Where to best out Y caps in offline flyback

[cupoftea]

Hi,
Customer sent us offline flybacj which fails EMC.
85-265,24VOUT, 66KHz, 20W
It has no Y caps, just CM choke.
Its in an earthed metal enclosure.
Served by L/N/E there is an earthing screw restring to which we will connect one of the Y caps.

We have only room for 2 Y caps....we can get one across the transformer.

The other one we can connect to primary HVDC or one downstream terminal of the CMC....we cannot connect to the other terminal of the CMC, as no room.

So where would you best connect this Y cap?...from Earth to HVDC, or from Earth to downstream terminal of CM Choke?

If i could connect a y cap to each downstream terminal of the CM Choke, then i would deffo do that, but i cannot as no room......so given that the CM filter will be unbalanced if i connect the 1 Y cap to the CM choke node, then where would you prefer to put the 1 Y cap?

 

[Rahul_d]

Are you sure that you are failing common mode EMC test & not differential mode EMC?
If you are sure that you are failing due to common mode noise, Y caps will help. Please note adding Y caps will also increase your chassis leakage current
& make sure you are meeting specifications for leakage current (specially if it is medical use)

If you add y cap across transformer you will have to test the power supply with system for line surge as you are allowing surge current to come to secondary side through this capacitor.

adding Y caps (one from line to chassis & other from nbeutral to chassis) to bridge rectifier side (on AC side & not on dc side) should help.

you still have to test EMI for effective location of Y caps depending on noise source.

for differential noise Y caps will not help.