Basic circuit diagram of switching power supply AC input and rectification and filtering

① Lightning protection circuit : When there is a lightning strike and high voltage is generated and introduced into the power supply through the power grid, the circuit composed of MOV 1 , MOV2, MOV 3 : F1, F2, F3, FDG1 provides protection. When the voltage applied to both ends of the varistor exceeds its working voltage, its resistance decreases, causing high-voltage energy to be consumed on the varistor. If the current is too large, F1, F2, and F3 will burn out the protection circuit.

② Input filter circuit: The double -type filter network composed of C1, L1, C2, and C3 is mainly used to suppress the electromagnetic noise and clutter signals of the input power supply to prevent interference with the power supply, and at the same time prevent high frequencies generated by the power supply itself. Clutter interferes with the power grid. When the power is turned on, C5 needs to be charged. Due to the large instantaneous current, adding RT1 (thermistor) can effectively prevent surge current. Since all instantaneous energy is consumed on the RT1 resistor, the resistance of RT1 decreases after the temperature rises after a certain period of time (RT1 is a negative temperature coefficient component). At this time, it consumes very little energy and the subsequent circuit can work normally.

③ Rectifier and filter circuit: After the AC voltage is rectified by BRG1, it is filtered by C5 to obtain a relatively pure DC voltage. If the capacity of C5 becomes smaller, the output AC ripple will increase.