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

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

1. Circuit composition of switching power supply
　　The main circuit of 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-under voltage protection circuit, output over-under voltage protection circuit, output over-current protection circuit, output short-circuit protection circuit, etc.
    The circuit block diagram of the switching power supply is as follows:
 
2. Principle 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, MOV1, MOV2, MOV3: The circuit composed of F1, F2, F3 and FDG1 is used for 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 π filter network composed of C1, L1, C2, and C3 mainly suppresses the electromagnetic noise and clutter signals of the input power supply to prevent interference with the power supply, and also prevents high-frequency clutter generated by the power supply itself. Interference 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, the energy consumed by it is very small, 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.
2. DC input filter circuit principle:
 
　　① Input filter circuit: The double π 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 power supply itself from The generated high-frequency clutter interferes 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 up, Q2 is not conducting due to the presence of C6, 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 C8 leaks or the downstream circuit is short-circuited, the voltage drop generated by the current on RT1 increases at the moment of starting up. Q1 turns on and Q2 does not conduct without gate voltage. RT1 will burn out in a short time. Protect the downstream circuit.
3. Power conversion circuit
1. Working principle of MOS tube: The most widely used insulated gate field effect tube at present is MOSFET (MOS tube), which uses the electroacoustic effect of the semiconductor surface to work. 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 gate-source voltage to change the amount of charge induced on the semiconductor surface, thereby controlling the drain current. .
2. Common schematic diagram:
 
3. Working principle:
　　R4, C3, R5, R6, C4, D1, D2 form a buffer and are connected in parallel with the switching MOS tube to reduce the voltage stress of the switching tube, reduce EMI, and prevent secondary breakdown. When the switch Q1 is turned off, the primary coil of the transformer is prone to generate peak voltages and peak currents. The combination of these components can absorb the peak voltages and currents well. The current peak signal measured from R3 participates in the duty cycle control of the current working cycle and is therefore the current limit of the current working cycle. When the voltage on R5 reaches 1V, UC3842 stops working and switch Q1 is turned off immediately. The junction capacitances CGS and CGD in R1 and Q1 together form an RC network. The charging and discharging of the capacitor directly affects the switching speed of the switching tube. If R1 is too small, it will easily cause oscillation and electromagnetic interference will be great; 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 below 18V, thereby protecting the MOS tube. The gate controlled voltage of Q1 is a saw-shaped wave. When the duty cycle is larger, the conduction time of Q1 is longer, and the energy stored in the transformer is more; when Q1 is turned off, the transformer passes through D1, D2, R5 , R4, and C3 release energy, and at the same time achieve the purpose of magnetic field reset, preparing the transformer for the next storage and transmission of energy. The IC constantly adjusts the duty cycle of the ⑥ pin saw-shaped wave according to the output voltage and current, thereby stabilizing the output current and voltage of the entire machine. C4 and R6 are spike voltage absorption circuits.
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 switching transformer, and TR1 is the current loop.
 
4. Output rectifier filter circuit:
1. Forward rectifier circuit:
 
　　T1 is a switching transformer, and the phases of its primary pole and secondary pole are in the same 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, and the phases of its primary pole and secondary pole are opposite. D1 is the 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 rectifier circuit:
 
　　Working principle: When the upper end of the secondary of the transformer is positive, the current passes through C2, R5, R6, and R7 to conduct Q2, and the circuit forms a loop, and Q2 is the rectifier tube. The Q1 gate is cut off because it is reverse biased. When the lower end of the secondary of the transformer is positive, the current passes through C3, R4, and R2 to conduct Q1, and Q1 is a freewheeling tube. The Q2 gate is cut off because it is reverse biased. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 are peak clipping circuits.