1. When the output terminal is short-circuited, the PWM control circuit can limit the output current within a safe range. It can use a variety of methods to realize the current-limiting circuit. When the power current-limiting does not work in the short-circuit, only another Add part of the circuit.

2. There are usually two types of short-circuit protection circuits. The figure below is a low-power short-circuit protection circuit. The 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 at pin UC3842 ① rises to about 5V, the partial pressure of R1 and R2 exceeds the TL431 reference, so that it is turned on, the VCC potential of pin UC3842 ⑦ is pulled down, and the IC stops working . After the UC3842 stops working, the potential of the pin disappears, and the TL431 is not turned on. The potential of the UC3842 rises, and the UC3842 restarts, repeating the cycle. When the short-circuit phenomenon disappears, the circuit can automatically return to the normal working state.

3. The figure below is a medium-power short-circuit protection circuit, and its principle is briefly described as follows:

When the output is short-circuited, the voltage on pin UC3842 ① rises, and the potential of pin U1 ③ is higher than pin ②, the comparator flips pin ① to output a high potential to charge C1. When the voltage across C1 exceeds the reference voltage of pin ⑤, pin U1 ⑦ outputs a low potential, and pin UC3842 ① 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 the time constants of charging and discharging, and the short-circuit protection will not work when the resistance value is wrong.

4. The figure below is a common current limiting and short circuit protection circuit. The working principle is briefly described as follows:

When the output circuit is short-circuited or over-current, the primary current of the transformer increases, the voltage drop across R3 increases, and the voltage at pin ③ rises, and the output duty cycle of pin UC3842 ⑥ gradually increases. When the voltage at pin ③ exceeds 1V, UC3842 turns off without output. .

5. The following figure is a protection circuit that uses a current transformer to sample the current. It has low power consumption, but is high in cost and more complicated.

The output circuit is short-circuited or the current is too large, the higher the voltage induced by the TR1 secondary coil will be. When the UC3842 ③ pin exceeds 1 volt, the UC3842 will stop working and repeat itself. When the short circuit or overload disappears, the circuit recovers by itself.