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Why is the common mode inductor connected to the AC side of the switching power supply?

Why is the common mode inductor connected to the AC side of the switching power supply?


1、 Efficacy and basic principle of common mode inductance

Presumably, many people are not unfamiliar with common mode inductors, but do you have a thorough understanding of its wiring method? Is the common mode inductor on your power circuit connected correctly? The first step is to know the common mode inductor.

Common mode inductance is a common mode influence suppression component with ferrite core as the transformer skeleton. It consists of two electromagnetic coils with the same specification and the same number of coil turns. The coil inductance symmetrically generates a four terminal component on the same ferrite core ring transformer skeleton. The common mode inductance has the effect of blocking the AC circuit. As for software inductors, UU type, EE type and their rings are commonly seen.

Under the condition of a certain AC frequency, the larger the inductance, the greater its blocking ability to AC current. Conversely, the smaller the inductance, the smaller its blocking ability. It suppresses the common mode data signal in the power supply circuit, and the inductance value of the common mode inductance can be accurately measured by the power bridge.

2、 Application of common mode inductance in switching power supply

Common mode inductor is sometimes called common mode choke coil because its function is suppression. It is often used in switching power supply circuit to form various filters to Filter EMI and suppress the outward transmission of radio waves caused by various high-speed data signals. As shown in the figure below, there is a group of parallel processing routes in the power supply circuit, All normal data signals will not be affected according to the situation. However, when there is common mode current flowing through, due to the isotropy of the common mode current, the electromagnetic field in the same direction will be caused in the electromagnetic coil, which will expand the inductive reactance of the electromagnetic coil, so that the electromagnetic coil mainly shows high characteristic impedance, resulting in strong vibration reduction effect. At this moment, the common mode current will be reduced by the attenuation coefficient, Achieve the goal of restraining the impact.

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