The common-mode choke, where two coils are wound on a single core, is useful for suppression of electromagnetic interference (EMI) and radio frequency interference (RFI) from power supplylines and for prevention of malfunctioning of power electronics device. It passes differential currents (equal but opposite), while blocking common-mode currents. The magnetic flux produced by differential-mode (DM) currents in the core tend to cancel each other out since the windings are negative coupled. Thus, the choke presents little inductance or impedance to DM currents. Normally this also means that the core will not saturate for large DM currents and the maximum current rating is instead determined by the heating effect of the winding resistance. The CM currents, however, see a high impedance because of the combined inductance of the positive coupled windings.
CM chokes are commonly used in industrial, electrical and telecommunications applications to remove or decrease noise and related electromagnetic interference.