Detailed explanation of the composition and principle of switching power supply circuit

1. Circuit composition of switching power supply
The main circuit of the switching power supply is composed of input electromagnetic interference filter (EMI), rectifier filter circuit, power conversion circuit, PWM controller circuit, and output rectifier filter circuit. Auxiliary circuits include input over-voltage and under-voltage protection circuit, output over-voltage and under-voltage protection circuit, output overcurrent protection circuit, output short-circuit protection circuit, etc.
The circuit composition block diagram of the switching power supply is as follows:

2. Principles of input circuit and common circuits
1. Principle of AC input rectifier filter circuit:

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 MOV1, MOV2, MOV3: F1, F2, F3, and FDG1 provides protection. When the voltage applied to the two ends of the varistor exceeds its working voltage, its resistance value decreases, so that the high voltage energy is consumed on the varistor. If the current is too large, F1, F2, and F3 will burn to protect the subsequent circuit.
② Input filter circuit: The double π-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 also prevent the 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. Because the instantaneous energy is completely consumed in the RT1 resistor, after a certain period of time, the temperature rises and the RT1 resistance decreases (RT1 is a negative temperature coefficient element). At this time, the energy it consumes is very small, and the subsequent circuit can work normally.
③ Rectification and filtering 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.
2. Principle of DC input filter circuit:

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 supply to prevent interference with the power supply, and also prevents the high-frequency clutter generated by the power supply itself from interfering with the power grid. 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, Q2 is not turned on due to the existence of C6, and the current forms a loop through RT1. When the voltage on C6 is charged to the regulated voltage value of Z1, Q2 is turned on. If C8 leaks or the subsequent circuit is short-circuited, the voltage drop generated by the instantaneous current on RT1 at the start-up increases, Q1 conducts and Q2 does not conduct without gate voltage, and RT1 will burn out in a very short time to protect the subsequent circuit.
3. Power conversion circuit
1. Working principle of MOS tube: The most widely used insulated gate field effect tube is MOSFET (MOS tube), which works by using the electroacoustic effect on the surface of the semiconductor. It is also called surface field effect device. Because its gate is in a non-conductive state, the input resistance can be greatly increased, up to 105 ohms. The MOS tube uses the size of the gate-source voltage to change the amount of induced charge on the semiconductor surface, thereby controlling the size of the drain current.
2. Common schematic diagram:

3. Working principle:
R4, C3, R5, R6, C4, D1, D2 form a buffer, which is connected in parallel with the switch MOS tube to reduce the voltage stress of the switch tube, reduce EMI, and prevent secondary breakdown. When the switch tube Q1 is turned off, the primary coil of the transformer is prone to generate spike voltage and spike current. These components combined together can absorb the spike voltage and current very well. The current peak signal measured from R3 participates in the duty cycle control of the current working cycle, so it is the current limit of the current working cycle. When the voltage on R5 reaches 1V, UC3842 stops working and the switch tube Q1 is immediately turned off. R1 and the junction capacitors CGS and CGD in Q1 together form an RC network. The charging and discharging of the capacitor directly affects the switching speed of the switch tube. If R1 is too small, it is easy to cause oscillation and electromagnetic interference will also be large; if R1 is too large, it will reduce the switching speed of the switch tube. Z1 usually limits the GS voltage of the MOS tube to below 18V, thereby protecting the MOS tube. The gate controlled voltage of Q1 is a saw wave. When its duty cycle is larger, the longer the Q1 conduction time is, the more energy the transformer stores. When Q1 is turned off, the transformer releases energy through D1, D2, R5, R4, and C3, and also achieves the purpose of magnetic field reset, preparing for the transformer to store and transfer energy next time. The IC adjusts the duty cycle of the saw wave of pin ⑥ according to the output voltage and current, thereby stabilizing the output current and voltage of the whole machine. C4 and R6 are the peak voltage absorption circuit.

4. Push-pull power conversion circuit:
Q1 and Q2 will be turned on in turn.

5. Power conversion circuit with drive transformer:
T2 is the drive transformer, T1 is the switch transformer, and TR1 is the current loop.

IV. Output rectifier filter circuit:
1. Forward rectifier circuit:

T1 is a switch transformer, and the phases of its primary and secondary poles are in phase. D1 is a rectifier diode, D2 is a freewheeling diode, and R1, C1, R2, and C2 are peak clipping circuits. L1 is a freewheeling inductor, and C4, L2, and C5 form a π-type filter.

2. Flyback rectifier circuit:

T1 is a switching transformer, the phases of the primary and secondary poles are opposite. D1 is a rectifier diode, R1 and C1 are peak clipping circuits. L1 is a freewheeling inductor, R2 is a dummy load, and C4, L2, and C5 form a π-type filter.

3. Synchronous rectification circuit:

Working principle: When the upper end of the transformer secondary is positive, the current passes through C2, R5, R6, and R7 to turn on Q2, and the circuit forms a loop. Q2 is a rectifier. The gate of Q1 is cut off because it is in reverse bias. When the lower end of the transformer secondary is positive, the current passes through C3, R4, and R2 to turn on Q1, and Q1 is a freewheeling tube. The gate of Q2 is cut off because it is in reverse bias. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 are a peak clipping circuit.