What is the Max value of Y cap (or what is max leakage current) should be select for EMI power Line Filter. understand increasing Y cap value will help to reduce Common mode noise but it will also increase the leakage current.

My product is Industrial grade, CISPR 11 CLASS A.


  • \$\begingroup\$ How big a Y capacitor have you found that is suitable for your AC supply? \$\endgroup\$
    – Andy aka
    Sep 20, 2015 at 12:21
  • \$\begingroup\$ 33nf Y capacitor \$\endgroup\$
    – Electra
    Sep 21, 2015 at 13:19

1 Answer 1


First you have to know the leakage current limit for your class of equipment. For IEC60950 IT equipment it is 250uA. Then the leakage current will be given by: Cy = (Il/ ωV) – Ciw where Cy is the Y-capacitor value, Ciw is the interwinding capacitance of your transformer (specified or measured) ω is the line frequency and V is the line voltage.

Of course you need to build in margin for all the tolerances involved (check temperature variation as well) and you need to measure the leakage current to be sure the calculation is correct.


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