The frequency range of common mode noise generated by switching power supply is 10 kHz to 50 MHz or even higher. In order to effectively attenuate these noise, it is required that the common mode inductor can provide sufficient inductance within this frequency range. Before explaining the working principle of a common mode inductor, one should understand the structure of the common mode inductor, which helps to understand the suppression of common mode high-frequency noise by the common mode inductor. What role does a common mode inductor play in a circuit?

(1) Analysis of normal AC current flowing through a common mode inductor. 220 V AC is a differential mode current that flows in the direction of common mode inductors L3 and L4. What role does a common mode inductor play in a circuit?

The magnetic field generated by the current in the two inductors is offset by the opposite direction. At this time, the normal signal current is mainly affected by the resistance of the inductor (which is very small), as well as a small amount of damping (inductance) caused by leakage inductance. In addition, the frequency of 220 V AC is only 50 Hz, and the inductance of the common mode inductor is not large. Therefore, the common mode inductor has very small inductance for normal 220 V AC, and does not affect the power supply of the entire machine by 220 V AC.

(2) Analysis of common mode current flowing through a common mode inductor. When the common mode current flows through the common mode inductor, the current generates a magnetic field in the same direction as the common mode current in the common mode inductor, L3 and L4. What role does the common mode inductor play in the circuit?

At this point, the inductance of the common mode inductors L3 and L4 is increased, which means that the inductance of L3 and L4 to the common mode current is increased, causing greater suppression of the common mode current, achieving the purpose of attenuating the common mode current and suppressing the common mode interference noise.