Offline Common mode choke winding

cupoftea said:

Hi,
Would you agree, its most usual for custom wound common mode chokes to be deliberately wound with different number of turns on each "limb"?...ie to provide both common mode and diff mode inductance, from the single part?

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No, Cores used for common mode choke usually saturate rather easily so generally arn't suitable for differential filtering, they typically use equal windings so the fields cancel each other out and prevent saturation. Typically a leakage inductance exists between the two windings which is usually speciified at the differential element. Some Manufacturers wind in particular manner to maximise leakage inductance to provide improved differential filteriing improvement on the same core but in this case the core is selected specifically. Generally as leakage inductance is usually small then it doesn't really factor until reasonably high frequencies so doesn't typically help with relatively low switching frequencies.. This is why filters are often made through both differential and common mode filtering