Do I need to EMC test a pre-approved power supply? – EMC Explained

One of the most common questions we get asked when we send an EMC Test Plan / quotation to our customers is along the lines of:

 

“Our equipment is powered from a pre-approved CE marked power supply so we don’t need to do any AC mains EMC testing… right?”

 

If a power supply has already been EMC tested (if it has a CE or UKCA mark you would hope that this was the case) then it is a fair question – why should we retest it?

Adding AC mains specific tests into the EMC Test Plan adds time and therefore cost, something that some of our customers would like to avoid. For smaller businesses, the cost of assessment for EMC might be one of the largest external costs incurred on a project.

The main assumption driving this question is that EMC emissions – the noise that is coming out of the power supply and either back onto the AC mains or radiated from the power supply – is the only EMC problem we have to worry about. It’s the main one, but not the only one.

The pre-approved power supply will have been tested for immunity, but only the immunity performance of the power supply itself, not the equipment that it is powering.

Some noise will get through the power supply and into the equipment being powered. How does your product respond to this noise?

Also, how low are the AC mains conducted emissions from the power supply? Have you seen a test report? How reputable is the vendor?

Testing is the most reliable way to find out.

 

Our Recommendations

We generally recommend to our customers that they perform all of the applicable tests to the product.

(What, a test lab recommending testing? I’m shocked!).

Firstly, the tests are called up in the EMC standards, and for CE/UKCA marking, testing to a Harmonised Standard gets you a “Presumption of Conformity” to the requirements of the Directives – a pass without any further Risk Assessment or justification on your part.

Deciding not to perform the testing puts the responsibility on you to assess the remaining EMC risks. If you needed us to do this assessment for you or advise on it, the cost of a few hours of consultancy time would be equivalent to just doing the tests in the first place.

Secondly, EMC performance is often dictated by parasitic capacitances and inductances, component values that are not on the datasheet or intentionally designed into the product. Even knowing their magnitude does not give a good understanding of how they will interact. Testing allows us to measure their interaction under standardized conditions.

 

 

Risk Assessment Factors

As discussed above, our recommendation is always to perform testing on applicable ports, the AC mains port included.

If you are worried about costs or time taken for testing, then you might decide to omit some of the specific tests. The below table outlines some of the factors you may wish to consider when making this decision.

The more items that apply from the Risk Increasing Factors column, the less strong your argument becomes for not carrying out testing.

 

Risk Reducing Factors Risk Increasing Factors
Class II power supply (un-earthed)

 

Class I power supply (earthed)

Especially if the DC negative of the power supply output is connected to Protective Earth in the system.

Power supply comes from reputable vendor (e.g. Meanwell, XP Power, Recom, Traco, TDK Lambda, Puls, etc) Power supply comes from cheap or from far east supplier
Power supply external to product Power supply internal to product
No analogue or sensitive circuitry Analogue circuitry e.g. audio, 0-10V I/O, 4-20mA I/O

Sensitive, low level signals e.g. thermocouple, RTD

No other long (>3m) cables connected to equipment One or more long (>3m) cables connected to equipment
Main use in Basic (residential, commercial) EM environment Flexible use, could be used in Light Industrial or Industrial EM environments

 

If you are at all unsure then you should test the AC mains port with your intended production power supply.

For the ultimate in performance, or if the equipment is for flexible use (could be powered from an AC/DC supply or from a distributed DC power supply) then we would recommend treating the DC power input to your product as a signal port with a length greater than 3m.

This would then call up Conducted RF Immunity (EN 61000-4-6) and Electrical Fast Transient (EFT, EN 61000-4-4) testing to the power port at the appropriate levels for the end EM environment (e.g. Basic or Industrial)

One step further would be to apply line-to-line and line-to-earth surges to the DC input, assuming that the design already contains a transient surge voltage suppressing element like a TVS diode or an MOV.

Let’s take a look at some of the technical justification behind the selection of these items.

 

AC Mains Port vs DC Power Port

If you typically derive your equipment power from an AC mains power supply, then it is unlikely that you will fall under the DC Power Port classification.

The term DC Power Port in EMC terms means a very specific classification of port. We discuss this in some length in this article.

 

Power supplies do not always meet the regulations

A scenario that we have experienced on several occasions: the power supplies that end up with our customers or in our test lab are not the same as the ones in the manufacturer supplied EMC test report.

 

Another customer had similar problems on  power supply that they had received samples of in that the EMC performance varied wildly. In this case the clue was that the weight of the two samples was significantly different.

 

 

These power supplies were almost identical on the outside but significantly different on the inside. Same manufacturer and model number, different components. Imagine the conversation:

“I’d like to order some HM-A132 power supplies please”

“Certainly sir, which ones?”

“Erm…”

 

This is mostly related to cheaper power supplies sourced from China. We often see significantly different results to those shown in the manufacturer test report.

The worrying thing is if changes like this are being made on the basis of EMC, what changes are being made that affect Electrical Safety that are going unchecked? We can check that for you as well.

 

Cable Routing

If your power supply is integrated into your equipment then there is the possibility of noise on the AC mains cable coupling onto other nearby cables.

It is also possible for noise to couple (both to and from) components connected to the AC mains and internal system components. This could be an emissions (noise getting out) or an immunity (noise getting in) risk.

 

This is particularly likely if you are using slotted trunking and mixing AC mains cabling in with other cables.

 

 

 

This is less important for an external power supply like a laptop type charger or a plug top power supply as the AC mains cable remains outside of the equipment enclosure.

 

 

Power Supply Common Mode Impedance

Electrical noise inevitably gets coupled onto the AC mains bus. Normally this noise is coupled onto the AC mains Common Mode. This means all the lines together in relation to a high frequency “ground” reference plane.

The noise current through the power supply and equipment will flow something like this:

The noise reaching the equipment will have been attenuated by the Common Mode impedance of the power supply and the currents diverted through the parasitic capacitance of the power supply relative to the HF ground reference plane used in the tests.

Crucially, some noise still gets through to the power supply and will flow through the product. The magnitude of this current can be estimated or measured but relies on electrical parameters that are not on the power supply datasheet.

It is this noise current that we are interested in. How does it affect your product? The only way to find out is to perform testing.

 

Class I vs Class II Power Supply CM Impedance

The construction of a typical switch mode AC/DC power supply is broadly similar across a wide range of topologies. One of the main EMC variations results from if the power supply is Class I (earthed) or Class II (unearthed).

 

Class II

A Class II power supply relies on Double or Reinforced insulation between Live parts and user accessible secondary low voltage parts for Electrical Safety. There is no connection to Protective Earth. This kind of power supply is usually identifiable by:

  • the square-in-a-square double insulation symbol (IEC 60417 symbol # 5172)
  • a plastic earth pin on a UK mains plug (technical name is an ISOD or Insulated Shutter Opening Device)
  • An IEC C8 “figure-8” AC mains inlet socket with just two pins

 

Looking at the typical internal structure of a Class II AC/DC SMPS we can see that the components providing Common Mode noise attenuation are

  • the inductive common mode filter (Lcm)
  • the components across the safety isolation barrier, transformer Tx and class Y capacitor Cy

The value of parasitic parallel capacitance of the choke or transformer (or wanted series capacitance of Cy) will reduce the impedance ( Xc = 1 / [ 2 * pi * f * C ] ) and allow more noise current to flow at higher frequencies.

This capacitance is usually a low value to prevent too high a touch / leakage current to flow which would compromise Electrical Safety.

However, at EMC frequencies of MHz and higher this presents a much lower impedance allowing noise currents to flow through the cable.

 

block diagram of a class II power supply showing EMC immunity noise current through the power supply

 

Because current always flows in a loop, and because current always returns to the source, to close this common mode current loop we need to have return currents flowing. We usually think of these coupling capacitively onto a nearby metallic element like a nearby metal structure.

In the test lab we simulate this with a nearby metal plate but in real life this could take a number of forms (building steelwork, conductive cable trays, other wiring).

 

 

Class I (Or Class II with Functional Earth) Power Supplies

With a Class I power supply, the Protective Earth is connected to accessible metalwork for Electrical Safety reasons (prevention of electric shock). Basic insulation (or higher) is required between the live parts and user accessible secondary parts.

Possibly the protective Earth is also connected to DC negative somewhere in the system as well.

A Class II with Functional Earth power supply is similar from an EMC point of view but very different from an Electrical Safety point of view. In this case, the Earth is connected for functional reasons (reducing noise or EMC emissions) but the power supply still relies on Double or Reinforced insulation for safety.

This isn’t a very common power supply topology choice, so I was surprised to see it marked on my laptop charger power supply.

 

 

In both cases, when we apply common mode noise to the AC mains input (L+N+E) then the Protective Earth conductor allows the noise to bypass the common mode impedance of the power supply. It is for this reason that we view the use of a Class I earthed power supply as a higher EMC risk for immunity reasons.

 

block diagram of a class I power supply showing EMC immunity noise current through the power supply

 

 

How this noise couples into the rest of the equipment, its magnitude, and how it affects it depends massively on the construction of the equipment. Again, testing is the best way to determine this.

 

Conclusion

Power supplies and the equipment they power are not perfect and can have varying EMC performance depending on how you connect them and how the equipment is designed.

It isn’t always easy to estimate how likely EMC issues are, even for experienced engineers and problem like us at Unit 3 Compliance. It is for this reason that we would always recommend testing to characterize the unknown EMC performance.

If you do decide to omit some testing, then the Risk Reducing or Increasing Factors above should help with that decision.

Again, we hope that this guide was useful to you in some way. Get in touch with us if you have any thoughts, questions, observations, or (obviously) a need for EMC or Electrical Safety testing.

All the best!