Common mode noise mitigation in offline power supplies

In offline power supplies, Common mode emissions are best dealt with by using common mode chokes in conjunction with Y capacitors.

However, do you agree that the “poor man’s” common mode noise reduction technique of using uncoupled inductors in both the live and neutral lines is also a somewhat worthy technique for mitigating common mode emissions? The idea is that the frequency of the noise in the live and neutral is reduced by the uncoupled inductances and goes below the 150khz level which is less penalised by EMC tests….so in other words, the overall common mode noise overall is not necessarily reduced in energy, but is reduced in frequency to levels where it is less penalised by the EMC regulations….do you agree with this “poor man’s” technique?

Also, when a company takes a product to an EMC test house for an investigation scan for conducted emissions….why do the test houses always present a “mixed” result graph with common mode and differential mode noise ‘lumped’ together into the same graph? It is , after all, very easy for them to use a splitter in the LISN and give separate graphs showing either differential mode emissions or common mode emissions on separate graphs……so why do the test houses not do this? A “Mixed” scan result graph, with common mode and differential mode levels “lumped” together is pretty useless, as the customer will not know whether common mode or differential mode filtering is needed and to what degree in each case.

Some EMC test houses actually give separate EMC conducted emissions result graphs with Live and Neutral lines shown on separate graphs. It is said that if the Live and Neutral graphs are the same then there are no common mode emissions, but this is not true…because if there are uncoupled inductances in both live and neutral then there could still be loads of common mode noise in there even if the live and neutral scan graphs look the same…do you agree?

Do you agree that it is strange that the regulatory limits for conducted EMC into an offline power supply concern both common mode and differential mode emissions taken together?…in other words, your noise can be either all_common_mode, or all_differential_mode, or any mixture of the two, as long as the overall level is below the limit line. Why are there not limits for common mode and differential mode noise separately?

Another point is common mode emissions…..these are caused by very high frequency ringing in the range above above 30MHz……..Though 30MHz is above the actual maximum limit of the conducted EMC test. So the lay person would think that common mode emissions would not manifest themselves on the conducted emissions graph, but rather on the radiated emissions graph instead. However, this is not so, common mode conducted noise problems can and do occur right down to 150kHz. This is due to the fact that the mains bridge rectifier can result in the low frequency manifestation of the common mode noise…also, the effect of any differential mode inductors can push the common mode noise manifestation down to as low as 150kHz.
Do you agree?

Hello,
In the attached AC filter we can see that L1 is a common mode choke.
We are trying to reduce the component count of this filter. Do you agree that we can get away with omitting the Cy1 capacitors , but that we must keep the Cy2 capacitors?
In fact, why would the Cy1 capacitors be needed at all?.....i mean, any noise that has escaped out out to earth, would always want to take the least resistance path, and that would definitely be through the Cy2 capacitors….because then the noise would be going back through the common mode choke and thus would not see the high common mode impedance of the common mode choke.
So why are the Cy1 capacitors included? If anything the Cy1 capacitors should just be used as extra capacitors at the same position as the Cy2 capacitors. Do you agree?

treez said:

So why are the Cy1 capacitors included?

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Cy1 are used as well in the 2nd order common mode filter for noise exiting the device and hence entering the mains. If you remove them, you will not have a 2nd order filter for common mode noise exiting the device.

If anything the Cy1 capacitors should just be used as extra capacitors at the same position as the Cy2 capacitors. Do you agree?

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If you want to increase the common mode filter for the noise entering the device, would be fine but then you would lack the advantage explained above.

From the schematic you are showing seems like the noise entering the device is not a concern for you since you are lacking a 2nd order differential mode filter at the very begining of the mains (only have the Cx1 cap)

From the schematic you are showing seems like the noise entering the device is not a concern for you since you are lacking a differential mode filter at the very begining of the mains.

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Thanks, the differential mode filter is from L2, L3 and the capacitors which are not shown but are downstream of this bit of the filter

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Cy1 are used as well in the 2nd order common mode filter for noise exiting the device and hence entering the mains. If you remove them, you will not have a 2nd order filter for common mode noise exiting the device.

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Sorry, but surely the Cy1 capacitors do not help to form a 2nd order filter for common mode noise?
..Imagine noise coming in to the product via the live line…..then going through the live half of the common mode choke, and then going through L2, then going through the diode bridge, then getting radiated out of the product by some means…then coupling to earth…..then going back to the neutral via the Cy1 capacitor (ie missing out the neutral half of the common mode choke)…….by doing this it would experience a high impedance because of the common mode choke, so surely it would simply just use the Cy2 route instead?...and go through the neutral half of the common mode choke where it would experience a low impedance path.

but surely the Cy1 capacitors do not help to form a 2nd order filter for common mode noise?

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Yes, they do help.

Cy1 help to filter the noise coming from the mains by giving a low impedance path as well as noise from exiting the device and going to the mains by forming a 2nd order filter with the common mode choke.

Cy1 and the common mode choke + Cy2 filter the common mode noise coming from the mains. CMC + Cy2 form a 2nd order filter for noise going from the mains to the device.
Cy2 and the common mode choke + Cy1 filter the common mode noise coming from the device. CMC + Cy1 form a 2nd order filter for noise going from the device to the mains.

That is why this structure is very symmetrical and another differential mode chokes to the mains input would make it even more symmetrical.

As you know, Common mode noise returns to earth. (as you know, it obviously came in to the product via live or neutral)
If noise returns via the mains live or neutral than its not common mode noise. Any noise going through any of the y capacitors will end up returning via live or neutral, and so it will not be common mode noise.
True common mode noise, which returns to earth does not get “LC filtered”. Common mode noise couples out of the product, and then returns to earth, probably outside the product….it certainly does not traverse the y capacitors, because if it did, then it would be going back in to live or neutral, which means it would not be common mode noise any more.
Common mode noise is reduced by having a common mode choke…..the common mode choke “forces” noise which has coupled out of the product to come back into the live or neutral wires, via the common mode chokes other half……so it makes the noise into differential noise, which gets acted on by the differential filter.
So, Y capacitors do not filter common mode noise……Y capacitors actually transport noise back into the mains live or neutral from earth…..making it “differential mode noise”.

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This is why I question why Cy1 is needed. Any noise which has coupled out of the product that will return back into live or neutral would choose the Cy2 route…because that takes it back through the other half of the common mode choke…where it sees the inductance of the common mode choke cancelled out…..if the noise were to return via Cy1, then it would miss out the other half of the common mode choke, and it would see a very high inductance, so therefore it would certainly not take that route…electricity generally takes the lowest impedance route, which in this case means returning back to mains via Cy2 and the other half of the common mode choke.
So why is Cy1 needed?

Continuing from my above comments on common mode noise……given the attached EMC scan result of a product which contains no AC mains filter, it is actually impossible to exactly calculate what combination of differential and common mode filtering will be needed to attenuate this noise down to below the pass/fail threshold.
The product of this scan is actually a 150W non-SMPS based lighting product……..but it does contain a 0.5W SMPS for bias power.
The only way you can approach this AC filter design (for the attached EMC scan result), is to make a Differential mode LC filter which attenuates by 35dB at 150kHz (that brings it below the line). You then have to more-or-less guess a common mode choke value, and guess accompanying Y capacitor values, and hope that that will convert most of the noise of the graph to differential mode noise….for which you can design a filter with known attenuation.
The problem with these conducted EMC scan result graphs, is that there is no way of knowing exactly how much of the noise shown is common mode and how much is differential mode.
It is thus impossible to exactly calculate filter components…you must simply design by emperical means (try it and see).
I have it on good authority that when returning to work this week, I will be sacked with immediate effect, due to my having first designed a pure diff mode only filter which attenuated down by 35dB at 150kHz. (but this did not solve it thereby I am due the sacking) The reason I did this initial pure diff mode design was because there is really not that much room for a common mode choke, so I needed to see how a pure diff mode only filter would perform. –Turns out that it doesn’t really attenuate much of what you see at all…which tells that most of the noise is actually common mode noise. If I was not going to be sacked, then I would put together a common mode/Diff Mode filter and try that out. However, the sack beckons, and I will be gone. Anyone fancy a job?

Do we all at least confess that it is not possible to actually calculate EMC filter component values for the above conducted EMC scan?, ....since it contains a mixture of common mode and differential mode noise in unknown proportions.

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Look at the "number of views" for this post...it has already surpassed 363 even though it was only posted a few hours ago....this is nearly an edaboard record, and shows how common mode noise filtering is such a mystery worldwide, that all are desperate to gather more on this area of massive industrial secrecy.
To be sacked (as i will be) for this is a worthy sacking.
I should have realised from that start that it was a common mode problem and would need a trial_and _error solution. Hindsight is wonderful.

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This is related to the above..

Page 17 (RHS of page) of the following document…
**broken link removed**

….states the following about EMI filter design for offline power supplies….

Physical component layout becomes increasingly critical above
1 MHz. Improper layout can lead to increased capacitor ESL.
It is also possible for noise voltages or currents to couple around
the EMI filter directly into the mains.

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….The wavelength at 1MHz is 300 metres, is it really the case that noise at around 1MHz could “couple around the EMI filter” and get straight back into the mains?

You then have to more-or-less guess a common mode choke value, and guess accompanying Y capacitor values, and hope that that will convert most of the noise of the graph to differential mode noise….for which you can design a filter with known attenuation.

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No.

The problem with these conducted EMC scan result graphs, is that there is no way of knowing exactly how much of the noise shown is common mode and how much is differential mode.

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Why dont you conduct CM test and DM test in order to get that?

It is thus impossible to exactly calculate filter components…you must simply design by emperical means (try it and see).

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Wrong. Obviously not exactly calculate, but not try and see either.

Do we all at least confess that it is not possible to actually calculate EMC filter component values for the above conducted EMC scan?, ....since it contains a mixture of common mode and differential mode noise in unknown proportions.

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Yes.

shows how common mode noise filtering is such a mystery worldwide

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I do not think so.

Please take a look at **broken link removed** so that you can design the filter properly without trial and fail (which is not engineering). Add some safety margins as well (i.e. some attenuation headroom).

Why dont you conduct CM test and DM test in order to get that?

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We asked them for it but they said no.

Measuring CM or DM mains EMI is not difficult.
Some LISN's already output both, but if not you can use a wideband-transformer (splitter) to separate the signals.
Or a resisitive summation LISN MATE

If that is too complicated, then use the EMC current-probe with a cancellation-loop.

If you don't have a LISN, spectrum analyser, clamp on probe etc then you are basically doomed as an engineer doing EMC design.
Then all you can do is get the EMC facility to test and you guess at a fix and re-submit, until management learns to empower their engineers, and that regulatory is much more than a formality.

Please take a look at Practical EMI filter design from IEEE so that you can design the filter properly without trial and fail

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Thanks, though we dont have an account with IEEE, it costs a lot of money we find, are their bargain memberships ever available?.

treez said:

True common mode noise, which returns to earth does not get “LC filtered”. Common mode noise couples out of the product, and then returns to earth, probably outside the product….it certainly does not traverse the y capacitors, because if it did, then it would be going back in to live or neutral, which means it would not be common mode noise any more.
Common mode noise is reduced by having a common mode choke…..the common mode choke “forces” noise which has coupled out of the product to come back into the live or neutral wires, via the common mode chokes other half……so it makes the noise into differential noise, which gets acted on by the differential filter.
So, Y capacitors do not filter common mode noise……Y capacitors actually transport noise back into the mains live or neutral from earth…..making it “differential mode noise”.

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This is totally wrong and you need to re-examine the filter circuit posted above. Y caps filter common mode noise because they are connected to earth ground. Common mode chokes do not act like baluns to convert common mode signals to differential signals. They block common mode noise.

prairiedog said:

Then all you can do is get the EMC facility to test and you guess at a fix and re-submit, until management learns to empower their engineers, and that regulatory is much more than a formality.

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You would be surprised -or perhaps not- at how many companies do not understand that;
they believe that regulatory compliance is a nuisance (regulatory red-tape, in corporate-speak) that prevents them from introducing their wonderful products quickly and cheaply to the market. And thus produce wonderful returns on their stock price.

This mentality pervades many industries, not only electronics. Look how many automotive companies have been caught cheating the fuel efficiency, the tailpipe emissions, etc.

Common mode chokes do not act like baluns to convert common mode signals to differential signals. They block common mode noise.

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Thanks, you use the word "block", but also i woudl say that common mode chokes offer high inductance to common mode currents, and low inductance to diff mode ones.

Its going to be the case that noise thats coupled out of the product, that then comes near a y capacitor, is likely to couple through that y capacitor and end up going through the other half of the common mode choke.
This is very likely because that is a low impedance path for it.
The common mode choke thus can be said, effectively , to force noise back down the neutral wire, instead of that noise coupling away to earth.

Are you saying that it is not possible for noise to come in to a product via the live wire, then couple out of the product, then couple back to the neutral wire through a y capacitor to neutral, then go through the second half of the common mode choke. Are you saying that that sequence of events is not possible? If you disagree with my above then surely that is what you are saying?

schmitt trigger said:

You would be surprised -or perhaps not- at how many companies do not understand that;
they believe that regulatory compliance is a nuisance (regulatory red-tape, in corporate-speak) that prevents them from introducing their wonderful products quickly and cheaply to the market. And thus produce wonderful returns on their stock price.
This mentality pervades many industries, not only electronics. Look how many automotive companies have been caught cheating the fuel efficiency, the tailpipe emissions, etc.

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The problem is the engineers are the ones "thrown under the bus".
There is an expectation they have divine knowledge and a perfect design- despite being rushed, lack of test equipment and training, unrealistic product requirements etc.

I've seen many companies deploy unsafe product or "cheat" regulatory just to get the product to market.
The management paradigm of "push" to get what you want, push your people to get them to be more productive- it's all something the great apes do.

Here, you can try learn EMC in the middle of this storm but treez is worried about being sacked.

Common mode chokes do not act like baluns to convert common mode signals to differential signals. They block common mode noise.

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Thanks, i agree that they dont convert common mode noise to differential mode noise.......they encourage the noise that couples out of the product to end up being more diff mode than common mode.

I do now agree that Y capcitors will LC filter common mode noise....the L being from the common mode choke...the thing is, noise that couples out of the product can couple back through a y capacitor and go to neutral and go through the second half of the common nmode choke.....and then it would go back via neutral, and it would be deemed diff mode noise.

We can argue our approach to getting an intuitive feel and explanation for EMC.
I worked with an engineer who had all the wrong theories about EMC and he ended up spending over $300,000 in the EMC lab farting around for a year and a half. In the end the product passed but had little capacitors everywhere, lol.

Don't forget on the other end of the common-mode emissions is the LED string and product enclosure where (stray) capacitive coupling to earth is part of the loop. The invisible Y-cap on the secondary side.
I'm assuming the EMC lab has a reasonably accurate setup for the PSU testing.

treez said:

Thanks, you use the word "block", but also i woudl say that common mode chokes offer high inductance to common mode currents, and low inductance to diff mode ones.

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Correct.

Its going to be the case that noise thats coupled out of the product, that then comes near a y capacitor, is likely to couple through that y capacitor and end up going through the other half of the common mode choke.
This is very likely because that is a low impedance path for it.
The common mode choke thus can be said, effectively , to force noise back down the neutral wire, instead of that noise coupling away to earth.

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What? No.

treez said:

Thanks, i agree that they dont convert common mode noise to differential mode noise.......they encourage the noise that couples out of the product to end up being more diff mode than common mode.

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No. So long as the filter is built symmetrically around earth potential (which is how it should be built), then common mode and differential signals are totally isolated from each other, and thus the filter can be analyzed as having different transfer functions for common mode an differential mode signals. It won't cause one mode to transform into the other.