Can a CM choke be used to suppress common mode current in a DC circuit?
I assume you refer to an EMC problem, high frequency currents.

A common-mode filter has no C1, that's for differential mode.

EMI is always solved by going way back to the basics. In a circuit as simple as yours, there is no common-mode current possible unless you consider the effect of the outside world, and antennas.

You think you have this:






simulate this circuit &ndash; Schematic created using CircuitLab

But in fact you have this:







simulate this circuit
The parasitic capacitance(s), ground connections, and even worse, antenna effects, are the only way it can generate common-mode currents.

Now you can start to think about what would help. Inductance in series definitely helps. Capacitance... but to what? Any asymmetry in the circuit will convert differential mode to common mode currents. Best to suppress the DM currents first, with series inductors (only on the +V side) and parallel capacitors.

If you must, connect the sense pin of the regulator to the far side of the filter, but it's unlikely that the regulator bandwidth is anywhere near the sort of frequencies you are failing on. So rather let the regulator do its job, and then filter and suppress those high frequencies. The filter impedance at high frequency is very low anyway, so the regulation will be good there.

So to answer your questions directly:


You can use the CM filter you have, as it has some series inductance
the second circuit looks like a bad idea, you don't want to have anything inside the buck converter's architecture. High frequency currents there!
Your third circuit looks a bit scary too, you need the inductance in series
You need to think about where you can put the parallel capacitors too.