Electronic – common mode choke inductance question


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.

Best Answer

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.

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