1. Circuit composition of switching power supply

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

2. Principles and common circuits of input 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, FDG1 will provide 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. 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.
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 conductive 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 transistor at present is the MOSFET (MOS tube), which uses the electroacoustic effect on the semiconductor surface to work; it is also called a 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 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 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 turn on in turn.

5. Power conversion circuit with drive transformer
T2 is the driving 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, causing Q1 to conduct, and the optocoupler OT1 emits light. The 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, thus 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.

6. Short circuit protection circuit

1. In the case of a short circuit at the output terminal, the PWM control circuit can limit the output current within a safe range. It can use a variety of methods to implement the current limiting circuit. When the power current limiting does not work during a short circuit, only another Add some circuits. ​
2. There are usually two types of short-circuit protection circuits. The figure below shows a low-power short-circuit protection circuit. Its principle is briefly described as follows:

When the output circuit is short-circuited, the output voltage disappears, the optocoupler OT1 does not conduct, the voltage of UC3842 pin ① rises to about 5V, the divided voltage of R1 and R2 exceeds the TL431 reference, causing it to conduct, the VCC potential of UC3842 pin ⑦ is pulled down, and the IC stops working . After UC3842 stops working, the ① pin potential disappears, TL431 does not conduct, UC3842 ⑦ pin potential rises, UC3842 restarts, and the cycle starts again. When the short circuit disappears, the circuit can automatically return to normal working condition. ​
3. The figure below is a medium power short circuit protection circuit. Its principle is briefly described as follows:

When the output is short-circuited and the voltage of pin ① of UC3842 rises, when the potential of pin ③ of U1 is higher than that of pin ②, the comparator flips pin ① and outputs high potential to charge C1. When the voltage at both ends of C1 exceeds the reference voltage of pin ⑤, pin U1 ⑦ outputs low potential, and UC3842① When the pin is lower than 1V, UCC3842 stops working and the output voltage is 0V, and the cycle starts again. When the short circuit disappears, the circuit works normally. R2 and C1 are charge and discharge time constants. If the resistance value is incorrect, the short circuit protection will not work. ​
4. The figure below shows a common current limiting and short-circuit protection circuit. Its working principle is briefly described as follows:

When the output circuit is short-circuited or overcurrent, the primary current of the transformer increases, the voltage drop across R3 increases, the voltage of pin ③ increases, and the output duty cycle of pin ⑥ of UC3842 gradually increases. When the voltage of pin ③ exceeds 1V, UC3842 turns off and has no output. . ​
5. The figure below is a protection circuit that uses a current transformer to sample current. It has low power consumption, but high cost and complicated circuit. Its working principle is briefly described as follows:

If the output circuit is short-circuited or the current is too large, the voltage induced by the secondary coil of TR1 will be higher. When the ③ pin of UC3842 exceeds 1 volt, UC3842 will stop working, and it will start over and over again. When the short-circuit or overload disappears, the circuit will recover on its own.

7. Output terminal current limiting protection

The above picture is a common output current limiting protection circuit. Its working principle is briefly described as above: when the output current is too large, the voltage at both ends of RS (manganese copper wire) rises, and the U1③ pin voltage is higher than the ② pin reference voltage. U1① pin Output high voltage, Q1 is turned on, the photoelectric effect occurs in the optocoupler, the voltage at pin 1 of UC3842 decreases, and the output voltage decreases, thereby achieving the purpose of output overload current limiting.

8. Principle of output overvoltage protection circuit

The function of the output overvoltage protection circuit is to limit the output voltage to a safe value when the output voltage exceeds the design value. When the internal voltage stabilizing loop of the switching power supply fails or the output overvoltage occurs due to improper user operation, the overvoltage protection circuit protects the downstream electrical equipment from damage.
The most commonly used overvoltage protection circuits are as follows:
1. SCR trigger protection circuit

As shown in the figure above, when the output of Uo1 rises, the voltage regulator tube (Z3) breaks down and turns on, and the control terminal of the silicon controlled rectifier (SCR1) gets the trigger voltage, so the thyristor turns on. When the Uo2 voltage is short-circuited to ground, the overcurrent protection circuit or short-circuit protection circuit will operate and stop the operation of the entire power circuit. When the output overvoltage phenomenon is eliminated, the control terminal trigger voltage of the thyristor is discharged to the ground through R, and the thyristor returns to the off state.
2. Photoelectric coupling protection circuit

As shown in the figure above, when Uo has an overvoltage phenomenon, the voltage regulator tube breaks down and conducts, and a current flows through the optocoupler (OT2) R6 to the ground. The light-emitting diode of the optocoupler emits light, thereby causing the phototransistor of the optocoupler to conduct Pass. The base of Q1 is electrically conductive, and the voltage of the ③ pin of 3842 is reduced, which turns off the IC and stops the operation of the entire power supply. Uo is zero, and the cycle repeats.
3. Output voltage limiting protection circuit
The output voltage limiting protection circuit is as shown below. When the output voltage rises, the voltage regulator tube is turned on and the optocoupler is turned on. The Q1 base has a driving voltage and the channel is connected. The voltage of UC3842③ increases, the output decreases, and the voltage regulator tube does not conduct. UC3842③ As the voltage decreases, the output voltage increases. Repeatedly, the output voltage will stabilize within a range (depending on the voltage regulation value of the voltage regulator tube).

4. Output overvoltage lockout circuit

The working principle of Figure A is that when the output voltage Uo increases, the voltage regulator tube is turned on, the optocoupler is turned on, and the base of Q2 is electrically conductive. Due to the conduction of Q2, the base voltage of Q1 is reduced and it is also turned on. The Vcc voltage passes through R1 , Q1, and R2 make Q2 always conductive, and the ③ pin of UC3842 is always at high level and stops working. In Figure B, when UO rises, the voltage of U1 ③ pin increases, and ① pin outputs high level. Due to the existence of D1 and R1, U1 ① pin always outputs high level. Q1 is always turned on. UC3842 ① pin is always low level and stops working. . Positive feedback?

9. Power factor correction circuit (PFC)

1. Principle diagram

2. Working principle
The input voltage passes through the EMI filter composed of L1, L2, L3, etc., and the BRG1 rectifies one path and sends it to the PFC inductor. The other path is divided by R1 and R2 and sent to the PFC controller as a sample of the input voltage to adjust the duty of the control signal. Ratio, that is, changing the on and off times of Q1 to stabilize the PFC output voltage. L4 is the PFC inductor, which stores energy when Q1 is on and releases energy when Q1 is off. D1 is the startup diode. D2 is the PFC rectifier diode, C6 and C7 filter. One channel of the PFC voltage is sent to the downstream circuit, and the other channel is divided by R3 and R4 and then sent to the PFC controller as a sample of the PFC output voltage to adjust the duty cycle of the control signal and stabilize the PFC output voltage.

10. Input over- and under-voltage protection

1. Schematic diagram

2. Working principle
The input overvoltage and undervoltage protection principles of AC input and DC input switching power supplies are roughly the same. The sampling voltage of the protection circuit comes from the input filtered voltage. The sampling voltage is divided into two channels. One channel is divided by R1, R2, R3, and R4 and then input to the comparator pin 3. If the sampling voltage is higher than the reference voltage of pin 2, the comparator pin 1 outputs a high level to control the main controller. Shut down, the power supply has no output.
The other channel is divided by R7, R8, R9, and R10 and then input to pin 6 of the comparator. If the sampling voltage is lower than the reference voltage of pin 5, the comparator pin 7 outputs a high level to control the main controller to shut down, and the power supply has no output. .