In a common mode choke, my understanding is that common mode currents will produce fluxes that add together, which increases the effective inductance between them and so attenuating these currents.

When differential currents are flowing through the common mode choke, the fluxes will flow in opposite directions, so that there is no coupling and the individual fluxes will cancel for a net 0 flux, effectively making the inductance appear very low if at all.

So when I am looking at specs for common mode chokes with an inductance specification, does this rating pertain to common mode signals only? That is my guess, but I'm still trying to get a better understanding.


1 Answer 1


Your assumptions are spot on.

Another thing to bear in mind is that any leakage inductance between the common-mode windings manifests itself as a parasitic differential mode inductor - this is why it's common to see toroidal common-mode chokes with the windings opposite each other (not interleaved) to maximize leakage inductance and get that 'free' differential mode filtering.

enter image description here

  • \$\begingroup\$ Thank you for the reply. I have a consideration I forgot to think about, and that is with a CM choke each winding by itself will have an inductance with the core independent of the other winding, and so is this inductance just ignored and assumed to be not applicable to the what its designed for and so not included in the datasheet? \$\endgroup\$ May 24, 2013 at 15:41
  • \$\begingroup\$ The datasheet inductance will be (generally) for one winding only. In the application, you get both windings' inductances providing your CM filtering. \$\endgroup\$ May 24, 2013 at 15:43
  • \$\begingroup\$ Actually, I think having separated windings like that has very little to do with "deliberate" differential-mode inductance, and very much to do with achieving the necessary winding-to-winding voltage standoff capability, such as required in mains line filtering. Note the spacer that's glued in between the windings in your photo. \$\endgroup\$
    – Dave Tweed
    May 24, 2013 at 15:50
  • \$\begingroup\$ Many power supplies don't have discrete DM inductors - the CM ones are made deliberately leaky enough to provide the DM filtering. Even in applications where isolation isn't required, like a telecom DC input converter, similar construction is often used (minus the spacer). \$\endgroup\$ May 24, 2013 at 17:08

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