The role and working principle of common mode inductance

1. The role and principle of common mode inductors are believed to be familiar to many people, but do you fully understand its connection? Is the common mode inductor on your circuit connected correctly? First, let's get to know the common mode inductor. Common mode inductor is a common mode interference suppression device with a ferrite core. It consists of two coils of the same size and the same number of turns wound symmetrically on the same ferrite toroidal core to form a four-terminal device, the common mode inductance plays a role in hindering the AC current. For plug-in inductors, we generally see more UU type, EE type and ring type. In the case of a certain AC frequency, the larger the inductance, the greater its ability to hinder the alternating current. On the contrary, the smaller the inductance, the smaller the hindering ability. It suppresses the common mode signal in the circuit, and the inductance value of the common mode inductance can be used for electricity. bridge to measure.

Second, the application of common mode inductance in switching power supply Common mode inductance is sometimes called common mode choke coil, because its function is to suppress, and it is mostly used in switching power supply circuits to form various filters to filter EMI. Suppress the electromagnetic waves generated by various high-speed signals from being emitted outward. As shown in the figure below, there is a set of parallel lines in the circuit. When normal signals pass through, it is basically unaffected. A magnetic field in the same direction will be generated in the coil to increase the inductive reactance of the coil, making the coil show high impedance and producing a strong damping effect. At this time, the common mode current will be attenuated to achieve the purpose of suppressing interference. The following figure is the schematic diagram of the front stage of the switching power supply, in which 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, and the noise it generates is ground noise, which is two The noise of the root line is respectively connected to the ground. If we understand the common mode noise, we will know the common mode inductance and we will know why the common mode inductance will be connected to the AC side.