The following is a schematic circuit of an electromagnetic interference filter. The five-terminal device has two input terminals, two output terminals and a ground terminal. When used, the outer casing should be connected to the ground. The circuit includes a common mode choke (also known as common mode inductor) L, and the filter capacitors C1~C4.L have no effect on the series mode interference, but when common mode interference occurs, since the flux directions of the two coils are the same, After coupling, the total inductance increases rapidly, so it has a large inductive reactance to the common mode signal, making it difficult to pass, so it is called a common mode choke. Its two coils are wound around a low-loss, high-permeability ferrite magnetic ring. When a current is passed, the magnetic fields on the two coils reinforce each other. When the rated current is large, the wire diameter of the common mode choke should be increased accordingly to withstand a large current.
In addition, by appropriately increasing the inductance, the low-frequency attenuation characteristics can be improved. C1 and C2 use film capacitors, the capacity range is roughly 0.01μF ~ 0.47μF, mainly used to filter the serial mode interference. C3 and C4 are connected across the output and ground the capacitor at the midpoint, effectively suppressing common mode interference. C3 and C4 can also be connected in parallel at the input end. Ceramic capacitors are still used. The capacity range is 2200pF~0.1μF. To reduce leakage current, the capacitance should not exceed 0.1μF, and the midpoint of the capacitor should be connected to the ground. The pressure values ​​of C1~C4 are both 630VDC or 250VAC.
Circuit design of EMI filter principle
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