Y2 capacitors downstream of common mode choke in offline SMPS

• Y2 capacitors downstream of common mode choke in offline SMPS
  Posted by ocset on 29 Jul, 2018 15:03
• 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
  

  ---

  Y2 capacitors in offline SMPS.pdf
• #1 Reply
  Posted by floobydust on 29 Jul, 2018 20:41
• 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.
• #2 Reply
  Posted by T3sl4co1l on 30 Jul, 2018 00:24
• 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
• #3 Reply
  Posted by ocset on 30 Jul, 2018 05:06
• Quote

  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?