Why are common mode inductors EMI resistant? To figure this out, we need to start with the structure of the common mode inductor.
The filter circuit of the common mode inductor, La and Lb are the common mode inductor coils. The two coils are wound on the same core with the same number of turns and phase
(winding in reverse). In this way, when the normal current in the circuit flows through the common mode inductor, the current will generate a reverse magnetic field in the inductor coil woud in the same phase and cancel each other. At this time, the normal signal current is mainly affected by the resistance of the coil (and a small amount of damping caused by leakage inductance). When the common mode current flows through the coil, due to the homogeneity of the common mode current, the magnetic field in the same direction will be generated in the coil and increase the inductive reactance of the coil, so that the coil shows high impedance and produces strong damping effect, so as to attenuatethe common mode current and achieve the purpose of filtering. In fact, if one end of the filter circuit is connected to the interference source and the other end is connected to the interfered device, La and C1, Lb and C2 constitute two groups of low-pass filters, which can control the common mode EMI signal on the line at a very low level. The circuit can not only inhibit
the external EMI signal incoming, but also attenuate the EMI signal generated when the line itself is working, which can effectively reduce the EMI interference intensity.
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