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

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

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?

There are many AC-DC power supplies and DC-DC converters with output power ratings that can vary dependent 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?

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

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

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 exceeds 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

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?

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

Choosing an Input EMI / EMC Filter for a Power Supply

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

1. Minimize outgoing electrical noise to avoid interfering with neighboring equipment
2. 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 RSEN (single stage) and RSHN (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 RSMN series of filters.

Droop Mode Current Share

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. TDK-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.