400Hz Power SupplyThe EMI noise filter is a passive low-pass filter that transmits AC power to the power supply without attenuation, while significantly attenuating the EMI noise that enters with the AC power; it also effectively suppresses EMI noise generated by power equipment, preventing it from entering the AC grid and interfering with other electronic devices.
The basic structure of a single-phase AC grid noise filter is a four-terminal passive network composed of lumped parameter components. The main components used are common-mode inductors L1 and L2, differential-mode inductors L3 and L4, as well as common-mode capacitors CY1 and CY2, and differential-mode capacitors CX. If this filter network is placed at the input of the power supply, L1 with CY1 and L2 with CY2 form low-pass filters between two independent ports on the AC line, capable of attenuating common-mode interference noise on the AC line and preventing it from entering the power equipment.
Common-mode inductor coils are used to attenuate common-mode noise on AC input lines. Typically, L1 and L2 are wound in the same direction with the same number of turns on the ferrite core within a closed magnetic circuit. When connected to the circuit, the magnetic flux generated by the alternating current in the L1 and L2 coils cancels each other out, preventing the core from causing magnetic flux saturation. This also ensures that the inductance values of these two coils are larger and remain constant in the common-mode state.
The differential inductor coils L3 and L4, along with the differential capacitor CX, form a low-pass filter between the independent ports of the AC input line. This filter is designed to suppress differential interference noise on the AC input line, preventing interference from affecting the power supply equipment.
The 400Hz power line noise filter is a passive network with bidirectional suppression capabilities. Placed between the AC power grid and the power source, it acts as a barrier against EMI noise between the two, effectively serving as a simple passive filter that suppresses noise bidirectionally, thereby gaining widespread application in various electronic devices.
