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

I believe that many people are familiar with common mode inductors, but do you fully understand its connection? Are the common mode inductors on your circuit connected correctly? First, let’s get to know the common mode inductance.

1. The role and principle of common mode inductance

I believe that many people are familiar with common mode inductors, but do you fully understand its connection? Are the common mode inductors on your circuit connected correctly? First, let’s get to know the common mode inductance.

Common mode inductance is a common mode interference suppression device with ferrite core. It is symmetrically wound on the same ferrite toroidal core by two coils of the same size and the same number of turns, forming a four-terminal For devices, the common mode inductance is an obstacle to AC current. For plug-in inductors, we generally see UU type, EE type and ring type.

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

In the case of a certain AC frequency, the greater the inductance, the greater the resistance to alternating current. On the contrary, the smaller the inductance, the lower the resistance. It suppresses the common mode signal in the circuit, and the inductance of the common mode inductor can use electricity. Bridge to measure.

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

Second, the application of common mode inductance in switching power supply

Common-mode inductors are sometimes called common-mode chokes because they are used to suppress them. They are mostly used in switching power supply circuits to form various filters to filter EMI and suppress electromagnetic waves generated by various high-speed signals. Transmission, as shown in the figure below, there is a set of parallel lines in the circuit, and the normal signal is basically unaffected when passing through, but when there is a common mode current flowing through, due to the isotropy of the common mode current, a magnetic field in the same direction will be generated in the coil. Increasing the inductive reactance of the coil makes the coil behave as high impedance and produces a strong damping effect. At this time, the common mode current will be attenuated to achieve the purpose of suppressing interference.

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

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

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

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