Switching power supply circuit composition and detailed explanation of each part (1)

With the rapid development of power electronics technology, power equipment is increasingly closely related to people's work and life, and electronic equipment is inseparable from reliable power supply.

switching power supply

Since the article is too long to read, it is divided into two parts. This article includes

1. Functional circuit diagram of switching power supply

2. Principles and common circuits of input circuits

3. Power conversion circuit

4. Output rectifier filter circuit

5. Principle of voltage stabilization loop

1. Functional circuit diagram of switching power supply:

The main circuit diagram of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an 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.

Switching power supply circuit structure

2. Principle of input circuit and common circuits:

1. AC input rectifier filter circuit principle:

Input filter and rectifier circuit schematic diagram

①Lightning protection circuit: When there is a lightning strike and high voltage is introduced into the power supply through the power grid, the circuit composed of MOV1, MOV2, MOV3: 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.

②Input 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, 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.

2. DC input filter circuit principle:

DC input filter circuit principle

①Input 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. It also prevents high-frequency clutter generated by the power supply itself from affecting the power grid. interference. C3 and C4 are safety capacitors, and L2 and L3 are differential mode inductors.

②R1, 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 charges 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. Since 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 half

The amount of charge induced on the conductor surface controls the drain current.

2. Common schematic diagrams:

3. Working principle:

R4, C3, R5, R6, C4, D1, and D2 form a buffer and are 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 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 cuts off, the transformer releases energy through D1, D2, R5, R4, and C3, and at the same time achieves the purpose of magnetic field reset, preparing the transformer for the next storage and transfer 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 turn 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 with the primary and secondary poles 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 with the primary and secondary poles in opposite phases. 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 a rectifier. 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.

5. Principle of voltage stabilization loop:

1. Feedback circuit schematic diagram:

2. Working principle: When the output U0 rises, after being divided by the sampling resistors R7, R8, R10, and VR1, the U1③ pin voltage rises. When it exceeds the U1② pin reference voltage, the U1① pin outputs a high level, turning Q1 on. , the optocoupler OT1 light-emitting diode emits light, the phototransistor is turned on, and the potential of UC3842 pin ① becomes low accordingly, thus changing the output duty cycle of pin U1 ⑥ and reducing U0.

When the output U0 decreases, the U1③ pin voltage decreases. When it is lower than the U1② pin reference voltage, the U1① pin outputs a low level, Q1 does not conduct, the optocoupler OT1 light-emitting diode does not emit light, the phototransistor does not conduct, and the UC3842① pin potential rises. High, thereby changing the U1⑥ pin output duty cycle to increase and U0 to decrease. Repeatedly, the output voltage remains stable. Adjusting VR1 can change the output voltage value.

The feedback loop is an important circuit that affects the stability of the switching power supply. For example, if the feedback resistor capacitor has errors, leakage, or weak soldering, it will produce self-excited oscillation. The fault phenomena include: abnormal waveform, empty and full load oscillation, unstable output voltage, etc.