Switching power supply circuit composition and detailed explanation of each part (2)

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, and 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 pin ① of UC3842 Below 1V, UCC3842 stops working, 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 is 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 at pin ③ increases, and the output duty cycle of pin ⑥ of UC3842 gradually increases. When the voltage at 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.

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.

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?

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 channel and sends it to the PFC inductor, and the other channel is divided by R1 and R2 and sent to the PFC controller as a sample of the input voltage for adjustment. Control the duty cycle of the signal, that is, change the on and off time 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.

Input over and under voltage protection

1. Schematic diagram:

2. Working principle: The input over- and under-voltage 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. .