Introduction to the working principle of AC/DC switching power supply
The main circuit of an AC/DC power adapter consists of an input electromagnetic interference filter (EMI), a rectification filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectification filter circuit. Auxiliary circuits include input overvoltage protection circuit, output overvoltage protection circuit, output overcurrent protection circuit, output short-circuit protection circuit, etc.
Principle of AC input rectification and filtering circuit
Lightning protection circuit: When there is a lightning strike and high voltage is generated and introduced into the power grid, the circuit composed of MOV1, MOV2, MOV3: F1, F2, F3, FDG1 is used for protection. When the voltage applied across the varistor exceeds its operating voltage, its resistance decreases, causing high-voltage energy to be consumed on the varistor. If the current is too high, F1, F2, F3 will burn out the protective circuit of the rear stage.
Input filtering circuit: The dual π - type filtering network composed of C1, L1, C2, and C3 is mainly used to suppress the electromagnetic noise and clutter signals of the input power supply, prevent interference with the power supply, and also prevent high-frequency clutter generated by the power supply itself from interfering 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. Due to the instantaneous energy being fully consumed by the RT1 resistor, after a certain period of time, the resistance of RT1 decreases as the temperature rises (RT1 is a negative temperature coefficient element). At this point, the energy consumed by it is very small, and the subsequent circuit can work normally.
Rectification and filtering circuit: The AC voltage is rectified by BRG1 and filtered by C5 to obtain a relatively pure DC voltage. If the capacity of C5 decreases, the output AC ripple will increase.
Principle of DC input filtering circuit
1. Input filtering circuit: The dual π - type filtering network composed of C1, L1, and C2 is mainly used to suppress the electromagnetic noise and clutter signals of the input power supply, prevent interference with the power supply, and also prevent high-frequency clutter generated by the power supply itself from interfering with the power grid. C3 and C4 are safety capacitors, while L2 and L3 are differential mode inductors.
2. R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, C7 form an anti surge circuit. At the moment of start-up, due to the presence of C6, Q2 does not conduct, and the current forms a circuit through RT1. When the voltage on C6 reaches the regulated value of Z1, Q2 conducts. If there is a leakage in C8 or a short circuit in the downstream circuit, the voltage drop generated by the current on RT1 at the moment of start-up will increase, causing Q1 to conduct and Q2 to not conduct without gate voltage. RT1 will burn out in a short time to protect the downstream circuit.
