DC switching power supply protection circuit

Based on the characteristics of DC switching power supplies and actual electrical conditions, in order to make the DC switching power supply work safely and reliably in harsh environments and sudden faults, this article designs a variety of protection circuits according to different situations.

1. Overcurrent protection circuit

In the DC switching power supply circuit, in order to protect the adjustment tube from being burned when the circuit is short-circuited and the current increases. The basic method is that when the output current exceeds a certain value, the adjustment tube is in a reverse bias state, thereby cutting off and automatically cutting off the circuit current.

The overcurrent protection circuit consists of a transistor BG2 and voltage divider resistors R4 and R5.

When the circuit works normally, the emitter potential of BG2 is higher than the base potential through the voltage divider effect of R4 and R5, and the emitter junction is subjected to reverse voltage. Therefore, BG2 is in a cut-off state (equivalent to an open circuit), which has no effect on the voltage stabilization circuit.

When the circuit is short-circuited, the output voltage is zero, and the emitter of BG2 is equivalent to grounding. Then BG2 is in a saturated conduction state (equivalent to a short circuit), so that the base and emitter of the adjustment tube BG1 are close to a short circuit and in a cut-off state, cutting off the circuit current, thereby achieving the purpose of protection.

2. Overvoltage protection circuit

The overvoltage protection of the switching regulator in the DC switching power supply includes input overvoltage protection and output overvoltage protection. If the voltage of the unregulated DC power supply (such as a battery and a rectifier) ​​used by the switching regulator is too high, the switching regulator will not work properly and even damage the internal components. Therefore, it is necessary to use an input overvoltage protection circuit in the switching power supply. A

protection circuit composed of transistors and relays. In this circuit, when the voltage of the input DC power supply is higher than the breakdown voltage value of the Zener diode, the Zener diode breaks down, and current flows through the resistor R, turning on the transistor T, the relay is activated, the normally closed contact is disconnected, and the input is cut off. The polarity protection circuit of the input power supply can be combined with the input overvoltage protection to form a polarity protection identification and overvoltage protection circuit.

3. Soft start protection circuit

The circuit of the switching power supply is relatively complex. The input end of the switching regulator is generally connected to an input filter with small inductance and large capacitance. At the moment of power on, a large surge current will flow through the filter capacitor, which can be several times the normal input current. Such a large surge current will melt the contacts of the ordinary power switch or the contacts of the relay and blow the input fuse. In addition, the surge current is based on the characteristics of the DC switching power supply and the actual electrical conditions. In order to make the DC switching power supply work safely and reliably in harsh environments and sudden faults, this article designs a variety of protection circuits according to different situations.

At the moment of power on, the input voltage charges the capacitor C through the rectifier bridge (D1~D4) and the current limiting resistor R1 to limit the surge current. When the capacitor C is charged to about 80% of the rated voltage, the inverter works normally. The trigger signal of the thyristor is generated through the auxiliary winding of the main transformer, which turns on the thyristor and short-circuits the current limiting resistor R1, and the switching power supply is in normal operation. In order to improve the accuracy of the delay time and prevent the relay from jittering and oscillating, the delay circuit can use a circuit to replace the RC delay circuit.

The high integration and light weight and small size of the switching regulator in the DC switching power supply greatly improve its power density per unit volume. Therefore, if the components inside the power supply device do not have corresponding requirements for the working environment temperature, the circuit performance will inevitably deteriorate and the components will fail prematurely. Therefore, an overheating protection circuit should be set in the high-power DC switching power supply to charge the capacitor through this current limiting resistor. In order to prevent the current limiting resistor from consuming too much power and affecting the normal operation of the switching regulator, a relay is used to automatically short it after the startup transient process is over, so that the DC power supply directly supplies power to the switching regulator. This circuit is called the "soft start" circuit of the DC switching power supply.

4. Overheating protection circuit

The high integration and light weight and small size of the switching regulator in the DC switching power supply greatly improve its power density per unit volume. Therefore, if the components inside the power supply device do not have corresponding requirements for the working environment temperature, the circuit performance will inevitably deteriorate and the components will fail prematurely. Therefore, an overheat protection circuit should be provided in the high-power DC switching power supply.