Common mode filtering of multiple supplies with common ground

Question

EMC question this day… when I have a circuit with some conducted noise on the supply line (maybe from a switcher, fast logic and so on) usually the solution is a common mode choke. One line for the ground, the other for the supply.

However, what about two or more supply lines with a common ground return? The current is not balanced anymore between a supply and the ground. Also being unbalanced the current compensation of the choke is not so effective anymore.

What is the correct solution in this case? given that's a DC supply and not a differential data line (i.e. no care for the differential signal content) I was thinking about a more traditional balanced LCL filter like the one typically used for harmonic suppression in RF:

^{simulate this circuit – Schematic created using CircuitLab}

Inductors would be rated for resonant frequency and saturation current but should work.

Is there a better solution? There are multiple (three or four) winding filter but they are quite rare and usually small (I think they are designed for audio)

Answer 1

The Common-mode-noise will be still common to all the supplies and their common ground. The capacitors in your schematic don't make a lot of sense because the noise is already common to all lines.

To effectively get rid of it, do as with all such noises (in order or priority):

1. return the responsible currents to their sources. I.e. decouple the switcher outputs (and output ground if its isolated) with caps of ~1 nF in a shorttest-possible loop to the input of the switcher. That way you keep the common-mode noise from spreading in the first place.

2. If the latter is not good enough, before looking at dissipative suppression using CMC, decouple the entire thing with ~nF caps to the RF-reference (usually the chassis or mains earth).

3. Only then, if the common-mode noise is still to large, you can use dissipative noise suppression as suggested in your post. There are the multiwinding common-mode chokes. You can get them custom-made if the right ones are not available. At quantities, that might be even worth it.

4. But also consider that the Common-Mode-Choke is just a special case of the Ferrite Bead, which makes use of DC current cancellation by running the return current through the same magnetic circuit. If this return current cancellation cannot be used, a ferrite bead is always possible. They will be much larger for the same current rating and impedance though.

Answer 2

However, what about two or more supply lines with a common ground return? The current is not balanced anymore between a supply and the ground.

Why do you think these products are useful: -

Image from here.

Any common mode current is reduced due to the increased impedance brought about by the ferrite clamp. Whereas the normal load currents are still balanced to zero and are unaffected by the presence of the clamp.

I was thinking about a more traditional balanced LCL filter like the one typically used for harmonic suppression in RF

Sure, you can use these (horses for courses) but, I just wanted to dispel your thoughts about being unable to use a CM choke.

Is there a better solution?

There's always a better solution but, without a design aim in terms of numbers, frequencies, amount of attenuation vs spectrum it's guesswork.