Common mode inductors’functions and principles

Common mode inductors, also known as commonmode chokes, are often used to filter commonmode electromagnetic interference signals in switching power supplies. In the board design, the common-mode inductor also plays the role of EMI filtering, which is used to suppress the electromagnetic wave generated by high-speed signal lines. In summary, common-mode inductors are mainly used to isolate high frequency crosstalk (EMI) between the inside and outside of an appliance. 

Why do common-mode inductors protect against EMI? In order to find this out, we need to analyze from the structure of common mode inductors. 

The filter circuit of common-mode inductor, La and LB are common-mode inductor coils. The two coils are wound around the same iron core with the same number of turns and phase (inverted). In this way, when the normal current in the circuit passes through the common-mode inductor, the current creates a reverse magnetic field in the inductor coil wound in the same phase and cancels each other out. In this case, the normal signal current is mainly affected by coil resistance (and damping due to a small amount of leakage); When the common-mode current passes through the coil, the codirectional magnetic field in the coil will increase due to the homogeneity of the common-mode current, so that the coil has high impedance and strong damping effect, so as to reduce the common-mode current and achieve the purpose of filtering. 

In fact, if one end of the filter circuit is connected to a source of interference and the other to a jamming device, La and C1, LB and C2 form two sets of low-pass filters that can keep common-mode EMI signals on the line at very low levels. This circuit can not only inhibit the incoming of external EMI signal, but also reduce the EMI signal generated when the line itself is working, and effectively reduce the EMI interference intensity.

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