Power Supply Leakage Current Testing to IEC60990

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?

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

1. The manufacturer has taken the appropriate steps on component evaluation, component derating and has utilized sound design techniques.
2. 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

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.

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

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

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.

Why Use DIN Rail Mount Power Supplies?

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

Operating Power Supplies in Series

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.

Maximizing the Life of Power Supply Fans

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?

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.