​Introduction and application of common mode inductors

Introduction and application of common mode inductors

Common mode Choke, also known as common-mode Choke, is a coil that is symmetrically wound on a closed magnetic ring in the opposite direction with the same number of turns. An ideal common-mode choke can suppress the common-mode interference between L (or N) and E, but has no inductance inhibition effect on the differential mode interference between L and N. But the incomplete symmetry of actual winding will lead to the generation of differential mode leakage inductance. The signal current or the supply current flows in opposite directions in the two windings, resulting in a magnetic flux that cancels each other and a low impedance in the choke. Common-mode noise current (including disturbance current caused by ground loop, also known as longitudinal current) flows through the two windings in the same direction, the magnetic flux generated is added in the same direction, the choke presents high impedance, thus playing the role of suppressing common-mode noise.

Common-mode inductor is essentially a two-way filter: on the one hand, it must filter out the common-mode electromagnetic interference on the signal line, on the other hand, it must suppress the electromagnetic interference that it does not emit to the outside, so as to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment.

Common-mode choke can transmit differential mode signals, both DC and very low frequency differential mode signals can pass through, while for high-frequency common-mode noise, it presents a large impedance, playing the role of an impedance, so it can be used to suppress common-mode current harassment.

Common mode choke operating principle and insertion loss characteristics (or impedance characteristics) :

1. Working principle:

Common mode choke coil is an important part of switching power supply, frequency converter, UPS power supply and other equipment. Its working principle: when the working current flows through two coils in opposite directions, two mutually offset magnetic fields H1 and H2 are generated. At this time, the working current is mainly affected by the ohm resistance of the coil and the damping of the small leakage inductor at a negligible working frequency. If there is interference signal flow through the coil, the coil will present a high impedance, produce a strong damping effect, to attenuate the interference signal.

2. Insertion loss characteristics:

The insertion loss characteristics of a common-mode choke are measured by its impedance characteristics in the interference spectrum.

When the frequency range is 0.01~1MHZ, the impedance depends mainly on the coil inductance L.

When the frequency range is 1~10MHZ, the impedance mainly depends on the distributed capacitance of the winding CK.

When the frequency range is > At 10MHZ, the impedance is related to the parallel circuit consisting of winding capacitance, inductance in the main circuit, leakage inductance and iron and copper losses in the magnetic core (ZS is the equivalent impedance).

Leakage inductance and differential inductance

For the ideal inductance model, all the flux is concentrated in the center of the coil when it is wound. But usually the toroidal coil will not be fully wound, or winding is not tight, which will cause leakage of magnetic flux. A common-mode inductor has two windings with a considerable gap between them, which produces flux leakage and forms a different-mode inductor. Therefore, common mode inductors generally have a certain attenuation ability of differential mode interference.

In filter design, we can also make use of leakage inductance. For example, in the ordinary filter, only one common-mode inductor is installed, and the leakage inductance of common-mode inductor is used to produce an appropriate amount of differential mode inductor to inhibit the differential mode current. Sometimes, it is necessary to increase the leakage inductance of common-mode choke to improve the differential inductance, so as to achieve better filtering effect.