As is known to all, the frequency range of common-mode noise generated by switching power supply is 10 kHz ~ 50 MHz or higher. In order to effectively attenuate or suppress these noises, it is required that the common mode inductors have sufficient inductance within this frequency range. So how does the common mode inductance suppress the noise?
First of all, the two coils of the common mode inductor are wound on the magnetic ring with the same number of turns and in the same direction, except that one set of coils is wound on the left side and the other set is wound on the right side. The common mode inductors use high permeability manganese zinc ferrite or amorphous materials to improve the performance of the common mode inductors.
Second, normal alternating current flows through the common mode inductor analysis. 220 V alternating current is the current differential mode, it flows through the common-mode inductance L3 and L4 direction shown in the diagram below, two inductor current produced by the magnetic field in opposite directions and offset, and then the normal signal current is mainly affected by the inductor resistance of little impact (a), and a small amount caused by the leakage inductance of the damping (inductance), plus 220 V alternating current frequency 50 Hz, common mode inductors, inductance is so common-mode inductor for normal 220 alternating current impedance is very small, do not affect the 220 V alternating current for power supply of the machine.