Electronic – Common mode choke in circuit with multiple ground connections

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 – 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.

You've asked quite a few questions. Let me try to answer them in the order you asked them.

1) "What I read about common mode noise is that it is a noise generated on all signal lines and is common to all lines. How can that be?" You should re-read that again as your first link specifically states that it is in a common direction on all the "signal lines." Basically what that means is that the lines where you care about noise have the same noise present on them, and the noise is in the same phase with the noise on the other lines.

2)"Now, my doubt being, how is the common mode noise generated ? What components generate common mode noise ?" CM noise is generated by anything with a varying signal, which includes noise, that is close enough to couple that signal into the line in phase with each other. That sounds complicated but lets break it down a bit more so that it's simpler to understand. Everything has some variation to it, be it noise or signal or anything else. These variations are called AC waves. These can couple to other lines either through capacitive coupling or through inductive coupling. You have two line that you care about. If the AC on the line near by the two you care about couples in it's outside AC onto your signal in phase (going the same direction) on both lines then that is a source of common mode noise.

3)"Now, in the link1 document it says power sources generate common mode noise. Is this applicable to both ac and dc power ?" Yes. Even DC power has some AC components to it, and all AC can be considered a noise source. Any noise source can be coupled commonly into any other signal.

4)"This means We connect one end to Vcc and gnd and then the other side to a dc rail and GND ? Is that it ? So that choke will have a current rating sufficing enough for the load ?" This is where you start getting a little fuzzy. The picture you included has a great example of how to hook up a common mode choke. The Vcc should pass through one inductor of the choke, and the GND should pass through the other inductor of the choke (Don't short them together). This also means that YES, the choke must have a current rating high enough for the power source's supplied current. Your understanding is correct.