EMC Question of the Week: February 4, 2019
Leakage inductance in a common-mode choke consisting of two wires wound around the same ferrite core results in
- leakage current
- equivalent series resistance
- radiated emissions
- differential-mode inductance
Answer
The best answer is “d.” When the currents flowing on the two wires of a common-mode choke are in opposite directions (i.e., differential-mode currents), magnetic flux in the ferrite core generated by one current is canceled by the flux from the other current. As a result, differential-mode signals experience little impedance as they pass through the choke. However, it is not possible to get perfect cancelation of the flux because there is always some flux wrapping the wires that is not captured in the core. This results in leakage inductance, which is the uncoupled flux divided by the source current.
Differential signals passing through a common-mode choke always experience some level of inductance due to this leakage. Sometimes filter designers rely on a certain amount of leakage inductance to provide a small amount of differential-mode filtering.
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