Explain the components of the switching power supply

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Summary:

First, the circuit composition of the switching power supply

The main circuit of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectification and filtering circuit, a power conversion circuit, a PWM controller circuit, and an output rectification and filtering circuit. The auxiliary circuit has an input over-voltage protection circuit, an output over-voltage protection circuit, an output over-current protection circuit, and an output short-circuit protection circuit.

The circuit block diagram of the switching power supply is as follows:

Second, the principle of the input circuit and common circuits

1, AC input rectifier filter circuit principle:

1 Lightning protection circuit: When there is lightning strike and high voltage is generated through the power grid, the circuit consists of MOV1, MOV2, MOV3: F1, F2, F3 and FDG1. When the voltage applied across the varistor exceeds its working voltage, its resistance decreases, so that the high-voltage energy is consumed on the varistor. If the current is too large, F1, F2, and F3 will burn and protect the latter circuit.

2 Input filter circuit: The double π-type filter network composed of C1, L1, C2 and C3 mainly suppresses the electromagnetic noise and clutter signals of the input power source to prevent interference to the power supply and also prevents high-frequency clutter generated by the power supply itself. Interference with the grid. When the power is turned on, the C5 should be charged. Because the instantaneous current is large, adding RT1 (thermistor) can effectively prevent the inrush current. Since the instantaneous energy is completely consumed on the RT1 resistor, the RT1 resistance decreases after the temperature rises after a certain time (RT1 is a negative temperature coefficient component), at which time the energy consumed is very small, and the latter circuit can work normally.

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

2, DC input filter circuit principle:

1 Input filter circuit: The double π-type filter network composed of C1, L1 and C2 mainly suppresses the electromagnetic noise and clutter signals of the input power source to prevent interference to the power supply, and also prevents high-frequency clutter generated by the power supply itself from the power grid. interference. C3 and C4 are safety capacitors, and L2 and L3 are differential mode inductors.

2 R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, and C7 form an anti-surge circuit. At the moment of starting, due to the presence of C6, Q2 is not conducting, and the current forms a loop through RT1. Q2 turns on when the voltage on C6 is charged to the regulated value of Z1. If the C8 leakage or the short circuit of the latter stage is short, the voltage drop generated by the current on RT1 increases at the moment of starting, Q1 is turned on so that the gate voltage of Q2 is not turned on, and RT1 will burn out in a short time. Protect the rear stage circuit.

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