Y capacitors downstream of common mode choke in an offline SMPS

Hello,

We have seen a report detailing that Y capacitors in an offline SMPS should be placed upstream of a common mode choke. (as in the attached schematic)

However, I always thought that it was better if they were downstream of it. If one only has two Y capacitors, then surely they should best be placed downstream of the Common mode choke, as in page 4 of the following….
Do you agree?

Page 4 shows Y capacitors downstream of common mode choke…
https://www.ti.com/lit/ug/sluu263c/sluu263c.pdf

Flyback said:

Do you agree?

Click to expand...

Not necessarily.

I guess it all depends if you want to stop noise entering the PSU, or, noise leaving the PSU.
The capacitors, in conjunction with the CM Choke will give different attenuation in each direction.

JimB