Does it matter how an EMC/EMI filter is wired to a power supply?

An email was recently received by TDK-Lambda’s Technical Support from a technician who was fault-finding a video system.  His questions about how an EMC filter should be connected were relevant and warranted a blog article on the subject!

As indicated in Figure 1, the EMC filter is situated between the AC input to the system and the power supply providing DC voltage(s) to the system load.  The filter’s function is to reduce incoming noise from the AC input and/or outgoing noise to the AC input from the power supply.

Figure 1: System block diagram

Regarding connection to the filter, let us now look at a typical EMC filter - the 250Vac 10A rated RSEN-2010 for example (see Figure 2 and 3).

Figure 2: RSEN-2010 filter

Figure 3: RSEN schematic

Reviewing Figure 2, the nomenclature “LINE/LOAD” on the left hand and right hand side of the label indicates that either set of the terminals can be used for the input or load connection.  It can be also noted that there is no indication of where the Line or Neutral should be wired to, just numbers 1, 2, 3 & 4.  Which connections are made to these terminals is very important.

On the right hand side of the label and schematic (Figure 3), two additional capacitors, known as “Y” capacitors can be seen.  These provide a low impedance path to earth ground to reduce high frequency common-mode noise.   When the filter is used to reduce system noise from the power supply reaching the AC source, terminals 3 and 4 should be connected to the power supply.  If the filter is being used to reduce externally generated high frequency noise from entering the system then terminals 3 and 4 should be connected to the AC source input.

It is essential that if the AC line is connected to terminal “1”, then terminal “4” should be connected to the power supply Line terminal.  Likewise with terminals “2” and “3” for the neutral wiring as shown in Figure 4.

Figure 4: Connection of the filter to the AC and the power supply

If the wiring to “3” and “4” is crossed, there could be safety issues for a power supply with a single input fuse situated internally in series with the AC Line terminal.  

If the AC is correctly wired (Figure 5a), in the event the internal power supply fuse was to open due to a fault, the internal circuitry is isolated.  It is important to note that at the breaker panel, where the AC first comes into the building, the neutral input is grounded to earth.

Figure 5a: Correct AC connection to a single fuse power supply

If the Line and Neutral is reversed to the power supply due to incorrect wiring (Figure 5b), in the event of the fuse opening the internal circuit is still live.  Two issues can arise with this scenario.  A service technician fault-finding the system could receive an electrical shock, particularly with an open frame power supply without a cover.  Secondly, if an internal short was to occur in the power supply between Line and the earthed chassis, the fuse would be out of circuit and not open.  Even the simple use of a mounting screw that is too long could cause this!

Figure 5b: Incorrect AC connection to a single fuse power supply

Always ensure that adequate inspection and testing techniques are in place with AC wiring.

Power Guy

Synchronizing the i6A DC-DC Converter Switching Frequencies

When multiple DC to DC converters are used to power sensitive circuitry, input and output noise can cause system issues, particularly when measuring very low signal voltages.  The problem is compounded if the converters’ switching frequencies vary with input voltage or output load. 

Even when the converter’s operating frequency is fixed, there will be a tolerance on the switching frequency timing circuit between the converters and just a few Hertz differences between the converters can cause sub-harmonic beat frequencies.

Filtering the inputs and outputs is one solution, but this can be complex over a wide frequency bandwidth.  If the DC-DC converters had a fixed switching frequency and could be synchronized to together or tied to a master clock, any board and/or system EMI filtering would be simpler.

Although somewhat uncommon, there are some DC-DC converters that have synchronization capability.  When the “full feature” option is specified on TDK-Lambda’s low cost, fixed frequency, i6A series of 250W non-isolated DC-DC converters, multiple units can have their operating frequency synchronized.

There are four ways to connect the modules:

1. Master / Slave, with no phase shift.  One i6A module is the “master” and the other modules will operate at the master’s switching frequency.  All the modules draw input current at the same time.

 

2. Master / Slave, with 180 degree phase shift.  One i6A module is the “master” and other modules will operate at the master’s switching frequency, but directly out of phase.  This can reduce the peak input ripple current from the supply, requiring less input capacitance.  Any module with a jumper between pins 4 and 33 will have the phase shift function activated.

 

3. An external clock is used with no phase shift.  All the i6A modules operate at the same frequency as the external clock (no master / slave). All the modules draw input current at the same time.

 

4. An external clock is used with a 180 degrees phase shift.  All the i6A modules operate at the same frequency as the external clock (no master / slave), but any module with a jumper between pins 4 and 33 will have the phase shift function activated.  This again reduces the peak input ripple current from the supply, requiring less input capacitance.

 

The i6A non-isolated DC-DC converters are a series of step down converters (the input voltage has to be higher than the output) in the industry standard 1/16^th brick footprint.  All models feature wide range input voltages, as high as 9 to 53V and have wide range output adjustment from 3.3 – 15V to 3.3 – 40V.  Operating efficiencies can be as high as 98%.

See http://www.us.tdk-lambda.com/lp/products/i6a-series.htm for further information.

Medical power supplies meeting IEC 60601-1-2 4th edition voltage dips and interruptions

Customers call TDK-Lambda wanting their medical product to meet the strict IEC 60601-1-2:2015 4^th edition immunity standard, and ask us if our medically certified power supplies fully comply.  In particular, concerns are raised about meeting the section dealing with voltage dips and short interruptions to the AC supply.

IEC 60601-1-2 is derived from the IEC 61000-4 standard, which covers Electromagnetic compatibility (EMC).  The testing and measurement methods are very similar, but some of the test levels for dips and interruptions in the section based on IEC 61000-4-11 are much tougher.  The interruption test of removing the AC supply for 5 seconds, without the loss of the output, is almost impossible without a custom solution with some form of battery back-up.  One may well question why are standard medical power-supplies being sold if they do not meet that standard.

Firstly, power supplies are not classified as medical devices, it is the customer’s product or system that is the medical device.

Secondly the term “essential performance” used in the standard has to be examined.  In the 3^rd edition of IEC 60601-1 it is defined as “the performance necessary to achieve freedom from unacceptable risk”.  To clarify, the designer/manufacturer has to determine if a loss of performance or functionality of their medical device product or system will result in an acceptable risk or an unacceptable risk.  That risk is the potential to harm a patient, operator or the environment.  Analysis must be made of the probability or the frequency of an event happening compared to the severity of that event.

Let’s give a simple example.  Diabetics check their blood glucose level on a regular basis and most use a handheld battery-operated meter that accepts disposable test strips.  If that meter was to stop working, say due to a faulty display, it would be classified as an acceptable risk.  Replacement meters are readily available from supermarkets and pharmacies and a short delay in testing would not normally cause harm.  An unacceptable risk would be if the internal sensor measuring the blood glucose level was to produce incorrect readings and the diabetic administered too much or too little insulin.

Power supplies, although not classified as medical devices, can have an impact on the IEC 60601-1-2 immunity performance of the device they are powering.  For the voltage dips and interruptions section of the standard, there are five tests performed.  Table 1 below shows the input voltage dip and the duration.  100Vac input and 50Hz conditions are shown as they could represent the worst case.

Test results are judged against four performance criteria levels:

Performance Criteria A – ‘Performance within specification limits’

This is the best result.  A very slight drop in output of a few milli-volts (within the regulation limits) should not cause the end device to malfunction.

Performance Criteria B – ‘Temporary degradation which is self-recoverable’

Criteria B is usually acceptable in the majority of cases.

Performance Criteria C – ‘Temporary degradation which requires operator intervention’

This would be classified as unacceptable from a user point of view, without even considering a risk analysis.  If the AC power was interrupted and the power supply had to be reset by a patient or operator, it would be much too inconvenient.

Performance Criteria D – ‘Loss of function which is not recoverable’

Criteria D is really a “fail” test result.  If a power supply is damaged and needs replacing after the test, it is very unlikely that a product with this performance level would be placed on the market.

AC Input Voltage	Actual Voltage Dip for 100Vac nominal	Voltage Dip by AC Input Cycle (50/60Hz)	Voltage Dip Time Period for 50Hz	Suggested Performance Criteria Level
Dip down to 0%	0Vac	0.5 of a cycle	10ms	A
Dip down to 0%	0Vac	1 cycle	20ms	A
Dip down to 40%	40Vac	10/12 cycles	200ms	B
Dip down to 70%	70Vac	25/30 cycles	500ms	A
Dip down to 0%	0Vac	250/300 cycles	5000ms (5s)	B

Table 1: Test Levels

Referring to Table 1, most power supplies will pass the first two tests with a Performance Criteria level A with some output derating to increase the hold-up time.

The third and fourth tests requires the power supply to continue to operate for 200ms when the input drops to 40% of nominal or for 500ms at 70% of nominal.  Criteria A could be achieved by having the power supply’s low voltage input protection circuitry modified to allow the power supply to operate at the lower input voltage for a short time.  As the AC input current will be higher, it is best to ensure that the power supply is not operated at full load.  As hold-up time is related to the actual output power drawn, operating the power supply at 50% load will result in a significant “ride through” capability during the interruption.

The fifth test of a 5 second interruption to the AC supply is usually met with the installation of battery back-up or a UPS (Uninterruptible Power Supply).  Adding sufficiently large energy storage inside the power supply would result in a significant increase in size.

In summary, the medical device designer/manufacturer must decide which performance criteria is needed, based on their risk analysis to meet IEC 60601-1.  Unless continuous performance is critical, most manufacturers will opt for the criteria in Table 1.

How will the Power Supply Industry be affected by EN 55032 replacing EN 55022?

In 2014 the Hazard-Based Safety Engineering (HBSE) standard IEC 62368-1 was announced combining the Information Technology Equipment (ITE) standard EN 60950-1 and the audio, video and similar electronic apparatus safety standard EN 60065.  This step was taken as there was no longer a clear definition between ITE and multimedia equipment with advent of internet connected TVs, smartphones and other home entertainment products.

Now the EMC standards are also being combined and as of March 5th, 2017, EN 55022, EN 55013 and EN 55103 will be replaced by one unified emission requirements standard called EN 55032.  The current “Electromagnetic compatibility of multimedia equipment” was first published in May 2012 as EN 55032:2012+AC 2013, will be withdrawn on May 5th 2018.  EN 55032:2015+AC:2016, which was announced May 2015 and published February 2016, has already superseded the 2012 standard.

The three standards that are being replaced by EN 55032 are:

EN 55022: Information Technology Equipment, Radio disturbance characteristics. Limits and methods of test.

EN 55013: Sound and Television Broadcast Receivers and Associated Equipment.

EN 55103: Audio, Video and Entertainment Lighting Equipment for Professional Use.

Fortunately for those in the power supply industry serving the ITE market who have relied on EN 55022 as their core standard for many years, there are no changes to the test requirements.  The multimedia equipment (MME) makers will have additional test requirements to interface ports, port type and emissions from cabling.  The individuals that prepare and sign their company’s CE Declaration of Conformity will be kept busy updating their forms though!

Power Guy

How does IEC 60601-1-2 EMC 4th Edition relate to power supplies?

The growing use of wirelessly connected devices like mobile phones, tablets, laptop computers and gaming consoles pose a risk to equipment sensitive to EMI and EMC.  On aircraft, restrictions on the use of these devices have long been in place and in general, the public are aware of that policy.  In the past, many of us have seen notices in hospitals asking visitors to not use their phones in intensive care, critical care pediatric units and where specialized medical equipment is located.  

With the growing popularity of home healthcare, enforcing such a policy is impossible.  The medical regulatory bodies, like the FDA (Food and Drug Administration), are now requiring equipment manufacturers to design and test their products to avoid any potential risk of patient harm.  This also includes electrostatic discharge (ESD), radio interference, voltage surges and power interruptions. 

In 2014 an update to IEC 60601-1-2 was published and it “applies to basic safety and essential performance of medical equipment and systems in the presence of electromagnetic disturbances and to electromagnetic disturbances emitted by that equipment and systems”.  Product categories were added and higher EMC test levels introduced.  Manufacturers must submit risk analysis documentation for both normal and abnormal use of their equipment and systems.  This standard is often referred to as the “4^th edition”.

The “life-supporting equipment” category has been removed from the standard, and it has been replaced by electromagnetic environments of “intended use”.  According to IEC 60601-1 (2012) it is defined as “use for which a product, process or service is intended according to the specifications, instructions and information provided by the manufacturer”.  These intended use environments are:

1)    Professional healthcare facilities with attending medical staff, and include hospitals, dental surgeries, surgery rooms and intensive care.

2)    Home healthcare which is defined by IEC 60601-1-11 as dwelling places where patients live or places where patients are present - excluding (1)

3)    “Special” environments are those that exclude (1) and (2), but include heavy industrial plants or medical treatment areas with high powered medical electrical equipment (such as short wave therapy equipment).

As far as timing for the update, EN 60601-1-2:2007 3rd Edition is scheduled to be withdrawn on December 31st, 2018, and will be replaced with the 2015 version of EN 60601-1-2.  This is also the FDA compliance date in the US, after several recent delays from July 2014, aligning it with the European Union Medical Devices Directive 93/42/EEC.  The FDA has urged manufacturers to test for compliance as quickly as possible.

Power supplies are not medical devices and the Medical Device Directive cannot be documented on the CE Declaration of Conformity, even for an external power supply.  It is highly recommended that power supply manufacturers comply with IEC 60601-1-2: 2014, to avoid failures in the end equipment or system.  Most are testing and working to meet the higher levels of susceptibility, as the changes to emissions are relatively minor.

The susceptibility changes are based on the IEC 61000-4 set of standards and include:

IEC 61000-4-2 (Electrostatic Discharge):  Test levels for contact discharge increased from ±6kV to ±8kV and air discharge levels nearly doubled to ±15kV from ±8kV.  This is to cover higher levels of ESD that will occur with home use.

IEC 61000-4-3 (Radiated RF Electromagnetic Fields):  Again this is aimed at home healthcare use where the 3V/m test has been extended to 10V/m. The RF susceptibility test has been extended from 80 MHz to 2.7 GHz, because of potential proximity to wireless communication equipment, including Bluetooth and WLAN.

IEC 61000-4-4 (Electrical Fast Transients):  The pulse repetition frequency rose from 5 kHz to 100 kHz, to reflect real operating environments.

IEC 61000-4-5 (Surge Immunity) + ISO 7637-2 (Electrical transient conduction along supply lines):  Changes here were made to include permanently connected DC input devices, for applications such as ambulances.

IEC 61000-4-6 (Conducted RF Immunity):  It is here where the differentiation has been eliminated between life support and industrial, scientific and medical.  Testing has to be made at a potential risk frequency, for example where the equipment might be used in proximity with ham radios.

IEC 61000-4-8 (Power Frequency Magnetic Fields):  Test levels for power frequency magnetic fields have risen from 3 A/m to 10 A/m for all environments, but only for equipment that may be sensitive to magnetic fields, containing relays or hard disc drives for example.

IEC 61000-4-11 (Voltage Dips and Interruptions):  This is where the risk management documentation will be often used.  Although tests must now be made at multiple phase-angles (not just at 0^o and 180^o) the percentage dip in line voltage, and number of periods, have also been changed for some devices.  The 5 second interruption requirement will need to be met at the equipment level as it is highly unlikely that a standard power supply will continue to operate with the input being removed for 5 seconds.  The equipment manufacturer for a heart rate monitor could document that this will not be a problem, since battery back-up is in place.

Power supply manufacturers will qualify their products as “compliant”, and provide a test report detailing the results.  For example, for the 5 second interruption in IEC 61000-4-11, it will be stated that the power supply will shut down, and automatically recover.

Power Guy

Reducing noise on open frame power supplies

We get a lot of questions on how to reduce noise, both output & EMI (ElectroMagnetic Interference), on open frame power supplies.  Usually it is a result of a failure to ground the product correctly.  With an enclosed power supply, encased in a metal box, it is simple as all the connections are made for the user by the chassis.  Connect up the input and output wiring and everything works fine.  With an open frame (pcb type) it is a little different.

TDK-Lambda’s ZPSA open frame power supply

First a look at what we are aiming to do.  Below is a simplified diagram of the noise decoupling capacitors in a typical power supply.  The Y capacitors on the left provide a low impedance path for high frequency noise to ground.  This avoids electrical noise (EMI) exiting the power supply and interfering with other devices on the AC input.   The capacitors on the right have the same function, but in this case stops electrical noise from appearing on the output of the power supply and interfering with the load.  In some cases, just one capacitor is sufficient.

You can see two blue Y capacitors on the ZPSA photograph, close to one of the mounting holes.

Looking at the underside of the ZPSA pcb, we can see the locations of those capacitors.

The red arrow shows the Y capacitors CY2 and CY3 are connected to a common trace that leads to the bottom left mounting hole.  This hole is in fact a plated through hole and the mounting screw and standoff will make a connection with that trace.
The blue arrow shows the output to ground capacitor CY1, again connected to a copper trace leading to the bottom right mounting hole.

Note that in the case of the ZPSA, there is no pcb trace between those two holes.  (The top two mounting holes do not have any traces going to them, so we can ignore them electrically.)

Looking at our schematic again, we need a connection from chassis ground to both the input and output capacitor traces to reduce the electrical noise.  This we do by mounting the power supply on a grounded metal plate, with metal standoffs and screws.

Follow these guidelines and the open frame power supply will meet the EMI and output noise specifications.

Power Guy

Reducing Switching Power Supply Radiated & Conducted EMI

One application issue that comes across my desk on a regular basis is where a customer has gone to an outside lab to certify their equipment for EMC, and they have failed conducted or radiated noise.

Usually the power supply in question is an open frame type, which does not have the shielding that a metal enclosed power supply has.  There are two areas that are worth checking; grounding points and wire harnesses.

1. Grounding Points

Power supplies utilize decoupling capacitors; two are typically connected from input to earth ground (see below). Likewise, two are connected from the output to earth ground. This keeps the noise currents circulating close to the power supply, rather than allowing them to radiate around the end user’s system.

 

In an enclosed power supply these capacitors will be grounded through the metal case, but with an open frame type, it is up to the user to connect these points to ground.  With the power density of products today, there often is more than one point on the power supply printed circuit board that needs to be connected.  A common mistake is to only connect one, which can cause excessive radiated and conducted noise.

The installation manual will show which mounting holes / points need to be grounded.  In the product below, three mounts should be connected (A, B & C).

 

 

The photo below shows the same power supply undergoing EMC testing, and it can be noted that the unit is connected to a metal plate with metal standoffs.

 

 

A quick glance of the underside of the printed circuit board will show which mounting holes have traces that need to be grounded. This smaller model has only one grounding point at the bottom right hand side of the board.

2. Wiring harnesses

In the test photo, it can be seen that the cable harnesses are neatly dressed and are kept away from the power supply.  Wiring that is positioned above or below the unit will pick up radiated noise, thus defeating the purpose of having the EMI filter components.

If I am assisting customers on site, I always pack some tie-wraps in my tool kit to re-route any offending harnesses.

An earlier blog covers the standards http://power-topics.blogspot.com/2007/11/guide-to-emc-standards-for-power.html

 

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.