Basic structure of noise filter

The power EMI noise filter is a passive low-pass filter, which transmits AC power to the power without attenuation, and greatly attenuates the EMI noise introduced with the AC power. At the same time, it can effectively suppress the EMI noise generated by the power supply equipment and prevent They enter the AC grid and interfere with other electronic equipment.

The basic structure of a single-phase AC grid noise filter is a four-terminal passive network composed of concentrated parameter components. The main components used are common mode inductors L1 and L2, differential mode inductors L3 and L4, and common mode capacitors CY1 and CY2. And differential mode capacitor CX. If this filter network is placed at the input end of the power supply, L1 and CY1 and L2 and CY2 respectively constitute a low-pass filter between two pairs of independent ports on the AC incoming line, which can attenuate common mode interference existing on the AC incoming line. Noise, preventing them from entering the power supply. The common mode inductor is used to attenuate the common mode noise on the AC input line. Among them, L1 and L2 are generally wound with the same number of turns in the same direction on the ferrite core of the closed magnetic circuit. The magnetic fluxes generated by the AC currents in the two coils cancel each other, which will not cause the magnetic core to saturate the magnetic flux, and also make the inductance values of the two coils larger in the common mode and remain unchanged.

The differential mode inductors L3, L4 and the differential mode capacitor CX form a low-pass filter between the independent ports of the AC incoming line, which is used to suppress the differential mode interference noise on the AC incoming line and prevent the power supply equipment from being interfered by it.

Passive networks have bidirectional suppression. Inserting it between the AC grid and the power supply is equivalent to adding a blocking barrier between the EMI noise of the two. Such a simple passive filter plays a role of bidirectional suppression of noise, thus in various electronic equipment. Has been widely used.