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What is a common mode inductor and what is its function?

2021年10月12日 already existingHuman browsing
Common mode inductor, also known as common mode inductor, is a coil that bypasses the closed magnetic ring in the symmetrical direction with the same number of turns. The ideal common mode choke can restrain the common mode interference between L (or n) and E, and whether it can restrain the differential mode interference between L and n. However, the incomplete symmetry in the actual coil winding will lead to differential leakage inductance. The direction of signal current or power current in the two windings is opposite, the generated magnetic flux offsets each other, and the wire coil has low resistance. When the common mode noise current (including the disturbance current caused by the ground ring, also known as the longitudinal current) passes through the two windings, the direction is the same, the generated magnetic flux increases in the same direction, and the choke impedance is high, which plays the role of suppressing the common mode noise.

In essence, the common mode choke is a bidirectional filter: on the one hand, it should filter the common mode choke on the signal line; on the other hand, it should suppress the electromagnetic interference it does not emit outward, so as to avoid affecting the normal operation of other electronic equipment in the same electromagnetic environment.
Common mode choke can transmit differential mode signal, DC differential mode signal and low-frequency differential mode signal can pass through, while high-frequency common mode noise has large impedance and acts as an impedance, so it can be used to suppress the disturbance of common mode current.
Working principle and insertion loss characteristics (or impedance characteristics) of common mode choke:
1. Working principle:
Common mode inductance resistance coil is an important part of switching power supply, frequency converter, ups and other power supplies. Its working principle is that when the working current flows through two opposite coils, two mutually canceling magnetic fields H1 and H2 are generated. At this time, the working current is mainly damped by the coil ohmic resistance, but the working frequency can be ignored. If an interference signal flows through the coil, the coil will have high impedance, produce strong damping effect and attenuate the interference signal.
2. Insertion loss characteristics:
The insertion loss characteristics of common mode choke are measured by the impedance characteristics in the interference spectrum.
In the frequency range of 0.01 ~ 1MHz, the impedance is mainly determined by the coil inductance L.
In the frequency range of 1 ~ 10MHz, the impedance mainly depends on the coil distributed capacitance CK.
When the frequency range is > 10 MHz, the impedance is related to the damage of winding capacitor, main circuit inductor, leakage inductor and magnet, and the parallel circuit composed of copper damage (ZS is the equivalent impedance).
Leakage inductance and differential mode inductance
For the ideal inductance model, when the coil is bypassed, all magnetic flux is concentrated in the center of the coil. However, under normal circumstances, the ring coil will not be wound for a full circle, nor will it be wound very tightly, resulting in magnetic flux leakage. The common mode inductor has two coils, and there is a large gap between the coils, resulting in magnetic leakage, forming a differential mode inductor. Therefore, in general, common mode inductors also have a certain ability to suppress differential mode interference.
Leakage inductance can also be used in filter design. For example, in an ordinary filter, only one common mode choke is installed, and an appropriate amount of differential mode choke is generated by its leakage inductance, so as to suppress the differential mode current. In some cases, the leakage inductance and differential mode inductance of common mode choke are manually increased to achieve better filtering effect.