Common-Mode Choke (EMC Filter Inductor)

An inductive component in which two (or more) conductors, typically line and neutral or all phases together, are wound in the same direction around a single common ferrite core, so that the magnetic fields of the normal, oppositely directed working current (differential mode) largely cancel out in the core and hardly saturate it for that current, while interference currents flowing in the same direction through all conductors (common mode, for example high-frequency noise leaking to earth via parasitic capacitances) do experience a large combined magnetic field and hence a high impedance. This distinguishes the common-mode choke from an ordinary (differential-mode) choke, which is instead intended to affect the normal working current, for example to damp switching ripple. Common-mode chokes are widely used at the input of variable-frequency drives, switch-mode power supplies and PV inverters to prevent high-frequency common-mode noise from being fed back onto the mains via the supply cables, which is needed to meet the EMC emission requirements of NEN-EN 61000-6-3, and at the same time help limit unwanted high-frequency leakage currents that could otherwise cause an RCD to trip unnecessarily.