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　　Overcurrent protection circuit design for switching power supply

　　1 Commonly used overcurrent protection circuits for switching power supplies

　　1.1 Use current sensors for current detection

　　The working principle of the overcurrent detection sensor is shown in Figure 1. The secondary current of the converter obtained through the converter is converted into voltage by I/V. After the voltage is converted into DC, it is compared with the set value by the voltage comparator. If the DC voltage is greater than the set value, an identification signal is issued. . However, this kind of detection sensor is generally used to monitor the load current of the induction power supply. For this reason, the following measures need to be taken. Since when the induction power supply starts, the starting current is several times the rated value, which is much larger than the current at the end of the start. Therefore, when the current battery is simply monitored, the necessary output signal should be obtained when the induction power supply starts. It must be used The timer sets the prohibition time so that the induction power supply does not output unnecessary signals before the end of startup. After the timer expires, it switches to the predetermined monitoring state.

　　1.2 Start the inrush current limiting circuit

　　When the switching power supply is powered on, it will generate a high surge current. Therefore, a soft-start device to prevent surge current must be installed at the input end of the power supply to effectively reduce the surge current to the allowable range. The surge current is mainly caused by the charging of the filter capacitor. At the moment when the switch tube starts to conduct, the capacitor shows a low impedance to the AC. If no protective measures are taken, the surge current can approach hundreds of A.

　　The input of the switching power supply generally uses a capacitor rectifier filter circuit as shown in Figure 2. The filter capacitor C can be a low-frequency or high-frequency capacitor. If a low-frequency capacitor is used, a high-frequency capacitor with the same capacity needs to be connected in parallel to bear the charge and discharge current. The current limiting resistor Rsc inserted in series between the rectification and filtering in the figure is to prevent the impact of surge current. When closing, Rsc limits the charging current of capacitor C. After a period of time, when the voltage on C reaches the preset value or the voltage on capacitor C1 reaches the action voltage of relay T, Rsc is short-circuited to complete the startup. At the same time, circuits such as thyristors can also be used to short-circuit Rsc. When closing, since the thyristor is turned off, the capacitor C is charged through Rsc. After a period of time, the thyristor is triggered to conduct, thus short-circuiting the current limiting resistor Rsc.

　　1.3 Current limiting circuit using base drive circuit

　　Under normal circumstances, the base drive circuit is used to isolate the control circuit of the power supply from the switching transistor. The control circuit and the output circuit share the same ground, and the current limiting circuit can be directly connected to the output circuit. The working principle is shown in Figure 3. When the output is overloaded or short-circuited, V1 is turned on, and the voltage at both ends of R3 increases, and is inverted with the comparator. terminal reference voltage comparison. Control PWM signal on and off.

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