I have a CMC that has is rated at about 1kR at 100Mhz at 3A

However my circuit uses only 1A. Do I need to derate my inductance since there is less current flowing through it ?


1 Answer 1


If your common mode choke is passing a regular circuit current of 1 amp then that will be a differential current i.e. it flows into one terminal on one side and returns in the opposite direction through the equivalent terminal on the other winding: -

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Under these circumstances the currents are in opposite directions and the fluxes cancel out hence there is no need to derate the inductance. In any case, inductance will only be reduced (usually) when the common mode current reaches a significant level: -

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Picture source.

  • 1
    \$\begingroup\$ So derating of inductance happens only for temperature and its independent of current through it ? \$\endgroup\$
    – Johan B.
    Commented Sep 5, 2018 at 10:11
  • \$\begingroup\$ For a pure differential current, there is no net flux (assuming the windings are identical and perfectly coupled) so inductance won't be affected by current however, in reality there will be a slight effect because the coupling is not 100% and the windings won't be identical. Temperature does affect core permeability but for a CM choke the inductance to differential signals is very small but is still affected by core permeability changes. \$\endgroup\$
    – Andy aka
    Commented Sep 5, 2018 at 11:28

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