Common mode choke (Common mode Choke), also known as common mode choke, is a coil that has the same number of turns in the opposite direction in a closed magnetic ring. The ideal common-mode choke has an inhibitory effect on the common-mode interference between L (or N) and E, and no inductive suppression on the differential-mode interference existing between L and N. However, the incomplete symmetry of the actual coil winding can lead to the formation of differential mode leakage inductance. When the signal current or supply current flows in the opposite direction of the two windings, the resulting magnetic flux cancels each other and the choke presents a low impedance. Common-mode noise current (including ground loop caused by harassment current, also known as the vertical current) flows through the two windings in the same direction, the magnetic flux generated in the same direction, the choke presents a high impedance, which play a total inhibition The role of mode noise.

Common mode inductor filter circuit, La and Lb is the common mode inductor. The two coils are wound on the same core with the same number of turns and phases (winding in the opposite direction). In this way, when the normal current in the circuit flows through the common-mode inductor, the current reverses the magnetic field in the inductors with the same phase and cancels each other. At this time, the normal signal current is mainly affected by the coil resistance When a common mode current flows through the coil, the same direction of the common mode magnetic field will be generated in the coil to increase the inductance of the coil due to the same direction of the common mode current, so that the coil will appear as a high impedance and generate a high impedance Strong damping effect, in order to attenuate the common-mode current, to achieve the purpose of filtering.

Common mode inductor is essentially a bi-directional filter: on the one hand to filter out the common-mode electromagnetic interference on the signal line, on the other hand but also to suppress itself does not issue electromagnetic interference to the outside to avoid affecting the normal work of other electronic devices under the same electromagnetic environment .

Common-mode choke can transmit differential mode signals. Both DC and low-frequency differential mode signals can pass through, while high-frequency common mode noise presents a large impedance. Therefore, it can be used to suppress common mode current disturbances.