EMC Question of the Week: April 20, 2026

A common-mode choke with bifilar windings generally has  

1. lower inter-winding capacitance
2. lower leakage inductance
3. lower resistance
4. all of the above

Answer

The best answer is “b.” In a common-mode choke with bifilar windings, the Signal+ and Signal- windings are wound together as a wire pair. This is illustrated in the right side of the figure. This winding technique reduces the amount of leakage inductance and therefore reduces the amount of impedance experienced by differential-mode signals. However, the close proximity of the primary and secondary winding tends to increase the amount of interwinding capacitance, which reduces the common-mode impedance at the higher frequencies.

For powerline filters, minimizing the interwinding capacitance is a priority while a little leakage inductance can help to filter the differential-mode noise. For this reason, common-mode chokes for powerline filtering tend to employ sectional windings as illustrated on the left side of the figure. This winding configuration has low interwinding capacitance and provides good voltage isolation. 

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