Principle circuit of electromagnetic interference filter

The following is the principle circuit of an electromagnetic interference filter. The five-terminal device has two input terminals, two output terminals and a ground terminal. When used, the shell should be connected to the earth. The circuit includes a common mode choke (also known as a common mode inductor) L and filter capacitors C1~C4.L which have no effect on series mode interference. However, when common mode interference occurs, since the magnetic flux directions of the two coils are the same, After coupling, the total inductance increases rapidly, so it presents a large inductive reactance to the common mode signal, making it difficult for it to pass through, so it is called a common mode choke. Its two coils are wound around low-loss, high-permeability ferrite magnetic rings. When current passes through, the magnetic fields on the two coils will strengthen each other. When the rated current is larger, the wire diameter of the common mode choke should also be increased accordingly so that it can withstand the larger current.

In addition, appropriately increasing the inductance can improve the low-frequency attenuation characteristics. C1 and C2 use film capacitors with a capacity range of roughly 0.01μF~0.47μF, which are mainly used to filter out series-mode interference. C3 and C4 are connected across the output end and the midpoint of the capacitor is grounded, which can effectively suppress common mode interference. C3 and C4 can also be connected in parallel at the input end, still using ceramic capacitors, with a capacity range of 2200pF~0.1μF. In order to reduce leakage current, the capacitance should not exceed 0.1μF, and the midpoint of the capacitor should be connected to the earth. The withstand voltage values ​​of C1~C4 are all 630VDC or 250VAC.