Many	Emission Limits (radiated and conducted)
ISA 12-12	US	Use in hazardous locations	Oil refinery, petrochemical	Protect against explosion
REACH	EU	Many	Many	Registration, Evaluation, Authorization and restriction of Chemicals
SEMIF47	US	Semiconductor fabrication	Die manufacturing, wafer fabs	Withstand dips in AC input
UL 60950	UL	Information Technology	Office machines, data & telecom networks, IT, Kiosks	Protect against fire, electric shock, injury
UL1310	UL	Power Supplies	DIN Rail, LED lighting, access controls, building automation	Protect against fire
UL508	UL	Industrial Control	Process control, factory automation	Safety
UL60065	UL	Audio, Video and Similar	Video, audio, projectors	Protect against fire, electric shock, injury
UL60601	UL	Medical Electrical	Surgical, monitoring, hospital equipment	Safety
UL61010-1	UL	Measurement, Control and Laboratory	Meters, Oscilloscopes	Protect against fire, electric shock, injury
VCCI	Japan	Information Technology	Office machines, data & telecom networks, IT, Kiosks	Limits and measurement of RF emissions
VDE0805	Germany	Information Technology	Office machines, data & telecom networks, IT, Kiosks	Protect against fire, electric shock, injury

Let me know if this table is helpful.

New video on the LS Series low cost power supply

2010-04-28T10:22:00.000-07:00

TDK-Lambda Americas just posted their new video on the LS Series, a low cost power supply backed by a three-year warranty. Check it out.

Cold Temperature Start Up of Low Cost Power Supplies with Inrush Thermistors

2010-04-26T11:24:00.000-07:00

I often get asked the question: "Regarding the TDK-Lambda low cost power supply that is rated from -25°C to 70°C, will it start up at -40°C?" I usually reply, "It depends."

Most low cost, low wattage power supplies avoid large surges of current being drawn when the AC input is first applied by using a thermistor in series with the AC line (see figure 1). This device is a type of resistor that when cold has a much higher resistance than when warm.

Figure 1: Thermistor

When the AC is first applied, the thermistor limits the amount of inrush current that charges the bulk storage capacitor. Once the power supply starts-up and delivers power, the thermistor self heats and decreases in resistance to improve the power supply efficiency and operation.

At ambient temperatures below freezing, these thermistors have very high resistances, and if the supply is “rated” to start-up at a cold temperature, it should have been tested during the design stage to ensure correct start-up at full load and at minimum AC input. If the power supply is not specifically rated for a cold temperature start-up, there is a possibility that the power supply will turn on, try to deliver power, but the thermistor will not have self-heated due to the very cold ambient and hence will have a large voltage drop across it, causing the power supply to switch off again. The power supply will try to restart again, causing “blips” on the output. (Fig. 2, top trace). In some circumstances, the power supply might not start up at all. These attempts to restart can cause system problems.

Figure 2: Cold Temperature Start Up

If the output load is light, however, the power supply may be able to start up correctly.

So back to my reply to the initial question! "It depends on what the loading will be at -40°C. If the application has say 20% loading, then usually the answer is yes." There are other issues with cold temperature operation, but I shall cover that in the future.

Hipot or Dielectric Strength Testing

2010-03-03T14:53:00.000-08:00

One area of confusion in production safety testing is the dielectric strength test, sometimes known the as dielectric withstand test or “hipot” test.

This test is usually applied between the secondary output and chassis ground and then between the AC connection (primary) and ground / secondary. This test can identify any assembly errors such as a pinched wire.

It is important to ensure to short the line and neutral together during the test, and when making the primary to secondary test, connect the secondary side to chassis. Short the output terminals together if testing a standalone power supply. Failure to do this can result in damage to the power supply.

A routine question is “should the test voltage be AC or DC?” The majority of power supply manufacturers use a DC voltage because the leakage current through the “Y” capacitors can mask another fault.

The “Y” capacitors are identified in a very simplified diagram below are used to reduce EMI and electrical noise.

As can be seen, applying an AC input to chassis hipot test would result in mill-Amps of current flowing through the capacitors

The majority of safety standards allow DC hipot voltages. Instead of applying 1500VAC, one would use the peak voltage of the AC, √2 x 1500 = 2121VDC. Add 10% to reduce the test time from 1 minute to 1 – 2 seconds.

Apply the DC voltage slowly to allow the capacitors to charge up without tripping the current limit of the test equipment. Remember to discharge the capacitors after the test.

Choosing Power Supplies for Medical Applications

2010-02-03T09:22:00.000-08:00

The selection and specification of power supplies for medical applications is a task that must be approached with great care; especially in these times where key safety and environmental standards for medical equipment are undergoing substantial changes that will affect large segments of the medical industry.

Modern switchmode power supplies are employed in a wide array of medical equipment including: MRI, X-ray, CT and PET scanners, blood analyzers, DNA equipment, patient monitors, ultrasound, robotic surgical devices, heart-lung machines, diagnostic equipment and automated pharmaceutical dispensers, to name but a few. As with all electronics, the trend in medical equipment is to make them smaller, lighter in weight, more efficient, more reliable and competitively priced. The safety standards for medical equipment vary dependent upon the application, proximity to patients and operators, and the location and environment of the equipment.

In the design of medical electronic equipment there is one consideration which takes precedence over all others, and this is the safety of the patient and operator. In some cases, it might be tempting to think that power supplies that have been designed and certified to be safe in industrial applications might be equally suitable for use in medical equipment. This is not usually the case because the risks involved are much different. Furthermore, much of the electronic equipment used in hospitals, such as patient monitors, operate with very low-level signals. Medical equipment like this tends to be more sensitive to electromagnetic interference (EMI) than most of the equipment used in industry, which also makes EMC (electromagnetic compatibility) compliance and performance a key concern in medical applications.

Protecting the Patient & Operator
Hospital patients are frequently in a weak condition. Exposure to even small leakage currents can have an adverse effect on their well-being. The same small leakage currents could have little to no effect on a healthy person and might be acceptable in industrial applications. Depending upon the application, the “allowed leakage current” from the end-product medical equipment (not the power supply alone) can vary from a few µA (microamps) to a few hundred µA. The “leakage current” can be defined as the unintended, and potentially harmful, electric current that may pass through the human body. Obviously, medical equipment that has direct physical contact with patients must limit their leakage current to the lowest prescribed levels.

Changing Medical Power Supply Safety Standards
The special requirements of medical equipment are reflected in international standards. For most of the world, including Europe and North America, the safety standards for medical power supplies are contained in the IEC60601-1 standards. As of the writing of this article, the present version of IEC60601-1 is the 2nd edition (originated in 1988). The 3rd edition of the IEC 60601-1 (originated in 2005) is being reviewed by power supply manufacturers and global safety certifying agencies for future adaptation. There are many differences between the 2nd and 3rd editions, foremost of which is the requirement in the 3rd edition for the establishment of a “Risk Management Process” and record/file retention in compliance with the ISO14971 standards. It is therefore expected that future product certifications to the IEC60601-1, 3rd edition may include an audit of the manufacturer’s compliance with ISO14971 (Risk Management Process). The exact date that the 3rd edition of IEC60601-1 may replace the 2nd has not yet been firmed up, but some predict it will occur as early as 2010.

The first and foremost requirement of the IEC60601-1 (both editions) is for the effective and reliable isolation between the AC input to the power supply, its internal high voltage stages, and its DC output, as any shortcoming in isolation would result in the risk of electric shock. Several factors contribute to effective isolation including the spacing between conductors and the electronic components. The IEC60601-1, 2nd edition sets minimum distances for spacing between these elements and it is important to note that these are greater than the spacing distances prescribed within the relevant standards for ITE (Information Technology Equipment) and industrial power supplies, which is covered by IEC60950-1.

In addition to adequate spacings between conductors/components, effective isolation also depends on reliable insulation. Most modern medical power supplies use double insulation or reinforced insulation, the effectiveness of which is verified by dielectric strength testing. This involves subjecting the insulation to a much higher voltage than that at which it operates, and ensuring that no failure occurs.

Medical requirements differ from those for standard power supplies. Reinforced or double insulation in supplies, which operate from a 240Vac mains for example, must withstand a dielectric test at 4kVac for medical applications, whereas the corresponding figure for ITE/industrial use is only 3kVac. As with the spacings, this difference must be taken into account when choosing a power supply. Power supplies that are approved to less than 4kVac may be used in medical applications as part of a reinforced barrier, provided that the insulation within the power supply is regarded as a lesser “basic” or “supplementary” barrier. In this case, additional isolation must be provided within the end-product medical equipment by the equipment’s manufacturer to achieve the requirements of a reinforced barrier between the AC mains supply and the patient. The 3rd edition of the IEC60601-1 separates the requirements for the patient and operator whereas the 2nd edition treated them as equal.

The leakage current requirements of the IEC60601-1, 2nd edition are difficult to achieve, while at the same time keeping the RFI low. The maximum permissible earth leakage is 300µA for worldwide approvals, but this figure applies to the end-product as a whole, not just the power supply. To allow for additional leakage in other components it is highly desirable for the power supply to have an even lower leakage current and/or for the medical OEM to install additional layers of insulation and isolation within their end product.

This leads to an interesting challenge since EMC performance is another crucial issue for medical power supplies. All modern power supplies are of the switchmode type, as these are smaller and more efficient than the old linear types. Switchmode supplies, however, generate electromagnetic interference (EMI), both conducted and radiated and require the incorporation of EMI filters to limit this unwanted electrical noise. The capacitors in these EMI filters allow a small amount of leakage current to flow and the more effective the filter at suppressing the interference, the more leakage it is likely to produce. Therefore, there is a trade-off between EMC performance and leakage current.

Tips for Selecting Medical Power Supplies
Modern medical equipment requires power supplies that are compact, lightweight, efficient, cost-effective, RoHS compliant, reliable and super-safe. Switch-mode power supplies can meet all of these needs, but not all supplies are created equal. OEMs of medical equipment should take care to choose power supplies from a reputable supplier, preferably with proven experience in the medical electronics field, and with a good understanding of the special demands and changing standards involved in this specialized industry.

Obviously, selecting medical power supplies based on the lowest price is foolish because of the high costs of potential law suits, product recalls, brand name damage, and warranty repairs far exceed any front end potential cost savings. Medical OEMs should also take care to ensure that their choice of power supplies fully satisfies, and is certified to meet, the prevailing edition of the IEC, EN, UL and CSA safety standards for medical supplies. Taking these precautions will make it much easier for newly developed medical equipment to be certified by the responsible safety agency and the FDA (Food & Drug Administration). More information concerning medical power supplies is available at: http://us.tdk-lambda.com/lp/products/medical.htm

LED Traffic Signals – A Double Edged Sword

2010-01-15T14:05:00.000-08:00

An unforeseen side effect of using high efficiency LED lights in traffic signals became apparent as winter storms swept across the country. Normally the heat generated by the incandescent bulbs used in the traffic signals is enough to melt snow. Since LED lighted signals consume 30% less energy, far less heat is generated. Therefore, the snow is not melting and the traffic signals became obstructed, leading to accidents.

Manufacturers are experimenting with alternative sloping lens and shields making it harder for snow to stick.

As electronics and their power supplies dissipate less power, it will be interesting to see if there will be other outdoor applications affected.

TDK-Lambda Launches 1kW Low-Profile Power Supplies

2009-12-15T10:11:00.000-08:00

San Diego, CA – December 15, 2009 – TDK-Lambda has introduced the RFE1000 series; a new 1kW single-output AC-DC power supply in an ultra low profile (only 1.61" high) 1U package for stand alone or distributed power architectures (DPA). These supplies are ideal for applications requiring reliable 24V, 32V or 48VDC bulk power. Up to +20% and -10% output voltage adjustment is possible, enabling the RFE1000 to be used in a variety of customer specific applications. Operating from a universal input of 85 to 265VAC with PFC, typical applications for the RFE1000 supplies include communications, factory automation, test and measurement, robotics and RF amplifiers. Efficiency of up to 88% minimizes heat dissipation.

The RFE1000-24, -32 and -48 power supplies can be used individually, or up to 8 units can be connected in parallel to form an N+1 redundant power system with built-in ORing diodes. Each power supply has variable-speed cooling fans and can operate in temperatures ranging from 0 to +70°C. The RFE1000 has a power density of 10.5W/in³ with dimensions of 12 x 5 x 1.61 inches.

Overvoltage, overcurrent and overtemperature protection are standard features, and for system monitoring there are opto-isolated signals for DC-OK, AC-fail and overtemperature warning, along with a LED indicator for DC-OK. Remote On/Off control is also standard, as is remote sense. Other standard features include single-wire current sharing and an auxiliary 12V/0.25A output.

As well as being EN55022 and FCC EMC compliant (achieving class B conducted and radiated emission), the RFE1000 series meets UL/EN 60950-1 safety approvals and carries the CE mark. Harmonic correction meets the EN61000-3-2 standard, and the power supplies are backed by a two-year factory warranty.

The new RFE1000 family of power supplies is available now and priced at $295.00 each in 1000 piece quantities. More information can be obtained at the following TDK-Lambda website http://us.tdk-lambda.com/lp/products/rfe-series.htm or by calling 800-LAMBDA-4.

TDK-Lambda is a subsidiary of the TDK Corporation, a leading global electronics company and has been a major provider of power solutions for over 60 years. The company designs and manufactures a wide range of AC-DC and DC-DC power products for Industrial, Medical, Telecom, Datacom, and Test & Measurement applications worldwide. For more information, please call TDK-Lambda directly at 619-575-4400 or visit the website at: www.us.tdk-lambda.com/lp

Why Use DIN Rail Mount Power Supplies?

2009-11-03T10:35:00.001-08:00

DIN Rails are metal strips that provide a convenient means for mounting electric and electronic devices in a compact and neat manner. For example, DIN Rails are frequently used for mounting circuit breakers (Fig #4), terminal strips (Fig #3), power supplies (see photo above) and all sorts of industrial control equipment within racks/enclosures or attached to backboards. In this way, any combination of devices can be mounted next to each other to meet the system requirements.

Standard DIN rails are shaped as shown in Figure #1 (end view) and #2 (photo). They typically measure 35mm from edge to edge. The distance from the back to the rail of the front bends can be either 7.5mm or 15mm. These metal DIN Rail strips can be provided in any length to suit the application and multiple rows of rails can be used.

Figure #1 – End view of typical DIN Rail

Figure #2 – DIN Rail with slotted mounting holes

Figure #3- Terminals Strips mounted on DIN Rail

Figure #4- Circuit Breaker mounted on DIN Rail

The use of DIN Rail mounting systems saves installation time since all devices just snap onto the metal rails. A complete system can be quickly put together in an organized configuration that provides high density, flexibility, safety and design time savings. Associated devices can be mounted adjacent to each other, thus reducing the length of interconnect wiring.

The DIN Rail concept is widely used in industrial control, instrumentation and automation applications. Today, even DIN Rail mountable micro-computers are available and being used.
DIN rail mounted AC-DC power supplies provide a convenient means for powering DC operated devices including sensors, transmitters/receivers, analyzers, programmable controllers, motors, actuators, solenoids, relays, etc., to mention a few. Since these power supplies are convection cooled, no cooling fans are needed. Output voltages from these supplies range from 5V up to 56V with power ratings from 7.5W up to 480W. Many of these supplies can be connected in parallel for higher power applications.

In some cases, conventional power supplies can be utilized in DIN Rail systems by means of “DIN Rail Mounting Kits/Adapters”. See Figure #5 below and more details at this web site: http://www.us.tdk-lambda.com/lp/products/ldin-series.htm

Figure #5 - DIN Rail Mounting Adapter Kit for conventional power supplies

Detailed information about TDK-Lambda’s wide range of DIN Rail mount power supplies is available at this web link: http://www.us.tdk-lambda.com/lp/products/finder6.htm

Is there a wave of power supply obsolescence coming?

2009-10-09T08:46:00.001-07:00

The old 2001 version of the safety agency specification UL60950-1 for power supplies expires in December 2010. Usually revision changes are minor for industrial power supplies, but one key component is affected this time. The 2005 version stipulates that any surge suppressor on the primary side shall be a VDR (Voltage Dependant Resistor) which must be comply with Annex Q and be approved to IEC 61051-2.

To comply, power supply manufacturers will have to modify their designs and retest to demonstrate compliance to the immunity specification EN61000-4. This is going to be costly in either Engineering labor costs (opportunity cost) or external test house fees.

Manufacturers may now decide to obsolete slow moving products before December 2010 and continue investing in new products.

TDK-Lambda's 100-150W External Power Supplies Meet New Energy Efficiency Standards

2009-08-18T09:32:00.000-07:00

TDK-Lambda has released a new range of AC-DC external power supplies with models rated from 100 to 150-watts that meet the latest Energy Star, EISA, and CEC standards. The DT100-C and DT150-C series feature active PFC (meets EN61000-3-2) and operate from a universal AC input of 90 to 264Vac (47-63Hz). Available output voltages include 12V, 16V, 19V, 24V, 36V and 48V.

These external power supplies are packaged in an insulated compact and lightweight enclosure measuring 3.35" wide by 6.7" long by only 1.73" high and are convection cooled (no fans needed). The operating temperature range is 0 to +40°C with no derating required. All models are fully isolated (3kVac, input to output) and meet the Energy Star 1.1 and the California Energy Commission (CEC) level IV efficiency standards. Plus, models with outputs of 24V to 48V meet the Energy Star 2.0 version level V standards.

These units include overvoltage and short-circuit protections and off/no-load standby power consumption of less than 0.50 watt as required by the green energy initiatives. In addition, these series include UL/EN/IEC60950-1 international safety agency certifications and meet EN55022-B and FCC Class B conducted and radiated EMI standards.

Moreover, these external power supplies feature a recessed IEC320-C14 AC input receptacle and a 3.4 foot long output cable with a molded 4-pin connector. Other output connector types can be special ordered. These 100-150W supplies are ideal for powering instrumentation, industrial devices and a wide range of other general purpose applications.

The DT100-C and DT150-C series are available now and priced from $40.50 each in OEM quantities of 1,000 units. For more information, please call TDK-Lambda directly at 1-800-LAMBDA-4 or see website: http://www.us.tdk-lambda.com/lp/products/dt-c-series.htm

About TDK-Lambda:

TDK-Lambda has been a major provider of power solutions for over 60 years. The company designs and manufactures a wide range of AC-DC and DC-DC power products and EMI filters for Industrial, Medical, Telecom, Datacom and Test & Measurement applications. TDK-Lambda is a subsidiary of the TDK Corporation (TSE, LSE: TDK), a leading global electronics company (www.tdk.com). For more information, please call TDK-Lambda at 619-575-4400 or 1-800-LAMBDA-4 (toll free), or visit our website at: www.us.tdk-lambda.com/lp

Operating Power Supplies in Series

2009-07-22T14:24:00.000-07:00

Although some users are nervous about operating power supplies in series, it is common practice in the industry. The benefit is that voltages greater than 60V can be obtained using off-the-shelf products.

It is possible to connect several power supplies in series, but please read the precautionary notes below:

Connect back-biased diodes across the power supply terminals as shown below.

Rate these diodes at the same output current as the power supplies.

In the event both power supplies do not turn on at the same time, or if the load becomes a short circuit, then the diodes will protect the power supplies from any applied reverse voltage.

Do not exceed the output to ground/chassis voltage rating. Inside most power supplies are noise filter capacitors connected from the output to ground. It is possible to exceed the operating voltage of those capacitors, particularly when configuring several units in series.

Avoid using “fold-back style” current limited power supplies as these may lock up the power supply during initial switch on.

TDK-Lambda Packs 15W into 1 x 1 Inch DC-DC Converter

2009-07-21T09:44:00.000-07:00

San Diego, CA – July 2009 – TDK-Lambda has added two new series to its already successful PX family of DC-DC converters. The fully-isolated PXA and PXB Series have a tiny 1 x 1 inch footprint and are designed for applications ranging from communications to factory automation equipment. They are available in open frame (PXA) and shielded metal case (PXB) versions.

The PXA Series offers single-output models with nominal inputs of 24V and 48VDC in 2:1 and wide 4:1 versions. The PXB Series offers both single and dual-output models with nominal inputs of 12VDC in a 2:1 version and inputs of 24V and 48VDC in 2:1 and wide 4:1 versions.

Available single-output voltages for the PXA and PXB series include 3.3V, 5V, 12V and 15VDC. In addition, the PXB series offers dual-output models that provide ± 5V, ± 12V and ± 15VDC outputs. Efficiency is up to 88%. Remote on/off and output adjustment (trim - single output models) features are standard, plus all models include overvoltage and overcurrent/short circuit protection. Operating temperature range is -40°C to +85°C.

The PXA is available in through-hole or SMD mounting formats and the PXB in a through-hole mounting format. All models are CE-Marked and are certified to UL/CSA/EN60950-1 safety standards as well as meeting the rigorous MIL-STD-810F thermal shock and vibration specifications.

The PXA and PXB series are available now and priced from $28.00 each in 500-unit quantities. For more information, please call TDK-Lambda directly at 1-800-LAMBDA-4 or visit the website at: www.us.tdk-lambda.com/lp.

About TDK-Lambda:
TDK-Lambda has been a major provider of power solutions for over 60 years. The company designs and manufactures a wide range of AC-DC and DC-DC power products and EMI filters for Industrial, Medical, Telecom, Datacom and Test & Measurement applications. TDK-Lambda is a subsidiary of the TDK Corporation (TSE, LSE: TDK), a leading global electronics company (www.tdk.com). For more information, please call TDK-Lambda at 619-575-4400 or 1-800-LAMBDA-4 (toll free), or visit our website at: www.us.tdk-lambda.com/lp

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NV Series Configurable Power Supply

2009-06-10T14:18:00.001-07:00

Comprehensive review of the powerful and adaptable NV Series of power supplies from TDK-Lambda Americas. These power supplies are 1U high, have up to 90% efficiency, active power factor correction, up to 8 outputs, no minimum loads and medical approvals.

PFE Series Brick Power Supply

2009-06-10T14:17:00.003-07:00

This video describes an innovative new AC-DC power module (PFE Series) that combines in a single pcb-mountable brick the major functions of an AC-DC power supply including input rectifiers, PFC circuits, switchmode converter, and provides a tightly regulated DC output.

Redundant Power Systems (FPS Series)

2009-06-10T14:17:00.001-07:00

Provide increased output power by connecting two or more power supplies in parallel. We have also covered how to construct an N+1 Redundant and Fault-Tolerant Power System and the features required of the supplies in order to accomplish this with the greatest amount of reliability and ease of system maintenance.

CE Mark and DC-DC converters

2009-05-03T11:47:00.000-07:00

The CE identification mark is accepted across the entire European Community. It is an indication that the product to which it is applied to conforms with the minimum requirements of all the applicable European Directives for that product, and that duly authorized assessment procedures (technical files) have been carried out on that product.

The most common power supply Directive is the Low Voltage Directive (LVD), EC number 73/23/EEC, which came into force in 1973. This applies to all electrical equipment with an AC input voltage of between 50 and 1000 V for alternating current and between 75 and 1500 V for direct current.

Many DC-DC board mount converters are hence exempt from this directive if they have a nominal input voltage of 5VDC, 12VDC or 24VDC provided the input range is not 75VDC or higher.

Most 48V input DC-DC converters have a DC input range of 36V to 75VDC and so fall under the Directive. 4 : 1 input range models (18 - 75VDC) would also be covered.

A link to the LVD can be found here http://ec.europa.eu/enterprise/electr_equipment/lv/index.htm

Nagaoka Fire Works 2008/08/02 TDK-Lambda

2009-04-24T16:22:00.001-07:00

Fireworks display from our sister company's fall festival. Fun to watch if you like fireworks.

Maximizing the Life of Power Supply Fans

2009-04-22T14:57:00.000-07:00

The vast majority of medium to high-power AC-DC power supplies have integral fans that are required to keep their internal components at safe operating temperatures. Since fans are electro-mechanical devices they are subject to wear out faster than any other component in the power supply.

The chart and diagram below illustrate this very well. As can be seen from the chart, if a power supply’s fan is operated with a high exhaust air temperature at perhaps 80°C (176°F) its life expectancy may be a short as 1.5 years. However, by reducing the exhaust air temperature (as measured 2-inches away) to perhaps 40°C (104°F) the fan’s life expectancy may now exceed 5 years.

Obviously the requirements of a specific application may require different operating temperatures. However, whenever possible, lowering the operating temperature of the power supply will increase the life of the fan as well as the components within the supply. Also, by derating a power supply below its maximum power rating will have a direct effect on its internally generated heat and, therefore, its exhausted air temperature, which will extend the life of its fan.

Positioning the power supply so cooler air is drawn in through the power supply from outside of the system will also help.

Also, a power supply fan’s life will naturally be extended if the supply is turned off when not needed. Some of the newer fans are thermally controlled so they turn on and off automatically. There are also variable-speed fans that increase or decrease the fan’s speed depending upon the sensed ambient temperature or the load required of the power supply. These have the advantage of extending the fan’s life as well as reducing the audible noise when the load current is low.

Another important factor for fan life maximization is to keep the area around the power supply (inlet and outlet) as free of dust and dirt as possible. Dust, metal and chemical particulates can sometimes kill a fan quicker than high temperatures.

If a fan starts making squeaking sounds, it’s a good indication that it should be replaced very soon, before it freezes up. Fan replacements should only be done by qualified electronic technicians who are familiar with the high voltages that can exist within power supplies even after the AC power is removed.

Why pay more for a power supply with a longer warranty?

2009-03-28T07:41:00.000-07:00

Since all power supplies contain similar electronic components such as capacitors, semiconductors, resistors, transformers, inductors, etc., why pay more for one with a longer warranty period? In today’s cost sensitive world, questions like this come up all the time. It’s easy to get caught up in the idea of buying a power supply with the lowest price rather than its warranty time-span.

It’s interesting to note that over 50% of TDK-Lambda’s standard power supplies that are sold each year carry a five-year or longer warranty. Is it that these customers have lots of money to fritter away on this luxury, or do they realize some hidden benefits?

One of the major cost drivers in power supplies is, not surprisingly, the component costs. For example, all power supplies use electrolytic capacitors, which are available with various capacitance, voltage and operating temperature ratings.

Electrolytic capacitors contain a paste-like electrolyte which will eventually dry out and cause the capacitor to fail. How quickly this process occurs depends heavily upon what materials are used to make these capacitors and how close to their maximum ratings these components are utilized.

Electrolytic capacitors used in industrial-rated power supplies are more costly than those used in light commercial applications, but they are made to last for many, many years without failing. It’s like comparing a professional mechanic’s tools to those sold in variety stores. You get what you pay for when it comes to high quality tools; the same holds true when buying power supplies.

Furthermore, the power supply designer can choose to operate the capacitors at or near their maximum ratings, which will result in a low-cost product, but with a shorter life. Or, if a longer field life is a consideration, the designer will “derate” the capacitors, which means he will make sure the capacitors are running at a lower voltage and operating at temperatures that are well below its maximum. In this way the designer can achieve a much more reliable and longer life design at a somewhat higher cost. The same trade-offs in design are made for the semiconductors, resistors and other components that comprise the power supply.

In addition to the above, the life span of a power supply depends a great deal on the operating environment. In an industrial environment where a manufacturing plant is running multiple shifts, the power supply may be operating 24 hours a day, 360 days a year, with an ambient temperature within the equipment of perhaps +50°C (+122°F) or higher. Compare this to an office or medical environment where the ambient temperature might be typically +30°C (+86°F) and the equipment is running 8 hours/day, 5 days a week. Obviously, in the industrial application a more robust and higher quality power supply would be required to handle the rigors of these applications.

Power supply manufacturers want to avoid paying the high costs associated with repairing a failed unit within its warranty period. Therefore, based on their predicted life calculations and field return data, they set the warranty period such that the power supply will, in the vast majority of cases, not fail within the warranty period. And, they usually ensure that their supplies have a buffer life-time of 6-months to a year or so beyond their warranty period. So, it turns out that the warranty period is a fairly good indicator of how long you can expect the power supply in your equipment to run without failing. If you purchase a low cost commercial power supply with perhaps a one year warranty and install it in your industrial equipment that may carry a 3 year warranty, that would be a big mistake. Your low-cost power supply would quickly lose its cost advantage when it fails prior to your OEM warranty expiring.

So, we now come to the answer of our headline question:
Why pay more for a power supply with a longer warranty?

Answer: Because it’s the most cost effective way for the OEM to avoid premature field failures, trouble calls, unhappy customers, and high field service/product repair costs.

HWS Series power supplies from TDK-Lambda come with a Limited Lifetime Warranty -- an industry first

Power Supply Considerations for Industrial Applications

2009-02-20T12:02:00.000-08:00

Although power supplies are among the most important components of any industrial application, they seldom receive any significant attention. Engineers often do not fully understand all of the variables that go into choosing the correct power supply, and may select a product that is insufficient or more costly than what is needed.

When considering a power supply for an industrial application, it's helpful if a designer has an understanding of the steady state output parameters of the product, as well as the electrical and physical environment that the equipment will operate in. Here are some critical considerations.

Unique Load Requirements
Motors, solenoids and relay controls require higher levels of current when they are turned on than they do for continuous operation. It is necessary for the designer to examine the magnitude of the pulses and either specify a power supply that is capable of providing the surge currents continuously, or use a product that can provide peak power for a limited time. Certain models, for example, can deliver up to 200% of the nominal rated current for up to 30 seconds. This enables the user to purchase a 240W unit to meet a 480W surge load, saving both money and space.

The designer should also anticipate potential mechanical failure of factory equipment. If a motor stalls or a relay "sticks", the current draw can rise dramatically. Using a power supply that is capable of protecting itself in overload conditions will both protect the unit and the system.

Input Line Disturbances
In most industrial environments the AC line is far from clean. This is because the same line that feeds a power supply is also being used to drive larger equipment. Large disturbances such as power sags and surges are commonplace.

High spikes on the AC line can damage a power supply in a similar way that ESD can damage semiconductors. On the surface, the unit can pass bench testing but long-term damage may occur to capacitors and power semiconductors, which leads to failure after just a few months of operation in the field. Industrial power supplies should meet EN61000-4 standards for immunity to line transients, and for extremely dirty AC line conditions the designer should consider using an external AC line EMI filter with high voltage pulse attenuation specs.

To prevent loss of DC power during sags, which is typical when a large piece of equipment is switched on that is in close proximity to our designer's system, it will help to specify a power supply that has a wide AC input range. If the AC line is 208VAC nominal, and sags down to 140VAC occur, utilizing a product that has an input range of 85 – 264VAC will allow DC power to be supplied without interruption. Even a short dip in the DC output can cause microcontrollers to reset and the host equipment to run through a reboot sequence.

Mounting Considerations
Most power supplies typically use electrolytic capacitors for filtering and energy storage. The higher the operating temperature of these capacitors, the shorter the life. As these parts age, the output ripple of the power supply increases, causing functional problems with the load equipment.

When mounting the power supply, ensure that adequate space is provided around the product to allow air to circulate. Do not block off heatsink fins with mounting brackets, restrict air inlet or exit from fan cooled units (1.5 to 2" clear space is a good rule of thumb), or mount the supply in a plane other than its standard-mounting orientations without consulting the installation manual.

In the event that other fans are in the enclosure, take note of the general system airflow direction, and be aware of any potential backpressure issues that may occur.

Operating Temperature and Life Effects
In addition to mounting considerations, the operating ambient temperature also plays a key part in the life of the power supply. The life of an electrolytic capacitor doubles for every 10°C reduction in temperature. The designer should be aware of the derating characteristics of the proposed power supply. Most AC/DC power supplies start to derate from 40°C or 50°C, and can only operate at 50% of its rated load at 70°C.

The derating calculations may indicate that a higher power unit is needed. Using a manufacturer with a broad base of products and a large number of models within a series will simplify this choice.

As a note, the ambient temperature is specified at the inlet of the fan or close proximity to the power supply. Designers should take into account any internal temperature rises in their system when considering potential derating.

To make an "apples-to-apples" comparison on competing products, also consider the warranty of the power supply. A product with a five-year warranty will have greater component deratings and higher quality components (use of 105°C rather than 85°C rated capacitors) for a longer field life than a product with a one-year warranty.

Operating Environment
Vibration and shock will also heavily influence the life of a power supply. A more rugged power supply will meet more stringent MIL-STD specifications. When considering the specifications, remember that how the power supply is mounted can cause mechanical resonance in the system. When the entire system is subjected to shock and vibration, a power shelf containing one or two supplies may start to vibrate at amplitudes greater than the system itself.

Think Ahead
While it is true that the power supply is only a small fraction of the size, complexity, and cost of industrial equipment, it is a key component that can have a disproportionate impact when the role in the system is not carefully considered. Because of the power supply's high unit cost compared to other electrical and electronic components, it is often targeted as an item for cost reduction. In the world of power supplies, you truly get what you pay for. Bargain-priced power supplies are not a bargain when the costs of field-failures, customer complaints, warranty repairs and potential damage to your company’s brand name are included in the equation.

Designers who consider their power applications carefully and early in the project are more likely to see their project go more smoothly, faster and most importantly protect their company's name and reputation with greater field reliability.

What Size Fan Do I Need?

2009-01-29T08:57:00.000-08:00

There are many AC-DC power supplies and DC-DC converters with output power ratings that can vary dependant upon the type of air cooling provided. “Convection air cooling” usually refers to situations where a power supply or converter is cooled by the prevailing ambient air temperature, adjacent to the power device, without forced-air-flow from fans or blowers. If the power device has two output power ratings, the “convection cooled” (still-air) power rating is lower than the “forced-air convection cooled” rating.

The power supply pictured above is an open frame switchmode supply with two output power ratings. For “convection cooled” applications, this supply can provide up to 151W of output power. However, with “forced-air-cooling” it can provide up to 201W of output power. The datasheet for this power supply indicates that for “forced-air-cooled” applications, 1.5 m/s (Meters per Second) must be provided by the user. 1.5 m/s equals 295 LFM (Linear Feet per Minute). Refer to conversion factors shown below.

Most fans are rated in CFM or Cubic Feet per Minute of air “Volume” flow. So, what size fan do you need to provide 295 LFM of air “Velocity” flow for the above application?

Most times the power supply is cooled by directing the air flow along its longest dimension; for example, from the input connector end to the output connector end. However, always read the power supply’s instruction manual to determine the manufacturer’s recommended axis for the cooling air-flow. The usual method for determining the required fan size is to first determine the height and width for the opening or port through which the air will flow around and through the power supply. In this instance the power supply is 3.15” wide and 1.46” high (and 8.2” long). We can consider the supply’s width times its height as the minimum area of the inlet port for forced air cooling of the supply. Then, we need to convert these dimensions from inches to feet by dividing by 12”. 3.15” = 0.26’ and 1.46” = 0.12’. So, the minimum “Area” of the port through which the air must flow to cool the power supply is 0.26’ x 0.12’ = 0.0312 square feet. The formula for determining the CFM (volume) rating of the fan when the required LFM (velocity) is known is as follows:

CFM = LFM x Area (in square feet)

Therefore, in this example:

CFM = 295 LFM x 0.0312 ft² = 9.2 CFM (min. fan rating)

Fans are rated in CFM based upon the expected free-flow of the air coming from them, without obstructions, which cause back-pressure. Of course, real world applications always include some obstructions. To ensure the least amount of back-pressure, it is best to have the exit ports in the enclosure about 1.5 times the area of the minimum entry port. In most applications there are other heat loads and components that can obstruct the path or free flow of the cooling air. It is therefore wise to select a fan with a higher rating than is calculated. Perhaps a 10 CFM or larger fan should be used in this application.

Tip: The use of a larger fan running at a slower speed can deliver the same airflow as a smaller fan running at a higher speed, but the larger fan will be much quieter.

Since most fans have round air outlets and square mounting patterns, the air-flow from the fan may require ducting within the end-product’s enclosure to direct the cooling air to the high power devices including the power supply.

The same process would be used to determine the correct fan rating for AC-DC power modules or DC-DC converters, with or without heatsinks that require forced-air-cooling. When heatsinks are used (see photo below), always direct the air flow in the same direction as the slots between the fins of the heatsink.
In all situations, the system must be tested with the selected fan and all other devices in-place to confirm that the power supply or converter and the load it drives do not exceed their maximum operating temperature, under worst case conditions (maximum ambient inlet air temperature, 100% power load, etc.). If problems are observed, a higher CFM rated fan or dual fans may be required.

In the metric world, fans are sometimes rated in “m³/hr” (Cubic Meters per Hour) and the air velocity is rated in “m/s” (Meters per Second). The following Metric to English conversion factors may be useful.

1 m³/hr = 36 ft³/hr ÷ 60 min. = 0.60 CFM (cubic feet per minute)
1 m/s = 3.28 ft/sec x 60 sec = 196.85 LFM (linear feet per minute)

Some fans and power supplies have dimensions in mm (millimeters).
Just remember that 1 inch = 25.4 mm, and 1 mm = 0.04”

There are a number of very good online calculators to assist you in determining the fan size and ratings required for various forced-air-cooling applications. Here are a few of those websites:

http://www.airperformancetech.com/conversion-tools.htm
http://www.aavidthermalloy.com/technical/airflow.shtml
http://www.calculatoredge.com/optical%20engg/air%20flow.htm

What is the SEMI F47 line sag spec all about?

2008-12-23T10:27:00.000-08:00

We have all seen lights dim at home or at work and this is an indication that the AC line voltage has been reduced or sagged. Although an occasional dimming of lights can be tolerated, when it comes to factory automation equipment, line sag can be the source of a production shutdown, resulting in significant revenue losses. Since the production of semiconductors, including microprocessors, is a very precise and expensive process, back in 1999 the Semiconductor Equipment and Materials Institute (SEMI), established standards relative to AC line sag immunity. This specification is called the SEMI F47 Voltage Sag Immunity Standard and has been revised periodically. Because many other factory automation processes are equally critical, some of these production products need to comply with the SEMI F47 standard as well.

Basically, this standard requires that the AC-DC power supply that is used in semiconductor production, or in other factory automation equipment, continue to provide the required output voltage and current, even if the input voltage dips below its specified limits. As can be seen in the chart below, in the blue area, the basic specs require that the power supply perform normally even if the input voltage sags down to 50% of its nominal voltage for up to 200 ms, or sags to 70% for up to 500 ms, and sags to 80% for up to one second. Since this sag percentage refers to the nominal line voltage, this means for example that with a nominal 220VAC input, the AC voltage can sag down to 50% or 110VAC for up to 200 ms, down to 70% or 154VAC for 500 ms, and down to 80% or 176VAC for up to one second.

There are additional sag ride-through “recommendations” within the latest version of the standard, which is the SEMI F47-0706 (these recommendations are not requirements) that includes operation of the power supply with 0% input power (no power) for up to 20 ms. This recommendation can be accommodated by insuring the selected power supply has a “hold-up time” specification of 20 ms or longer. Other newly recommended thresholds within the SEMI F47-0706 include sags of 80% for 10 seconds, and continuous sags of 90%. Most power supplies that meet the previous versions of this standard, the SEMI F47-0200, and have a hold-up spec of 20 ms or greater, should be able to meet the new recommendations (not requirements) as well.

The simplest and lowest cost method of complying with the SEMI F47 standard is to use a power supply with a universal input, such as 90 to 264VAC, and operate it from a 220VAC or higher line input. In this way you automatically meet and exceed the standard since this type of power supply can operate down to 90VAC (even lower than the 50% line sag spec of 110VAC). Note that this method does apply to auto-strap power supplies.

Another way of meeting the SEMI F47 is to draw less power than the supply can normally provide (de-rate the supply). If you do this, always check with the manufacturer to confirm that your reduced load will allow the supply to fully meet the SEMI F47 standard. This may require extra testing to confirm compliance, either by the power supply manufacturer or the end-product OEM. Alternatively, the power supply manufacturer may be able to modify the supply to meet the SEMI F47 standard.

Some factory automation equipment require the use of SEMI F47 “certified” power supplies, which means the supplies were tested by an outside agency or laboratory and found to fully comply with the standard (similar to UL certification). If this is a requirement, always look for supplies that have existing certifications from a reliable manufacture, because the cost of getting this type of certification can amount to $2,000 or more. There is a grandfather clause in the updated standards that provides for equipment that was tested or certified under the previous versions of the standard to not require re-testing or re-certification.

Many industrial-type power supplies are designed and/or certified to meet the SEMI F47. These supplies may be a bit more expensive, but it will be the lowest cost solution, especially if you compare it to the cost of adding an external constant voltage transformer or UPS to the input of the power supply.

Power supply manufacturers such as TDK-Lambda offer supplies that are SEMI F47 certified and supplies that operate with a wide universal input of 90 to 264VAC. In addition, modified supplies can be provided that meet this and other prevailing power supply specifications.

Ripple & Noise Specs and Measurements

2008-11-21T10:28:00.000-08:00

AC-DC power supply and DC-DC converter datasheets should always include output “Ripple & Noise” specifications. The Ripple & Noise spec is sometimes referred to as Periodic And Random Disturbances or PARD. The following drawing shows how ripple and noise may look when viewed on an oscilloscope that is attached to the output of a typical switchmode power supply.

The output “Ripple” frequency is primarily determined by the switching frequency of the power supply. The higher frequency “Noise” spikes are generated by the fast rise and fall times of the pulses associated with the switching and rectification components of the power supply. Typical ripple and noise specs are defined as peak-to-peak measurements in mV units.

Ripple & Noise Measurements
Unfortunately, there is no universally accepted method for measuring ripple and noise. It seems that each manufacture, and sometimes different products from the same manufacturer, may have varying methods for these measurements. In some cases the bandwidth of the test oscilloscope is defined as 20MHz or 100MHz. In addition, added components such as capacitors, resistors, twisted wires, and/or coax are sometimes required in the test set-ups that are defined by the manufacturer. In order to meet the power product’s specified ripple and noise specs, care must be taken to follow the manufacture’s defined test set-up. There are a few standardized methods for ripple and noise measurements; one of which is the JEITA-RC9131A standard.

Fig 1: JEITA-RC9131A Ripple & Noise Test Set-Up

The above drawing (Fig 1) shows the test set-up per JEITA-RC9131A. This standard defines a custom oscilloscope connection comprised of a length of 50 ohm coax that is connected to the output of the power supply with the other end terminated at the scope with a 50 ohm resistor in series with a 4700pF capacitor. Notice that the coax is attached to the output of the power supply within 150mm or 6 inches of the output terminals and has two added capacitors (22uF electrolytic and 0.47uF film type) soldered across those points. The 50 ohm coax should not exceed 1.5M or 5 feet in length. All coax pigtails and added component’s lead lengths should be kept to a minimum to prevent pick-up of radiated noise.

Other Measurement Precautions
Some ripple and noise measurements can be made with the use of a standard oscilloscope scope probe that has been modified by removing the plastic tip cover and ground clip wire and replacing the ground connection with a short length of bare copper wire that is wound around the probe’s ground ring. In this way the probe’s tip and ground connections are kept to a minimum length, thereby reducing the chance of the ground lead acting as an antenna and picking up radiated noise signals, which can result in out-of-spec measurements.

Figures (a), (b), and (c) below show incorrect set-ups for ripple and noise measurements.

When making ripple and noise measurements a standard load should be used. This precaution is to prevent any noise from the power supply’s normal system load, which may contain noisy digital or RF circuits, from feeding noise back to the output of the supply, which again can result in out-of-spec test measurements. In some cases, to reduce ground loops, it may be necessary to isolate or float the oscilloscope from the AC source by plugging it into an isolation transformer.

Unless otherwise stated, the ripple and noise specifications are usually based on measurements taken while operating the power supply with its nominal input voltage, at the rated output voltage and current load, and at or near room temperature (typically 72°F to 77°F).

Over Current Protection in Power Supplies & Converters

2008-09-19T10:45:00.000-07:00

Most AC-DC power supplies and DC-DC converters have internal current-limiting circuits to protect the power device, and to some degree its load. The majority of over-current-protections include an automatic recovery feature. In practice, the current limit feature typically starts operating when the output current excedes it maximum rating by 10 to 20%.

In many cases, should an overload (e.g., short circuit) be allowed to exist for a prolonged period, it can reduce the product’s field life by temperature stressing the electrolytic capacitors, and in extreme cases, it can damage the user’s printed circuit traces. Therefore, always check the power supply’s “Instruction Manual” to be sure you understand the precautions associated with the power product’s over-current-protection feature. Also, if the power product has an Output Good signal, this can be used as an indication that the power supply is either faulty or could be in an over-current mode.

There are a number of ways to implement over-current-protection (OCP), and below are descriptions of the most common methods.

Fold-Back Current Limiting: When this method is employed if an overload condition exists, the output voltage and current reduce to safe levels. As can be seen from the following curve, should an overload occur the supply will provide current up its current limit point (aka ‘knee’), and then the output current will fold-back to a lower value as the output voltage reduces towards zero.

This technique is employed in linear power supplies because it reduces the strain on the supply’s internal power devices to minimum. One drawback of fold-back current limiting is that if the supply turns on into a heavy capacitive load, it could latch-up at a reduced current before reaching its full output voltage. Depending upon the design, recovery from a fold-back current limit condition can be automatic, or after a built-in time delay when the overload condition is removed.

Fold-Back Current Limiting

Fold-Forward Current Limiting: In this method, when an overload is sensed the output voltage reduces towards zero, but the current increases. When driving motors, pumps, or highly capacitive loads, employing a fold-forward current feature can help overcome the electrical inertia of these loads. Recovery from a fold-forward current limiting situation is usually automatic when the overload is removed.

Fold-Forward Current Limit

Constant Current Limiting: In this method, should an overload occur, the output current stays at its limit point and the output voltage reduces towards zero in a somewhat linear fashion. This technique is used in many switchmode power supply designs. Typically, the supply will automatically return to its normal output voltage when the overload condition is no longer present.

Constant Current Limiting

Current Limit Shutdown: In some power supply designs, when an overload occurs the power supply will begin to go into a constant-current limit mode, but when the output reaches a preset reduced voltage, the supply will shutdown. Recovery from this condition can be automatic or require recycling of the input power.

Hiccup Mode Current Limiting: Some low power supplies have what is termed a hiccupcurrent-limit feature. As the name implies, if a current limit is sensed, the supply will reduce its output voltage to zero and then, after a short time, it will attempt to provide its normal voltage. These On-Off attempts at operation are referred to as a hiccup-mode. Should the overload condition be removed, the supply will again operate normally.

Peak-Current Power Supplies
It should be mentioned that some power supplies are designed specifically to provide large peak-currents, which can range from 200 to 300% of the maximum current rating for a short duration, without going into a current-limit condition. These are especially useful when powering loads that include electric motors such as computer hard drives, fans, actuators, pumps, etc. When using this type of power supply it is important to limit the “average power” that is delivered to load. More information about peak-current-rated supplies will be provided in a separate article.

Power Supplies with Wide Range Adjustable Outputs

2008-08-05T09:25:00.000-07:00

For some power supply applications it is desirable to change the output voltage over a wide range. There are a number of ways to control the output voltage of power supplies that are designed to provide wide adjustment ranges. Remotely adjustable output voltages can be implemented by using one of the following methods.

Variable Voltage Control
In this case an external variable control voltage (e.g., 1-6V) is connected to the designated input of the power supply, sometimes called the PV input. As the input control voltage is varied it will cause the output voltage to change in a fairly linear fashion over a wide range (e.g., 20% to 120% of the nominal output voltage). For some applications this is a low cost method of providing a programmable power supply. Below are diagrams showing an example of this type of remote voltage adjustment for Lambda’s HWS/PV and SWS-L series of power supplies.

External Variable Voltage Control (1-6V)

Output Voltage Change (20-120%) with Ext. Variable Voltage Control (1-6V)

Variable Resistive Control
Some power supplies can be remotely adjusted via a variable resistive control (external potentiometer). This method has the advantage that an external voltage is not required since an internal Ref. voltage is provided by the supply. As the resistance changes, it will cause the output voltage to change in a non-linear fashion over a wide range (e.g., 20% to 120% of the nominal output voltage) as shown in the diagrams below (Lambda’s HWS/PV series). For some applications this is a low cost method of providing a programmable power supply.

External Variable Resistive Control (50k ohm pot.)

Output Voltage Change (20-120%) with Ext. Variable Resistive Control (50k ohm pot.)

Serial Digital Control
Programmable Power Supplies can be remotely controlled via a serial digital port such as RS232 or RS485. Both the output voltage and current can be controlled from zero to the maximum output ratings. In addition, alarm signals from the supplies can be sent back to the remote computer or controller via the same digital link. Programmable Power Supplies are more expensive than wide adjustable supplies mentioned above, but they have a large array of local and remote control features that are not found elsewhere. Lambda’s ZUP series is a good example of a feature-rich Programmable Power Supply.

Up to 31 ZUP Series Programmable Supplies can be Remotely Controlled via RS485 Interfaces

What is Remote Sensing?

2008-07-03T11:21:00.000-07:00

Most medium to high power AC-DC power supplies and DC-DC converters have “Remote Sense” connection points (+/- Sense) that are used to regulate the supply’s output voltage at the load. Since the cables that connect a power supply’s output to its load have some resistance, as current flows it will cause a voltage drop in the cables. Since it is best to regulate the voltage at the load site, the use of the two Remote Sense wires connected from the supply to the load will compensate for these voltage drops.

Typical remote sensing circuits are capable of correcting from 0.3V to 1.0V of voltage-drop in the output cables. However, to be sure, always check your power supply’s instruction manual to determine the maximum remote sense compensating range. If the voltage drop across the cables exceed the range of the remote sense circuits, this can be remedied by either reducing the length of the cables or increasing the size of the cable’s conductors. The remote sense leads carry very little current, so light gauge wires can be used. Steps should be taken to ensure the remote sense wires do not pick up noise by either twisting the +/- Sense wires together and/or shielding the wires from noise. It is important to observe the correct polarities, i.e., the +Sense wire should connect at the load to the +V output cable and the –Sense wire should connect at the load to the –V output cable. Refer to Figure 1.

Fig. 1: Power Supply with Remote Sense Wires Connected at the Load

When not using the remote sense feature, Local Sense (LS) connections should be used. In this case the +/-Sense points should be connected to their corresponding output or local sense terminals at the power supply (+Sense to +V output or +LS and, –Sense to –V output or -LS). Most power supplies are shipped from the factory with these “Local Sense” connections in place.
Refer to Figure 2.

Fig. 2: Power Supply with Local Sense Jumpers Installed

Harvesting More Energy without Building More Power Plants

2008-06-13T13:38:00.000-07:00

An AC-DC switchmode power supply without Power Factor Correction (PFC) can draw approximately 950 Watts from a typical 115VAC wall socket protected by a 15A circuit breaker before exceeding the UL mandated limit of 12A. A simple load like a toaster can draw almost 1400 Watts. The difference between the two is due to the higher Power Factor (PF) of the toaster, which presents a resistive load to the power line. If we correct the Power Factor of the switchmode supply it can then draw about as much power as the toaster, allowing it to provide more output power to its load from the same 115VAC/15A wall socket.

What is Power Factor Correction (PFC)?
Power Factor (PF) is technically the ratio of real power consumed to the apparent power (Volts-RMS x Amps-RMS), and is expressed as a decimal fraction between 0 and 1. PF is traditionally known as the phase difference between sinusoidal voltage and current waveforms. When the AC load is partly capacitive or inductive, the current waveform is out of phase with the voltage (Fig. 1, the dotted line is the current waveform). This requires additional AC current to be generated that isn't consumed by the load, creating wasted I2R (wattage) losses in the power lines.

An electric motor is inductive, especially when it is starting. The current waveform lags behind the voltage waveform, dropping the PF to well below 1 (similar to Fig. 1). This is why many motors have “starting” capacitors installed to counteract the inductance, and therefore correct the PF during motor startup.

Figure 1. Voltage and current waveforms are sinusoidal but out-of-phase; PF <1.

A simple resistive load has the highest PF of 1. An AC voltage across the resistor causes an AC current which is identical to and in-phase with the voltage waveform (Fig. 2).

Figure 2. Voltage and Current waveforms are sinusoidal and in-phase; PF=1.

A switchmode power supply when viewed as an AC load is neither capacitive nor inductive, but non-linear. A switchmode supply conducts current in short pulses or spikes that are in-phase with the line voltage (Fig. 3). The product of “Volts-RMS x Amps-RMS” is considerable higher than the real power consumed, and thus the PF is much less than 1, typically around 0.65 or less.

Figure 3. Voltage waveform is sinusoidal, current waveform is non-sinusoidal but in-phase; PF<1.

Improving the Power Factor
Low Power Factors can be improved via Power Factor Correction (PFC) circuits. The types used for switchmode power supplies “smooth out” the pulsating AC current, lowering its RMS value, improving the PF and reducing the chances of a circuit breaker tripping. There are two basic types of PFC: Active and Passive. Active PFC is more effective, a bit more expensive, generally integrated into the switchmode power supply, and can achieve a PF of about 0.98 or better. Passive PFC is less expensive and typically corrects the PF to about 0.85.

Harvesting Additional Output Power
To determine just how much more power is available from the AC line and a power supply with PFC, the user needs to understand the following equation, which defines the amount of output power (Pout) available from a switchmode supply:

Pout = VL-RMS x IL-RMS x PF x Eff

For example, UL limits a system's line current to 80% of the circuit breaker's rating. For a typical 15A breaker, 12A is the maximum allowed, and the best-case power available is therefore 120VAC x 12A = 1440 Watts. Referring to the above equation, here are two examples of supplies with different power factors:

A switch-mode supply with 0.65 PF and 85% efficiency can only deliver (120 x 12 x 0.65 x 0.85) = 796 Watts (Pout).
However, if the power factor is corrected to 0.98, the same power supply can now deliver (120 x 12 x 0.98 x 0.85) = 1200 Watts (Pout), a 51% increase.

From the examples above, it can be seen that by employing power supply’s with PFC, more output power can be delivered to the OEM’s end-product, without the need to increase the AC power wire sizes, increase the circuit breaker’s rating or draw more current from the power plants. Thus, PFC has a significant effect on our environment relative to reducing the pollutants coming from electric power plants.

Meeting International Regulations
Since switchmode power supplies without PFC tend to draw the AC line current in a non-linear fashion, many unwanted harmonic currents are generated and reflected back on the AC power lines. These reflected harmonic currents are “pollutants” to the power grid that have a negative affect on other devices connected to the same power lines. These unwanted harmonic currents can range in frequency from the 100 Hz on up to over 2,000 Hz, and have a direct relationship to the Power Factor of switchmode power supplies.

An important reason to have PFC within your power supply is to comply with international regulations, especially if you intend to sell your equipment in Europe. Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. These regulations were put in place to maximize the available power generated each day by electric power plants located worldwide. The intent was to make the most of the power we have today without expanding our carbon footprint. Today, the most important regulation is the “European Norm” EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies. Power supplies with PFC circuits that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or better.

Summary
As previously mentioned, "Power Line Harmonics" are created whenever the line current is not a pure sinewave, as is the case with a switchmode power supply's input, which tend to have “pulsed” currents (Fig 3). Measuring power line harmonics is a mathematical means to describe a complex waveform's Power Factor by resolving it into a fundamental frequency and its many harmonics.

The harmonic currents do not contribute to the output load power, but cause unwanted heating in the wall socket, wiring, circuit breaker, and distribution transformers, resulting in wasted energy. When personal computers first hit the mainstream market, their power supplies lacked PFC. As a result, circuit breakers that seemed to be sized correctly for the load were tripping for no apparent reason. After investigation, it was determined that the poor power factor of the PC’s power supplies was the culprit.

Today new “green initiatives” are dictating that personal computers include power supplies which must have Power Factor Correction (PFC) and high efficiencies. PFC significantly reduces harmonics, resulting in almost a pure “fundamental” current frequency that will be in-phase with the voltage waveform (Fig. 2). International regulations dictate the substantial attenuation of harmonic currents. The vast majorities of AC-DC power supplies manufactured by Lambda Americas employ active PFC, are in accordance with EN61000-3-2 and provide typical power factors in the range of 0.97 to 0.99.

Lambda’s RTW Series of switchmode power supplies range in output power from 50 to 300-Watts. All units include PFC and have typical Power Factors of 0.98 (meets EN61000-3-2). Efficiencies reach up to 89%.

Choosing an Input EMI Filter for a Power Supply

2008-05-19T15:42:00.000-07:00

There are two primary functions that an input EMI filter can perform:

Minimize outgoing electrical noise to avoid interfering with neighboring equipment
Attenuate (reduce) incoming electrical noise that could damage the system

Regarding outgoing noise, although most power supplies meet the governmental regulations for EMI, noise is additive and if there are multiple power supplies or high speed processor boards, it can result in a failing grade.

If the noise is only slightly out of specification, then a (lower cost) single stage filter may suffice. If the noise is considerably out of specification then a higher performance two stage filter will be required.

An example of these would be Lambda’s MA (single stage) and MX (two stage) filters. Look for the terms “wideband” or “low frequency attenuation” in the features.

Incoming electrical noise is usually in the form of a spike or burst of energy. It can be generated from natural causes such as a lightning storm or man made by a large piece of industrial equipment.

This type of filter may have “high pulse attenuation” listed as a feature and will have internal values optimized to reduce these potentially harmful spikes from reaching the power supply. The filter will also have some outgoing noise attenuation, but may not be as effective. An example would be Lambda’s MZ series of filters.

Benefits of Environmentally Friendly Power Supplies

2008-04-01T16:17:00.000-07:00

Increasingly, Lambda’s customers are asking for and receiving Environmentally Friendly (EF) or “Green” power supplies. What constitutes an EF/Green power supply? There are a number of factors which contribute to a power supply being considered EF/Green. These include: RoHS compliance, electrical noise (EMI) suppression, high efficiency and power factor correction (PFC).

RoHS Compliance: Lead, mercury and other “hazardous” elements and chemicals used in the production of electronic devices have been identified, and by-law (since July 2006), have been banned or severely limited to their content in these devices, by the European Union (EU). These limitations are spelled out in what is called the RoHS Directives (Restriction of Hazardous Substances). Currently, all products sold to the countries within the EU must comply with the RoHS Directives. Many of the States here in America have adopted similar restrictions. Lambda has been a leader in providing RoHS-compliant power supplies even to their US customers who may not currently require them. This represents Lambda’s dedication to making the earth a greener and cleaner place to live. Over 99% of Lambda’s power products are RoHS-compliant with a few exceptions being for those industries that are RoHS-exempt such as specific military applications.

EMI Suppression: All electrical devices contribute in some way to the electrical noise that “pollutes” our power lines and airways in the form of unwanted noise spikes and radio interference. These unwanted noise generators are restricted to the amount of EMI (Electro-Magnetic Interference) they are allowed to emit by strict standards that are produced and maintained by the FCC and international organizations (e.g., IEC, EN). Power supplies are among the electrical devices that have an inherent electrical noise problem (both conducted and radiated) which must be addressed by the power supply designer and manufacturer. Without properly designed internal EMI filters and RF shielding, power supplies would become huge polluters of our electrical and electronic environment. Lambda power supplies comply with the most rigorous EMI suppression standards. For example, the very restrictive FCC Class-B EMI compliance is available, as standard, on many Lambda power supplies.

High Efficiency: The efficiency rating of a power supply is measured by comparing the AC power going into the power supply divided by the DC power coming out of the supply. For example, if 100 watts of AC power is used by a power supply for it to provide 90 watt of DC output power, the efficiency of the supply is calculated by dividing 90W by 100W, which equals 0.90 or 90% efficiency. When comparing a 75% to a 90% efficient power supply, the savings in electricity usage and wasted energy (in the form of heat) is quite significant.

Power Factor Correction (PFC): Modern switchmode power supplies should include active Power Factor Correction circuits in order to maximize the AC power that is used by the supply. The Power Factor of a power supply is technically the ratio of the real power consumed to the apparent power (Volts-rms x Amps-rms) and is a decimal number between 0 and 1.0. If left uncorrected the Power Factor (PF) of switchmode supplies will generally be around 0.65 or less. With active PFC, switchmode power supplies can achieve power factors from 0.96 to 0.99. Without PFC, switchmode supplies would draw their AC line current in the form of spikes or pulses, instead as a clean sinewave; the net result being that the AC power wires, circuit breakers and power generating plants need to be sized larger than if they are driving products that contain PFC power supplies.

So what does High Efficiency and PFC mean relative to being Environmentally Friendly?
Most electricity used within the USA is generated by burning fossil fuels such as oil, natural gas or coal. It has been shown that there is a green-house-effect that is increasing at an alarming rate due to the large amount of carbon dioxide that power generation plants produce. So, whenever we can reduce the amount of electricity used, we are contributing to a cleaner environment, a reduction of CO2 in the atmosphere and our dependence on, and the transportation costs of foreign oil from the OPEC countries (the oil tankers burn diesel fuel, which further adds to the pollution).

In addition, there can be significant money savings to the power supply user. Basically the savings of just 1-watt dissipated in the power supply = 365 days x 24 hrs x 1W = 8.76kW/hrs per year. If electricity costs $0.30 per kW/hr, that would amount to $2.63/year per each watt saved. If we boost the efficiency of a power supply by only 5% on a 150W unit from 85% to 90%, that saves 7.5W [(150W x 0.15 = 22.5W) - (150 x 0.10 = 15W) = 7.5W]. That translates to $19.73 per year savings (7.5W x $2.63/year = $19.73/year), which is more than the purchase price of the power supply over its typical lifespan (4 to 5 years), plus, we have reduced the amount of carbon dioxide in the air. If we consider an OEM who uses a 90% efficient power supply (instead of 85%) in their end product and produces 100,000 units/year, the “total electric power savings” would amount to 100,000 units x 7.5W = 750,000 watts per hour, a significant power savings.

Furthermore, the higher the efficiency of the power supply, the less power is wasted in the supply and the less energy is needed to remove that wasted energy (in the form of heat), by cooling the supply or the room in which the equipment is used (via use of electromechanical fans, blowers, air conditioning, etc). The combination of high efficiency and PFC in power supplies allows the use of smaller gauge power distribution wires, lower-rated circuit breakers and fewer power generating plants (air polluters).

Lambda’s HWS Series, 1000-Watt Switchmode Power Supply is fully RoHS-Compliant, Meets FCC Class B EMI, has integral active PFC (Power Factor =0.98) and has a typical operating Efficiency of 88%.

In Summary: As mentioned above, there are many factors that comprise an “environmentally friendly/green” power supply. And, the benefits to the end-user are significant. Lambda is a leader in the provision of these advanced power devices and our international presence insures that we will continue to be at the forefront of these “Green & Clean” power products.

Droop Mode Current Share

2008-03-05T11:15:00.000-08:00

If two power supplies are to be connected together to produce more power or share the load, then a parallel capable model should be selected. Lambda’s DPP100, 120, 240 and 480 models are all parallel capable. On the front of each power supply is a small black switch. For parallel operation this switch should be set to “parallel” (Fig. 1).

In single mode the load regulation (the amount the output voltages changes with load) is minimal, the difference being less than 0.24V from zero load to full load for a 24V output power supply.

In parallel mode that load regulation is artificially increased to 1.2V using internal circuitry (Fig. 2).

The extra voltage drop or “droop” is proportional to the load drawn, so that when two or more power supplies are connected in parallel the output load is shared between the power supplies. If one of the paralleled power supplies tries to provide more current, its output will droop slightly and the other supplies will balance.

For optimal performance, all power supplies should have their outputs set to the same voltage.

What is PFC and why do I need it?

2008-02-08T09:07:00.000-08:00

Switchmode power supplies without Power Factor Correction (PFC) tend to draw the AC input current in short bursts or spikes relative to the line voltage, as shown in Fig. 1. The Power Factor of a power supply is technically the ratio of the real power consumed to the apparent power (Voltsrms x Ampsrms) and is a decimal between 0 and 1.0. If left uncorrected the Power Factor (PF) of switchmode supplies will generally be around 0.65 or less.

Figure 1. Input of switchmode power supplies without PFC. The voltage waveform is a sinewave and the current waveform is a pulse or spike. PF<1

The Power Factor can be improved by using PFC circuits. These circuits “smooth out” the pulsating AC current, improving the PF, and reducing the chances of a circuit breaker tripping prematurely. There are two basic types of PFC, passive and active. Passive PFC circuits are less expensive and typically can correct the PF to about 0.85. Active PFC circuits are the most popular, are built into the switchmode power supply and can increase the PF to 0.98 or higher. The closer the PF comes to being 1.0, the better the performance of the power supply. Ideally, we want to end up with the input voltage and current waveforms being sinusoidal and in phase with each other as shown in Fig. 2.

Figure 2. Voltage and Current waveforms are sinusoidal and in-phase. PF=1.

PFC is Required by International Regulations
An important reason to have PFC in your power supply is to comply with international regulations, especially if you intend to sell your equipment in Europe. Since 2001, the European Union (EU) established limits on harmonic currents that can appear on the mains (AC line) of switchmode power supplies. Today, the most important regulation is the “European Norm” EN61000-3-2. This regulation applies to power supplies with input power of 75 watts or greater, and that pull up to 16 amps off the mains. It sets severe limits on the harmonic currents up to the 39th, when measured at the input of switchmode power supplies.

For example, the first harmonic is the primary input frequency, typically 50 Hz for the EU countries. The third harmonic is 150 Hz, and the 39th harmonic is 1,950 Hz. These unwanted harmonic currents have a direct relationship to the Power Factor of switchmode power supplies. Therefore, power supplies that meet EN61000-3-2 inherently have high power factors that are typically 0.97 or higher.

PFC Increases the Supply’s Output Power Capability
The PF, much like the supply’s efficiency rating, determines the amount of useful power a switchmode power supply can draw from the AC line and then deliver to its output load. Specifically, the formula that determines this is:

VLrms x ILrms x PF x Eff = Pout

As an example, if a power supply is operating off of 120VAC line, which is protected by 15A circuit breaker, UL guidelines say you should not draw more than 12A. So, using the formula above, we can compare two power supply examples with different Power Factors, as follows:

Example A: No PFC, PF = 0.65, 85% Efficiency, 120VAC input, 12A max. current:
Therefore: 120VAC x 12A x 0.65 x 0.85 = 796 Watts Output Power

Example B: PFC used, PF=0.98, 85% Efficiency, 120VAC input, 12A max. current:
Therefore: 120VAC x 12A x 0.98 x 0.85 = 1200 Watts Output Power

As can be seen above, the power supply in Example B (with PFC) can deliver 404 Watts or 51% more power to its output load than the non-PFC supply, a significant increase.

Why do I need PFC?
A power supply with PFC can supply higher output load currents than those without PFC. PFC significantly reduces the AC current harmonics, leaving mainly the “fundamental” current frequency that is in-phase with the voltage waveform (Fig. 2). International regulations dictate the substantial reduction of harmonic currents. The vast majority of AC-DC power supplies manufactured by Lambda Americas has active PFC, is in accordance with EN61000-3-2 and provides typical power factors in the range of 0.97 to 0.99.

What does 1U, 2U or 3U mean?

2008-01-03T16:16:00.000-08:00

Many rack-mounted power systems are specified as being 1U, 2U, 3U, etc. What does this mean? For electronic equipment racks (e.g., 19 or 23 inches wide), the term 1U is used to define one rack unit of height.

1U equals 1.75-inches (44.45mm) of rack height. Therefore, a 2U rack mount height would be 2 x 1.75", which equals 3.5-inches high. A 3U height would be 3 x 1.75" = 5.25-inches.

It should be noted that the 1U, 2U, 3U, etc., heights are maximum dimensions. In order to allow for mechanical tolerances and to provide some space between panels, typically, for each 1U of height manufacturers may deduct about 0.03" (see Photo #1). For example, a 2U panel, which has a nominal height of 3.50" may be only 3.44" high [3.50" – (2 x 0.03") = 3.44"].

Individual power supplies are sometimes mounted within rack-mounted enclosures that require integral power. In these cases, the power supply needs to be a bit shorter than the equipment’s overall height to allow for the top and bottom covers. So a 1U high enclosure-mountable power supply needs to be shorter than 1.75-inches; a 2U enclosure-mountable supply needs to be shorter than 3.5-inches, and so forth (see Photo #2).

Examples

Photo #1: This 19" rack-mountable power system can hold up to 3 plug-in, hot-swap and redundant power supplies. The enclosure with mounting ears is 1.72" high (= 1.75" minus 0.03") and is therefore considered 1U high.

Photo #2: This 1000-watt switch-mode power supply is 3.25” high and, therefore, can be mounted in a 2U rack-mountable enclosure, which can vary between 3.44" to 3.50" high.

Since we still live in an English and Metric measurement world, here are a couple of handy conversion factors: 1 inch = 25.4 millimeters (mm), 1 mm = 0.03937 inch

As a side note, Lambda ran a clever ad campaign that those who understand what “1U” or “2U” really means would appreciate. Here is a copy of that ad, which hopefully you will find humorous.

What do they mean by Output Power Derating?

2007-12-05T09:52:00.000-08:00

All power supplies have a specified “Operating Temperature Range”. For example, Lambda’s AC-DC switchmode SWS600L series of 600 watt, single output power supplies have an operating temperature range from “-20°C to +74°C”. However, the spec also states: “…derating linearly to 50% load above 50°C”. What does this mean?

Please refer to Figure 1 below. Most power supply manufacturers provide this type of curve to make it easier for the end user to determine the maximum output power that can provided by a power supply at various operating or ambient temperatures. Ta = Temperature of the Ambient Air, or, the temperature of the air surrounding the power supply, especially the air at the intake of a fan-cooled supply. By comparing the “Operating Temperature Range” specification listed above to the derating curve, the following information can be seen:

The supply can deliver 100% of its rated output power load (600 watts) from -20°C to +50°C ambient temperatures
Above 50°C ambient, the supply can deliver a reduced amount of power
At 60°C ambient, the supply can provide about 80% of its max. rated power (0.80 x 600 = 480 watts)
At 74°C ambient, the supply can provide 50% of its max. rated power load (0.50 x 600 = 300 watts)

Figure 1: SWS600L Output Power Derating Curve

In addition to the supply’s normal “operating temperature range” and output derating-curve, some supplies like this one, have a specified low-temperature “start-up” capability (i.e., -40°C). This means that the supply can “start-up” or be “turned-on” with an ambient temperature as low as -40°C (below the -20°C spec) and deliver 100% of its rated power, however, the supply’s output regulation, hold-up time, ripple & noise, and other specifications cannot be fully guaranteed until the supply warms up to at least -20°C. This cold temperature start-up is a nice feature to have, especially for outdoor-mounted applications. Once the supply is turned-on it will usually self-heat due to the heat generated by its internal electronic power components.

What are the differences between Conduction, Convection and Radiant Cooling of Power Devices?

2007-11-26T08:19:00.000-08:00

All power devices generate heat. This is due to the unavoidable internal losses of all power circuits due to their inefficiencies. The higher the efficiency rating of the power device, the less internal heat is generated within it. If we could achieve 100% efficiency, there would be no heat generated within the power device and no cooling required.

There are three methods of transferring or removing heat from power devices: These are conduction, convection and radiant. In all cases, the heat is being transferred from the power device to another medium that is at a lower temperature. Heat is constantly seeking to move to any object or medium that is cooler.

Conduction Cooling: This is defined as the transfer of heat from one hot part to another cooler part by direct contact. For example, many DC-DC converters have a flat surface that is designed to mount directly to an external heat sink or cold plate that will conduct the heat away from the power device by direct contact, thereby cooling it. Conduction is the most widely used method of heat transfer. All power supplies use internal heatsinks to help conduct the heat away from the hot devices.

Convection Cooling: This involves the transfer of heat from a power device by the action of the natural air flow (a low density fluid) surrounding and contacting the device. Many power devices are rated for natural convection cooling as long as the air surrounding the unit remains within a limited temperature range that is cooler than the device. The advantage of this method of cooling is that no electromechanical fans are required.

Another type of convection cooling requires forced-air-flow via fans or blowers across the power device. Many power supplies come with a build-in fan to provide this forced air type of convection cooling. Other types of power supplies specify the amount of air flow that must pass through or around the device (in cubic-feet-per-minute) in order for the supply to provide its maximum rated output power.

Some power devices with heat sinks depend on convection cooling (with or without forced air) to assist in transferring the heat away from the power devices to the cooler air.

Radiant Cooling: This is the transfer of heat by means of electromagnetic radiation (energy waves) that flow from a hot object (power device) to a cooler object. True radiant heat transfer can take place in a vacuum and does not require air. It should be noted that conduction cooled power devices also give off radiant heat; however, radiant heat transfer is less effective as a means to cool a power device than are conduction or convection cooling described above.

Guide to EMC Standards for Power Supplies

2007-11-05T15:35:00.000-08:00

Introduction:
EMC refers to ElectroMagnetic Compatibility. Electrical equipment that takes power from a distributed AC or DC source which is connected to other equipment, such as the AC mains in a building, has to have minimal influence on that source. It also has to have minimal influence on other equipment through electromagnetic radiation. A power converter which incorporates switching devices operating at high frequency needs to employ special means to keep the electromagnetic interference within internationally agreed upon limits. In general, electrical equipment has to operate in its environment with minimal disturbance to its environment. The limits to disturbances are defined by the international standards described below.

Types of Standards:
1) Generic Standards:
A top level standard for a type of equipment which encompasses specific basic standards in their references. The current relevant standard for power supplies is EN61204-3: 2000. This covers the EMC requirements for power supply units with DC output(s) of up to 200V, at power levels up to 30kW, and operating from AC or DC. source voltages of up to 600V. The EN refers to Euro Norm or European standard. Europe has led the field in establishing standards for EMC and many other areas which have been adopted worldwide, with some local deviations.

2) Basic Standards List:
The relevant basic standards called up in EN61204-3 are:
EN55022 and EN55011. Conducted and radiated electromagnetic interference emitted by the power supply. This is also known as CISPR22. The FCC has similar standards in the USA. There are two levels for the emission limits, Class A and Class B. Class B is normally required which puts a lower limit on allowed emissions.
EN61000-4-2. Immunity to electrostatic discharge.
EN61000-4-3. Immunity to radiated radio frequencies.
EN61000-4-4. Immunity to fast transient voltages on the input lines.
EN61000-4-5. Immunity to lightning surges on the input lines.
EN61000-4-6. Immunity to conducted radio frequencies.
EN61000-4-8. Immunity to power frequency magnetic fields.
EN61000-4-11. Immunity to damage from input line voltage reductions.
EN61000-3-2. Limits to the harmonic currents that can be taken from the input line.
EN61000-3-3. Limits to the voltage fluctuations that the power supply can cause to the line input voltage.

3) Performance Criteria:
In immunity testing, there are four classes by which passing or failure are assessed.
Class A. No loss of function or performance due to the testing.
Class B. Temporary loss of function or performance, self recoverable.
Class C. Loss of function or performance which needs intervention to restore.
Class D. Permanent loss of function or performance due to damage. This would always represent a failure.

Basic Emissions Standards

EN55022 (IT equipment), EN55011 (Industrial equipment), and FCC Class A or B (in the USA):
Conducted and radiated emission limits.
Conducted EMI (electromagnetic interference) is radio frequency energy that the power supply couples into the input power lines. The power supply input incorporates filtering to reduce the conducted emissions as necessary. The radio frequency noise is measured between 150kHz and 30 MHz using a spectrum analyzer or special receiver.

Radiated EMI is radio frequency energy emitted from the enclosure and input and output wiring of the power supply and is measured in the 30MHz to 1,000MHz frequency range. The measurement is usually performed at an “open” site which is an open air location selected to be in a radio frequency quiet zone where television and radio transmissions are weaker. The unit to be tested is placed on a wooden table above a large ground plane 10 meters away from a suitable receiving antenna connected to a spectrum analyzer.

EN61000-3-2
Puts limits on the harmonic currents that the power supply is allowed to take from the AC mains source. The standard applies to power supplies with rated power between 75W and input line current of up to 16 amps per phase.

A power supply which is not power factor corrected will take a current from the source which is not the same shape as the voltage waveform. This is because the input storage capacitors can only charge when the input voltage is higher than the capacitor voltage. Thus the input current flows for only part of the cycle, and has a high peak value which causes currents which are harmonics of the line frequency. With three phase power distribution the absence of harmonic currents ensures that the neutral current is zero. This was not the case when large numbers of personal computers without power factor correction began to be used in office buildings, and the neutral wire would burn out. Most power supplies now incorporate power factor correction circuitry to ensure that the harmonic currents are low.

EN6100-3-3
Limits voltage changes that the unit under test can impose upon the input power source. This is referred to as the flicker test.

Although this is not normally a problem with power supplies, some types of electrical equipment, especially in process control, can load the power source at regular or semi-random intervals. This can cause voltage changes that can affect the brightness of electric lighting and cause flicker. A survey was performed to determine what rates of flicker were the most disturbing to human subjects, and a curve of maximum percentage voltage variation at various frequencies was established. The most disturbing rate was just over 1,000 changes per minute, and the curve reflects the smallest percentage change at this frequency. Above 1,800 changes per minute the flicker is not noticed.

Basic Immunity Standards

EN61000-4-2
Tests immunity to electrostatic discharge from a simulated human body capacitance of 150pF. By walking across a carpet of artificial fiber in a low humidity condition, a person can build up a charge of several thousand volts. This can be discharged to electrical/electronic equipment, and so it is important that the equipment is immune to these discharges. The test is performed at a voltage of up to 8kV by discharging a probe to the chassis at various locations by direct contact, and at up to 15kV through the air, with the power supply operating. Test levels of 4kV and 8kV are common. Class B performance criterion applies.

EN61000-4-3
Checks immunity to incident radio frequency energy in the frequency range of 80MHz to 1,000MHz, and a separate test at 800 MHz to 960MHz to simulate the effect of digital cellular telephone transmissions. The test is performed in an anechoic chamber which is a shielded room with cone shaped plastic moldings on the inside wall surfaces which absorb radio frequency energy, so there are no echoes. The field strength is 10V/m for the carrier. Class A performance criterion applies.

EN61000-4-4
Tests the effect of a fast voltage transient or burst applied between each input line and ground in turn. The applied voltage has a peak level of 2kV, and rises to maximum in 5 nanoseconds, and falls back to zero in 50 nanoseconds. It is applied at a repetition rate of 5kHz. Class B performance criterion applies.

EN61000-4-5
Simulates the effect of a lightning surge voltage applied to the input power lines. Surge voltages are applied between each line and ground, and also between lines. The line to ground peak voltage is normally twice that applied from line to line. 4kV and 2kV are typical test voltages. The voltage has a rise time of 1.2 microseconds, and a fall time of 50 microseconds. Class B performance criterion applies.

EN61000-4-6
Tests the effect of conducted radio frequency energy which is inductively coupled into the input cables with a ground return. The frequency range is 150kHz to 80MHz at 10Vrms amplitude, and the frequency is increased in 1% steps. The carrier is 80% amplitude modulated at 1 kHz. Class A performance criterion applies.

EN61000-4-8
Electromagnetic compatibility, testing and measurement techniques for power frequency magnetic fields. Criterion A, using Helmholtz coil at 50 Hz, to 30 amps (rms) per meter.

EN6100-4-11
Checks the effect of input voltage dips on A.C. input power supplies only.
There are three different degrees of test severity, a 30% reduction of input voltage for 0.5 period, a 60% reduction for 5 periods and a 95% reduction for 250 periods. For the first test, the unit should continue working with no change of output voltage because most units have a hold-up time of one period, which corresponds to 20 milliseconds at 50Hz. The other two tests will cause reduction or loss of output voltage, and intervention may be needed to restore the output. The unit should not be damaged by the testing. Class B and C performance criteria apply.

Isolated & Non-Isolated DC-DC Converters

2007-10-26T08:53:00.000-07:00

There are two frequently used terms for types of DC-DC converters; non-isolated and isolated. This “isolation” refers to the existence of an electrical barrier between the input and output of the DC-DC converter.

The simplest example of a non isolated “converter” is the popular LM317 three terminal linear regulator. One terminal for unregulated input, one for the regulated output and one for the common.

Source National Semiconductor

Note there is no isolation between the input and output.

Today, non-isolated switching regulators are very common, or Point of Load (POL) converters.

Although low cost and simple, these converters suffer from one disadvantage in that there is an electrical connection between the input and output. Many safety agency bodies and/or customers require a separation from the applied input voltage and the output voltage which is often user accessible.

An isolated DC-DC converter will have a high frequency transformer providing that barrier. This barrier can withstand anything from a few hundred volts to several thousand volts, as is required for medical application.

A second advantage of an isolated converter is that the output can be configured to be either positive or negative.

Where many users get confused concerns how to connect the input up, particularly with the differences between a datacom system (input negative connected to chassis) and a telecom system (input positive connected to chassis).

Below are four scenarios, be aware - figures 3 & 4 will result in failed converters! Most DC-DC converters cannot withstand reversed input connections.

Why is my power supply input only rated from 100-240VAC?

2007-10-08T08:47:00.000-07:00

Most power supplies have a rating label that looks something like this:

However, a close look at the power supply’s datasheet will usually show the absolute AC input voltage range, from minimum to maximum. This is usually 90-264VAC, or occasionally 85-264VAC if the power supply has been designed for Japanese use.

Japan uses the lowest AC mains voltage, which is 100VAC nominal; however, short duration AC line droops or brown-out conditions often mandate a rating down to 85VAC. The UK is among the countries that use the highest AC mains, with a nominal rating of 240VAC.

The safety certification bodies (UL, CSA, TUV, etc.) mandate that a rating of 100-240VAC be listed on the power supply’s label. However, they factor in a +/-10% tolerance for the power generation and transmission utilities. -10% of 100VAC is 90VAC, and +10% of 240VAC is 264VAC. All safety testing is performed at the high and low limits as listed on the power supply’s datasheet.

So, if the power supply label states 100-240VAC, it can usually operate over a wider AC operating input range. However, always check with the manufacturer’s datasheet to confirm this. Continuous operation of the power supply over the datasheet’s specified AC input range will not normally cause any problems. In some cases, however, the maximum output power (total watts) of the power supply may need to be derated if the supply is operating off an input voltage that is on the low-end of the specified range. Always check the power supply’s datasheet for the specified minimum AC input voltage with various output load levels. Deratings may also apply depending upon the power supply’s operating ambient temperatures.

Should a label state 100/240VAC (note the slash) it “may” indicate that there is a voltage select switch or jumper that is required to be set for the correct operating input voltage range. Newer products tend to not have an AC select switch or jumper.

Worldwide, the AC mains power has a nominal frequency of either 50 or 60 Hz (cycles per second). However, these frequencies are subject to variations by the power generators in different countries (especially third world) and so the typical AC frequency range for power supplies is 47-63Hz.

A “Beginner’s Guide” to Fault Tolerant Power Supplies

2007-09-18T10:44:00.000-07:00

The effectiveness of having a fault tolerant power strategy was demonstrated after hurricane Katrina hit the Gulf Coast in 2005. A financial news television station interviewed the heads of two telecom carriers to find out when their telephone services would be operational again. The interview was very short – “we never lost service” they replied.

The telephone systems we take for granted have expensive and complex back up systems. Fault tolerant power supplies are supported by battery banks, generators and uninterruptible power supplies. Large Industrial complexes have also implemented similar systems - having an oil refinery stop production can result in enormous sums of money being lost!

For those with less extensive budgets, this brief article will explain the benefits, terminology and tips on how to implement a relatively low cost, but effective system.

Why have redundant power supplies?
Imagine a 24VDC 10A power supply driving motors and sensors on a conveyor based production line. For two or three years everything works fine, then one Friday (always at the end of the month), the power supply fails causing the conveyor to stop. Even if a spare part is in stock, it could still result in 30 minutes of expensive lost production.

If two identical power supplies had been installed in a fault tolerant, redundant mode, the remaining (good) unit would have continued to power the production line. The failed power supply could then be replaced at a more convenient time during routine maintenance.

Frequently Used Terminology

N+1
An expression where N is the number of power supplies needed to run the system. The simple two power supply system mentioned above would be considered 1+1. A triple redundant system (where two failures would have to occur to shut the system down) would be designated 1+2.

Hot-swap
Some equipment is operated 24 hours a day, 7 days a week, allowing no time to bring the system down for maintenance. In this case the failed power supply must be “swapped” out and a new one inserted without disruption to equipment operation.

ORing diodes
In the rare event of a power supply failing with a shorted output, low voltage-drop ORing diodes block that short from bringing down the system power.

Current share
Some power systems employ a method of balancing the current between the power supplies to increase field life. This can be an electronic signal wire that links the power supplies together or a switch* on the power supply that initiates a slight drop in the output voltage as more current is drawn. (*Common on high power DIN rail units)

Two Ways of Implementing Fault Tolerance

DIN Rail mount
For the example listed above, the simplest off-the-shelf solution is to use a diode “ORing” module and two power supplies. Here we are using Lambda’s DIN rail mount DLP-PU module and two 24V 10A DLP240-24-1/E power supplies.

Tip: When wiring the system, ensure that the cable lengths from the output of the power supplies to the ORing module are equal. This will help optimize the performance and life of the power supplies.
Inside the diode ORing module are two diodes and two alarm relays. Even in the event of one power supply failing with an internal short circuit, the remaining unit will continue to deliver power. See below.

Tip: - It is important to identify power supply failure using the relay alarms to flag the need for maintenance. Engineers sometimes overlook this which can result in a second failure unexpectedly bringing the system down!

Rack Mount
System Engineers requiring more power are turning to the communications style racks. These sophisticated low cost systems allow power supplies to be hot-swapped and come completely self contained. An example of such a product is Lambda’s FPS series.
Advantages of this solution include:

Easy mounting into a standard 19” rack
All in one solution
Hotswap capable (ORing diodes or MOSFET switches built-in)
No tools are required for replacement of a supply
High density, low profile (1.75”)
Off the shelf parts
Fully safety approved
All necessary warning signals included
12V, 24V, 32-36V and 48V outputs

Click on http://www.lambdapower.com/products/fps-series.htm for an animated example.

Finally, one important note
A company wanted to ensure that in the event of a power supply failure their system would continue to operate. A battery was installed across the power supply output to give 24 hours uptime in the event of a power supply failure.

Unfortunately no thought was given to how anyone would know that the system needed maintenance! The power supply did eventually fail and the battery kept the system up for 24 hours before it discharged resulting in a system shutdown. A simple alarm circuit could have prevented that.

If you take Lambda’s recommendation to invest a little extra money up front to make your power system more secure, test your system to make sure you have it right!

Advances in Power Supplies for Automated Electrochemical Mini-Plants

2007-09-07T10:38:00.000-07:00

On-site and on-demand production of disinfectants, biocides and water purification chemicals including sodium hypochlorite, chlorine dioxide has been substantially improved via the use of advanced switchmode power supplies that provide the power for automated electrochemical generators.

Many municipal water, food processing, and wastewater treatment plants are switching over from the use of chlorine disinfectants and biocides to safer and more environmentally friendly point-of-use and on-demand generated chemicals. The primary reasons for this change are that conventional chlorine agents require transport by tankers on accident-prone highways or railroads, ever increasing safety and environmental regulations regarding toxic gases and chemical spills, and the required bulk storage of these hazardous materials at the sites where they are used. Safer and in many cases more effective chemicals have been developed that can replace chlorine. For example, after many trial and error attempts to find a way to effectively control Legionnaires’ disease, it was found that chlorine dioxide (CIO2) was one the few chemical agents that could consistently and safely disinfect Legionella bacteria (see References). Add to this the ability to manufacture these safer chemicals at the locations that use them, and only when needed, and the advantages in total become obvious.

Two popular substitute chemicals for chlorine are sodium hypochlorite (NaOCI) and chlorine dioxide (CIO2) both of which can be manufactured via mini-plants (aka, generators) that are delivered to the end users’ site as a complete package and provide the disinfectants on-demand and as needed. In many cases, these mini-plants operate automatically and can be employed in unmanned locations such as municipal water treatment sites.

These electrochemical generators use the process of electrolysis as the basis for the production of these disinfecting and biocide chemicals. Recalling our science classes, electrolysis is a common method of separating bonded elements and compounds by passing an electric current through them. It involves applying a voltage between two electrodes (anode and cathode) which are submerged in a conductive solution (electrolyte). When a voltage is applied to the electrodes, electric current flows and in turn breaks down the molecules within the solution into its components (Figure 1).

Figure 1 shows part of the process that is used to produce sodium hypochlorite (NaOCI), which is more commonly known as household bleach when sold as a solution containing 5-6% of NaOCI. However, instead of a static vessel as shown in Figure 1, modern electrochemical generators pump the electrolyte solution continuously through one or more tubes that have the electrodes mounted within them. As the electrolytic solution flows through these tubes (electrolytic cells), the electrolysis process continuously separates the molecular components. In some instances, the solution is run through the dual-electrode electrolytic cells more than once to further refine and separate the resulting chemicals. (Note: Batteries operate by a reverse process from electrolysis; they generate electricity by means of galvanic or voltaic cells that contain anode and cathode electrodes that are in contact with an electrolyte solution or gel.)

Historically, the power supplies that provide the driving force for electrochemical generators have evolved from basic transformer and diode rectifiers, to transformer and SCR (silicon-controlled-rectifier) power sources, to modern and more sophisticated power sources. The development of the switchmode power supply greatly reduced the size and substantially improved the efficiency of these power sources. In addition, switchmode power supplies have the ability to provide electronic signals for status information (volts, amps, temperature, etc.), remote control, and communications to and from a PLC (Programmable Logic Controller) or a local/remote computerized controller.

The vast majority of switchmode power supplies are designed to operate as regulated voltage power sources. These supplies regulate the output voltage very precisely regardless of the amount of current drawn from the supply, up to its design limit. For example, a 1500-watt supply can provide a 12-volts output while providing from 0 to 125 amperes of current. Once the maximum current of 125-amps is reached, the supply is designed to go into a current-limit mode (where the output voltage is automatically reduced or the supply shuts down).

Figure 2 above shows the loop diagram of a typical sodium hypochlorite generator. The part of the system shown above that is called the Electrolyzer consists of multiple electrolytic cells (tubes containing electrodes), connected in series, through which the electrolytic solution is pumped and in turn separated into its primary chemical components (e.g., sodium hypochlorite solution and hydrogen gas) via electrolysis. The electrochemical process for manufacturing chlorine dioxide is similar to the above except it starts with a solution of sodium chlorite.

It has been found that in many electrochemical processes, including the production of disinfecting agents, that standard voltage-regulated power supplies do not always provide the ideal power profile for these processes. In fact, in many instances, the power supplies are being forced to operate at a fixed voltage and at close to their maximum current rating. If these operating conditions are maintained for long periods of time, the supply will internally heat-up and prematurely fail, thus shutting down the production of the disinfecting agents.

Why does this happen and how can it be avoided? As described above, during the electrochemical process, in order to keep up with the continuous electrolysis process with constantly flowing electrolyte solutions, the power supply must provide a high enough voltage to overcome the impedance between the two electrodes and the solution surrounding them, and, more importantly, to provide a high enough current density (amperes) to effectively separate the molecules during the short time (determined by the flow rate) that the solution comes in contact with the electrodes. By using a switchmode power supply that is designed to operate in a “constant-current” mode (instead of constant-voltage, as is the norm) the electrochemical process has been found to produce chemicals much faster, with consistent high quality, without forcing the power supply into an overload state.

There are a number of ways of providing current-mode power supplies for enhanced electrochemical applications. One method is to use Programmable Power Supplies. These supplies are designed to be manually or remotely programmed to operate in a voltage-mode and/or a current mode, at a specific voltage and current range, along with other specified parameters. As an added bonus these supplies usually include a serial digital communications port that allows it “talk” to local or remote computer controllers. Additionally, these supplies can be connected in parallel to the electrodes, or to groups of electrodes, when an electrochemical process requires more current than one supply can provide. For example, Lambda Americas’ model ZUP10-80/U programmable power supplies is adjustable from 0 to 10-volts with 0 to 80-amps (800 watts total). This type of supply is being used in its “constant-current” mode to efficiently produce disinfectant and biocide chemicals at many unmanned, non-air conditioned, municipal water treatment sites. In some applications, two or more ZUP supplies are connected in parallel to provide the necessary amount of current for the electrochemical process.

Another method of providing a “constant-current” mode power supply is to modify the design of a voltage regulated supply. This can be done by adding circuits that monitor the supply’s output current to prevent an overload, yet maintain a “constant-current” profile from the supply. For example, Lambda Americas has produced modified versions its HWS-CC 1500-watt supply to do exactly this. In electrochemical applications that produce disinfectant and biocide chemicals, a number of these “current-mode” supplies are connected to different sets of electrodes, and/or, in parallel, to support different output current requirements for various models of electrochemical generators. Obviously, generators that produce higher output rates of chemicals require higher current levels.

Lambda’s HWS-CC Series Power Supplies

This paper has focused on the techniques and benefits related to advanced switchmode power supplies for mini-electrochemical generators (self-contained plants) that produce disinfecting and biocide chemicals on-site. It should be noted that electrolysis processes are used extensively in many other chemical and industrial areas, some of which are listed below.

Production of aluminum, copper, sodium
Anodizing
Production of hydrogen (e.g., for the cars and fuel cells of the future)
Electroplating and polishing
Large waste water treatment plants
Factory and power plant cooling towers - recirculating water treatments

Many of these electrochemical processes require power levels that far exceed the range of the switchmode power supplies described above. These high power rectifier systems (ranging from 300 to 30,000 kW) are very specialized, large, heavy, and are usually comprised of huge transformers, rectifiers, thyristors, SCRs, capacitors, regulating controllers, and water cooling systems. Some of these high power sources are as large as a typical bathroom, kitchen, and larger. There is no doubt that as technologies advance, these huge power sources will see reductions in size and improvements in efficiencies.

In summary, the application of switchmode power supplies operating in a “constant-current” mode has been shown to provide significant improvements in electrochemical self contained mini-plant generators that are used to produce disinfecting and biocide chemicals. These benefits include:

Improved current-density control for consistent electrolysis
Enhanced quality of the resulting chemicals
Higher efficiencies and improved regulation of the power sources
Reduced space and weight
Power Supplies meet international Safety and Power Factor Correction (PFC) standards
Availability of digital communications, remote control, and status signals
Substantial reduction of downtime

References:
http://en.wikipedia.org/wiki/Electrolysis
http://www.medscape.com/viewarticle/520378
http://www.lambdapower.com/ http://www.doh.wa.gov/ehp/dw/Publications/alternate_disinfectants.htm

What’s all this stuff about "Digital Power"?

2007-09-06T13:33:00.000-07:00

It seems that every 2-3 weeks an article or news announcement about “Digital Power” appears in electronic design periodicals or online news links. In fact, one industry newsletter seems to be having a love affair with digital power, as it mentions it in just about every issue.

So what is all this fuss about Digital Power and what is it anyway? Well, the simple answer is that there are two basic types of digital power. These are Digital Control (used internal to the power devices) and Digital Power Management (provides external control and communications between power devices and a master controller).

Digital Control
The majority of switchmode AC-DC and DC-DC power supplies/converters use analog techniques to regulate/control the output voltage, current, and power factor correction circuits, etc. The closest that most of these devices come to looking a bit digital (On/Off states) in nature is by employing Pulse Width Modulation (PWM) in their switching regulator circuits; but even that is a bit of a stretch.

In recent years, new integrated circuits (ICs) have been developed that can replace “analog” control ICs and discrete circuits, which are used extensively in all power devices, with those that are, at least in part, “digital” in nature. These internal ICs and circuits perform such control functions as: voltage regulation (VR), power factor correction (PFC), pulse width modulation (PWM) control, internal monitoring/alarms, and external communications.

The advantage of these digital ICs is that they can be programmed by engineers with digital or analog electronics training. And, since the Universities are pumping out more digital (e.g., computer science) than analog engineers these days, these digital ICs are becoming attractive. However, at present the cost of these digital ICs (along with NRE for the equipment needed to program the devices) is still higher than for the mature analog ICs. Nonetheless, some predict that these IC costs will become equal within the next 12 months or so. A potential disadvantage of these digital ICs is that, by their nature, they require a high speed clock to operate, which can add to the radiated and conducted noise coming from the power supply or converter. However, advanced functions such as fault diagnostics/prevention and improved power efficiencies are among the promises of the new digital control ICs.

Digital Power Management
As mentioned above, Digital Power Management (DPM) involves the external control and communications between power supplies (or converters) and a master controller. Currently, many analog-based power solutions already have the ability to communicate with an external computer or controller via digital communications links (e.g., RS232, RS485, GPIB, or I2C bus).

Newer DPM control and communications formats have evolved that are designed to operate with the new digitally-controlled power devices. These include DPM technologies such as PMBus (Power Management Bus) and Z-One. Sadly, these technologies are not compatible or interchangeable. In fact, currently there are lawsuits between the backers of both of these technologies.

If I were a potential user of these Digital Power Management schemes, I would stay clear of them until the lawsuits are settled (expected to occur within the next 12 months), rather than find out later that the cost of these DPM ICs or controllers have substantially increased due to royalties that must now be paid to the company that won the lawsuit.

The potential advantages of the DPM and digital power technologies in general, include enhanced bidirectional communications, fault diagnostics, remote programming of the linked power supplies/converters, automatic compensation of dynamic input and output load changes, and overall improvements in efficiencies that relate to green-power.

In Summary
Although “Digital Power” is a popular buzzword these days, especially by those companies who have developed or adopted the technologies, the bottom line will always be: “What do I get for my money?” At present, there are hardly any power supply or converter applications that “must have” digital power when compared to the many lower cost and field-proven analog solutions that exist.

For example, during their new power-product design and development process, Lambda has designed in-parallel devices that employed both analog along with those that use digital control ICs and technologies. In all cases, the final decision on which technology ultimately goes into production has been based on comparative price/performance factors; which is the dominant decision factor for their customers.

When the time comes that digital power products offer the same or better performance and reliability, along with the ”needed features”, at the same or lower price as analog-based products, that is when Digital Power will become the winning technology. Realistically, someday digital power will provide a price/performance advantage over purely analog power devices. Who knows for sure when that time will come.

Can I Operate my AC-DC Power Supply with a DC Input?

2007-09-04T13:15:00.000-07:00

The answer is yes, sometimes.

Many standard AC-DC switch mode power supplies (most of Lambda’s products) specify a high voltage DC input range in addition to the more common AC input range of 90-264VAC. We receive many questions about how and where to connect the DC input to an AC-DC supply that is spec’d to operate off of DC as well as an AC inputs.

Where and why is high voltage DC power used? It turns out that many power generation facilities provide a high voltage DC to power the plant’s equipment rather than the regular 115VAC or 208VAC power grid. This high voltage DC (typically 120 or 130-330VDC) can be easily used with batteries to provide a secure source of power rather than using expensive centralized or local UPS systems.

Now back to the subject. The topology of many switch-mode power supplies actually lends itself to operation from either AC or DC input. Important Note: Always check your power supply’s Operations Manual or spec sheet to confirm that it is designed to operate from either an AC or DC input.

Referring to the simplified power supply schematic below:

When powered by an AC sine wave, during the first half cycle the current flows from the Line terminal through the input filter and charges capacitor C1 through diodes D1 and D3. During the second (negative) cycle, current flows from the Neutral terminal and capacitor C1 is charged through diodes D2 and D4.

When powered from a high voltage DC source, the polarity of the connection is not critical as far as the operation of the power supply is concerned. If the positive connection is made to the Line terminal, C1 is charged through diodes D1 and D3. If the positive connection is made to the Neutral terminal, then C1 is charged through diodes D2 and D4.
An important note of caution to insert here is about the protective fusing of the power supply. Internally most power supplies have a fast acting AC rated fuse in series with the Line terminal. It is recommended that a DC rated fuse be installed external to the power supply. If one side of the high voltage DC buss is connected to ground, then the fuse is usually positioned in series with the “hot” side (the ungrounded side). It is recommended that you consult with your local safety engineer to be sure.

What size and type of output wires should I use?

2007-08-17T13:22:00.000-07:00

There are two main considerations for sizing DC wiring from the output of a power supply to its load. They are ampacity (fancy term for the number of Amps) and voltage-drop (remember ohms law: V = I x R). Ampacity refers to a safe current carrying level as specified by safety organizations such as Underwriters Laboratories and the National Fire Prevention Association, which publishes the National Electric Code (NEC).

AWG stands for American Wire Gauge and defines the diameter and cross sectional area of the wire. The smaller the AWG number, the larger the diameter, cross-sectional area, and current carrying capacity of the wire. Always use insulated wires with solid or stranded pure copper conductors (do not use aluminum or copper-clad steel wires). The voltage-drop is simply the amount of voltage lost in a length of wire due to the resistance of the conductor.

DC wires may be sized for either ampacity or voltage drop depending on the wire length and conductor heating. In general, ampacity considerations will drive wire selection for short wire lengths (less than 50 feet) and voltage drop will drive wire selection for longer lengths (greater than 50 feet). Note: If you are using the Remote Sense feature of the power supply, remember to stay within the maximum voltage drop across the cables that the Remote Sense is designed to compensate for, which can range from 0.3V to 1.0V (check the power supply’s user-manual for details).

The National Electric Code table 310.16 provides ampacity values for various sizes, bundles, and insulation temperature rated wires. ALWAYS FOLLOW THE NEC RULES, LOCAL CODES, AND YOUR COMPANY’S PRACTICES WHEN SELECTING DC WIRING.

Table 1 shows the MINIMUM recommended wire sizes for different load currents. The use of larger diameter wires (with a smaller AWG number) would reduce the voltage drop (and heat generated) across the wires. The current ratings in Table 1 are based upon using 90° C rated insulated wire. If using a lower temperature rated insulated wire (e.g., 60° C), the wire diameter would need to be larger. Refer to the following web site for more information about wire gauges: http://en.wikipedia.org/wiki/American_wire_gauge .

For example, per Table 1 below, a load current of 200 Amps would require a minimum of two # 2 AWG wires connected in parallel for each of the output connections (one pair or wires for the positive (+) and one pair for negative (-) output connections to the load). Again, larger diameter wires would decrease the voltage drops across these wires.

FAQ Modular power supplies

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I often get asked the same questions by our customers concerning Modular Power Supplies that I thought it would be helpful to list them here:

Q) What is a Modular Power Supply?
A) A Modular Power Supply is one that is assembled using a variety of pre-built building blocks (modules). This gives the customer a maximum choice for his voltage, current, signals & case style.

Q) What is the minimum quantity I have to buy?
A) At Lambda you can buy as few as one. Their modular power supplies are built using sophisticated manufacturing methods. The bills of materials are automatically generated, fed down to the shop floor and then quickly assembled using pre-built assemblies. Stock of the pre-built assemblies is managed by Kan Ban methods that trigger replenishment orders from the various workcells in the facility.

Q) Is there a set up charge for production?
A) None

Q) How long does it take to get all the safety approvals?
A) A good modular power supply series has all the safety certifications in-place to cover all of the potential configurations. Often these include EN61010 (test & measurement) and EN60601 (medical) certifications.

Q) How soon can I get a unit?
A) Because the modular units use pre-built assemblies, lead-time can be as short as 48 hours. Several US distributors have arrangements with power supply manufacturers to do the assembly. These are often called VARs (Value Added Resellers). Lambda has several VARs who can quickly build supplies to your needs.

Q) Are these units expensive?
A) Not really. Prices have dropped substantially since 1980 when Lambda first introduced modular supplies. It is not unheard of for our customers to be using in excess of 1000 units/year.

Q) Why should I use a modular over a modified standard or custom?
A) As these modular products are sold to a huge number of different customers, who are using them over a wide range of applications, the likelihood of a problem is minimal. With a custom or modified standard that risk goes up dramatically as you will be the "beta" site.

Q) Can I get odd output voltages?
A) Yes, many output "modules" feature a wide adjustment range, often 2:1. Recently, we had a customer ask for five (5) floating 18V outputs and we were able to meet their needs with a modular power supply.

If you have any questions regarding Modular Power Supplies, please let us know.

Types of Distributed Power Architectures

2007-08-09T16:03:00.000-07:00

Compact DC-DC converters have made their way into millions of electronic products and systems. The vast majority of these depend upon an AC front-end-box to convert the AC power source into a DC voltage from which the converters operate. In addition, international regulations have mandated that these front-end-boxes include Power Factor & Harmonic Correction (PFHC) to maximize the available power from the power grid.

Traditional Distributed Power Solutions

Traditional designs that employ distributed power architecture place DC-DC converters on PC boards very close to the point-of-load to maximize system speeds and efficiencies. To power the DC-DC converters, the required AC-DC power supply with PFHC is typically mounted somewhere in the system’s enclosure, external to the main pc-board (Figure 1).

This technique is quite reasonable for most applications. However, when it comes to equipment that must be mounted outdoors and occupy the smallest possible volume, there are now improved power products available.

Improved Power Distribution Methods

Typical medium power (400-700 watts) PCB mounted DC-DC converters are packaged in “full brick” sizes (e.g., 2.4” W x 4.6” L x 0.5” H). A number of major manufacturers of DC-DC converters have seen the need for, and are now providing AC input PFHC front ends in brick-formats that are PCB mountable near to the DC-DC converter(s). This has the advantage of placing all the power components on the same pc-board thus reducing the end products size and eliminating the power interconnect wires (Figure 2).

These AC-DC w/PFHC front-end bricks require some external components (capacitors, resistors, etc.), but the space required for these items is small in comparison to the elimination of the external “metal boxed AC front end”. And, these external components can be robotically inserted during the production of the pc-board. An added benefit of utilizing these brick packages is that they can be cooled without fans, by means of heat sinks or cold plates (e.g., mounting the brick bases against the system’s metal enclosure).

The Latest AC-DC Power “Brick” Solutions

Power Supply manufacturers have not stopped developing smaller and better power solutions. In fact, in recent times the AC/PFHC brick mentioned above has been merged with a DC-DC converter to form the ultimate power solution; an AC/PFHC/DC integrated brick. These 2-in-1 devices accept wide range 85 to 265 VAC inputs, correct the power factor, and provide the DC output(s) to the system. All this is accomplished within the same size constraints of a single “full brick” package measuring only 2.4” W x 4.6” L x 0.5” H, thus providing a 50% board space savings (Figure 3).

These integrated 2-in-1 pcb-mounted Power Bricks are ideal for Distributed Power Architectures where POL (Point of Load) Converters are needed. Since the 2-in-1 Power Bricks provide the conversion from AC to DC (with PFHC) along with the needed isolation, and the Intermediate Bus Voltage, the use of multiple low-cost, non-isolated POL converters becomes quite practical (Figure 4).

Recent advances in components and power design technologies have made these new
2-in-1 pcb-mount power bricks possible. In order to increase power densities, special Permalloy cores have been developed and employed in the inductors. New substrates and innovative transformer winding techniques have facilitated component height compressions and improved thermal management. And, of course, advances in integrated and hybrid circuits have contributed greatly to this next generation of power products.

Applications of 2-in-1 AC-DC Power Bricks

These new “2-in-1” AC-DC power bricks are ideal for many outdoor and indoor applications including:

Custom Power Supplies
PCB Mounted Bulk Power for Multiple DC-DC or POL Converters
Large LED & Liquid Crystal Displays
Traffic Information, Control, & Signaling Equipment
Toll Devices
Pico & Cell Phone Repeaters
WiFi, Telecom Sub-Stations
Underwater Surveying Devices
Automatic Pass-Reading-Devices for FastTrac Car Lanes
Oil Pumping & Pipeline Monitoring Devices
Security Systems

New 2-in-1 AC-DC Power Bricks

Lambda, a unit of TDK Corp., is currently one of the manufacturers of a new range of integrated “single-brick” AC-DC power bricks. These “2-in-1” pcb-mount devices are so innovative, they have seven patents pending.

Some of the salient features of Lambda’s single-brick AC-DC PFE Series power modules include:

Operates from Universal 85 to 265VAC, 47-63Hz Input
Power Factor & Harmonic Correction Meets EN61000-3-2
Low Profile, Single-Brick Footprint
High Power Density (up to 129W/in3) & Efficiency (up to 90%)
Regulated and Isolated DC Outputs with Wide Operating Temperatures (at baseplate)

PFE500-12: 12VDC Output, 400 Watts, -40 to +85°C
PFE500-28: 24 to 28VDC Output, 500 Watts, -40 to +100°C
PFE500-48: 48VDC Output, 500 Watts, -40 to +100°C
PFE700-48: 51VDC Output (semi-regulated), 714 Watts, -40 to +85°C

±20% Output Voltage Adjustment Range
Over Voltage/Current/Temperature Protection
Approved to UL/CSA/EN60950-1, CE Marked, & RoHS Compliant
Optional Heatsinks & Evaluation Kits Available

Linear vs. Switch-mode Power Supplies

2007-08-08T10:20:00.000-07:00

The Power Guy blog focuses on modern switch-mode power supplies and converters. However, to provide the newbie (newcomer) with some background information, we have included the following discussion.

Introduction
Linear power supplies were the mainstay of power conversion until the late 1970’s when the first commercial switch-mode became available. Now apart from very low power wall mount linear power supplies used for powering consumer items like cell phones and toys, switch-mode power supplies are dominant.

What are the differences and how do they work?
Linear power supplies have a bulky steel or iron laminated transformer. It provides a safety barrier between for the high voltage AC input and the low voltage DC output. The transformer also reduces and the AC input from typically 115V or 230VAC to a much lower voltage, perhaps around 30VAC. The lower voltage AC is then rectified by two or four diodes and smoothed into low voltage DC by large electrolytic capacitors. That low voltage DC is then regulated into the output voltage by dropping the difference in voltage across a transistor or IC (the shunt regulator).

Switch-mode supplies are a lot more complicated. The 115V or 230VAC voltage is rectified and smoothed by diodes and capacitors resulting in a high voltage DC. That DC is then converted into a safe, low voltage, high frequency (typically switching at 200kHz to 500kHz) voltage using a much smaller ferrite transformer and FETs or transistors. That voltage is then converted into the DC output voltage of choice by another set of diodes, capacitors and inductors. Corrections to the output voltage due to load or input changes are achieved by adjusting the pulse width of the high frequency waveform.

Comparisons of both technologies
Size: - A 50W linear power supply is typically 3 x 5 x 5.5”, whereas a 50W switch-mode can be as small as 3 x 5 x 1”. That’s a size reduction of 80%.

Weight: - A 50W linear weighs 4lbs; a corresponding switcher is 0.62 or less. As the power level increases, so does the weight. I personally remember a two-man lift needed for a 1000W linear.

Input Voltage Range: - A linear has a very limited input range requiring that the transformer taps be changed between different countries. Normally on the specification you will see 100/120/220/230/240VAC. This is because when the input voltage drops more than 10%, the DC voltage to the shunt regulator drops too low & the power supply cannot deliver the required output voltage. At input voltages greater than 10%, too much voltage is delivered to the regulator resulting in over heating. If a piece of equipment is tested in the US and shipped to Europe, or even to Mexico in some cases, the transformer “taps” have to be manually changed. Forget to set the taps? The power supply will most certainly blow the fuse, or may well be damaged.

Most switch-mode supplies can operate anywhere in the world (85 to 264VAC), from industrial areas in Japan to the outback of Australia without any adjustment. The switch-mode supply is also able to withstand small losses of AC power in the range of 10-20 milliseconds without affecting the outputs. A linear will not. No one will care if the AC goes missing for 1/100th of a second when charging your cell phone, it will take 100 of these interruptions to delay the charge by one second. However, having your computerized equipment shutdown or reboot 100 times a day will cause a great deal of heartburn.

Efficiency: - A linear power supply because of its design will normally operate at around 60% efficiency for 24V outputs, whereas a switch-mode is normally 80% or more. Efficiency is a measure of how much energy the power supply wastes. This has to be removed with fans or heatsinks from the system. For a 100W output linear, that waste would be 67W. A 100W switch-mode would be just 25W. Therefore, 67W – 25W = 42W is the extra power lost by a linear supply. Doesn’t sound much, but don’t try touching a 40W light bulb. If the equipment were running 24 hours a day, then the extra losses would be 367kW hours, at the current average cost of $0.10 per kW hour; that’s an extra $37 a year for a power supply that costs around $80.

As a quick note, in Europe, they are trying to limit those losses of all power supplies used by consumers particularly when operating in the “Off” mode (as many products are left plugged in 24 hours a day). Imagine 250 million power supplies eating up a couple watts. That equates to the output of a whole power station.

About Power Topics

2007-08-08T10:17:00.000-07:00

The purpose of this blog is to provide end-users of AC-DC Power Supplies and DC-DC Converters with useful information regarding product applications, helpful hints, news, comments, and answers to your questions. My focus is on modern switch-mode power products in the range of 1 to 3,000-watts.

My goal is to provide OEM designers, who select power products for their end-products, and purchasing agents, who buy these devices, with a valuable resource to assist you in making decisions involving power solutions for your next and/or existing products.

I hope you find this website and blog informative and useful. You are invited to contact me with your questions and comments.

Thank You,
Power Guy