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# Function and working principle of common mode inductor

2021-12-04

1、 Function and principle of common mode inductance

I believe many people are familiar with common mode inductors, but do you fully understand its connection? Is the common mode inductor on your circuit connected correctly? First, let's talk about common mode inductors.

Common mode inductor is a common mode interference suppression device with ferrite as the magnetic core. It consists of two coils with the same size and the same number of turns symmetrically wound on the same ferrite ring magnetic core to form a four terminal device. The common mode inductor hinders the AC current. For plug-in inductors, we generally see UU type, EE type and ring type.

When the AC frequency is certain, the larger the inductance, the greater the resistance to ac. on the contrary, the smaller the inductance, the smaller the resistance. It suppresses the common mode signal in the circuit, and the inductance value of the common mode inductance can be measured by the bridge.

2、 Application of common mode inductance in switching power supply

Common mode inductors are sometimes called common mode chokes because they are mostly used in switching power supply circuits to form various filters to Filter EMI and suppress the outward emission of electromagnetic waves generated by various high-speed signals. As shown in the figure below, there are a group of parallel lines in the circuit, which are basically unaffected when normal signals pass through, but when common mode current flows through, Due to the isotropy of the common mode current, the magnetic field in the same direction will be generated in the coil to increase the inductive reactance of the coil, so that the coil will show high impedance and produce strong damping effect. At this time, the common mode current will be attenuated to suppress interference.

The following figure is the schematic diagram of the front stage of the switching power supply, in which L1 is the common mode inductor. In order to suppress the common mode interference, we know that the common mode signal is a signal with equal amplitude and the same phase, and the noise it produces is the noise to the ground, which is the noise of two wires to the ground respectively. After understanding the common mode noise, we know the common mode inductor and know why the common mode inductor is connected to the AC side.

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