Location of Y capacitors when common mode choke used in SMPS.

[cupoftea]

Hi,
Say in a DCDC SMPS with an input common mode choke....do you agree, the Y capacitors must be connected NOT on the input supply side of the common mode choke?
After all, the noise that needs filtering comes from the switching power stage.
The idea is that the common mode choke offers high Z to the common mode, and so the common mode emissions need that Y capacitor path to channel them back into the Rail and ground nets.......its no good putting y caps upstream of the input common mode choke, because the common mode emissions wont actually get there in much "quantity" anyway. (due to the common mode choke)......Y capacitors must always go downstream of an input common mode choke......would you agree?

 

[std_match]

Murata agrees with you:

https://www.murata.com/~/media/webrenewal/products/emc/emifil/knowhow/26to30.ashx

 

[mtwieg]

I agree in the sense that if you had to choose between input and output side, you should always choose output side, so long as the impedance of the CMC is larger than the common mode impedance of the AC mains (which should always be the case).

But putting Y caps on the input side isn't some sort of disaster, just not as effective.

Oh and the above applies only mitigating emissions from the DUT. For susceptibility/immunity, the opposite may be true! But immunity is much more dependent on the specifics of the DUT, so hard to generalize here...