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When do you use high value Y capacitors?

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cupoftea

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Hi,
What are the SMPS's , where when designing them, you would use high value Y capacitors (say four 100nF Y capacitors).
....And simply not use any common mode choke at all.
(The Y capacitors being connected from chassis metal to circuit ground)

Why would you ever do this?
(Apologies, i searched but cant find docs on high value Y cap EMC performance, when no common mode choke is used)
 

Thanks, sorry i should have said, this converter feeds off isolated DC input of 5V..and gives out 24V at 90W. As such, we are not in a position to be able to inject leakage current back into the mains...if thats what was meant?
 

I think this should be better understood from the aspect of AC impedance. When using a larger Y capacitor, EMC is better based on the input common mode inductance. If there is no input common mode inductance, its impedance is still not small enough. Therefore, using a large capacity Y capacitor can only use a smaller input common mode inductance, but not cancel the inductance.
 
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