Y2 capacitors downstream of common mode choke in offline SMPS

[ocset]

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…
http://www.ti.com/lit/ug/sluu263c/sluu263c.pdf

[floobydust]

It depends on which direction the EMI is going, through a filter.

To attenuate mains EMI from entering a piece of equipment, Y caps after CM choke.
To attenuate SMPS EMI from leaving a piece of equipment, Y caps before CM choke.

[T3sl4co1l]

Generalizations are utterly meaningless in EMC.  All that can be analyzed is each special case.

Try it both ways and see which one works best.  During layout, place both footprints.

Tim

[ocset]

To attenuate mains EMI from entering a piece of equipment, Y caps after CM choke.
To attenuate SMPS EMI from leaving a piece of equipment, Y caps before CM choke.

so to pass an emc test we need them upstream
..to pass an immunity test we need them downstream
would you agree?