Protection of DC switching power supply
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 failures, 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 regulating 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 biased state and thus cuts off, automatically cutting off the circuit current. As shown in Figure 1, the overcurrent protection circuit consists of transistor BG2 and voltage dividing resistors R4 and R5. When the circuit is working normally, the base potential of BG2 is higher than the emitter potential through the pressure effect of R4 and R5, and the emitter junction withstands the reverse voltage. So BG2 is in a cut-off state (equivalent to an open circuit) and has no impact on the voltage stabilizing 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 are in a cut-off state. Cut off the circuit current to achieve protection purposes.
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 batteries and rectifiers) used by the switching regulator is too high, it will cause the switching regulator to not work properly or even damage the internal components. Therefore, it is necessary to use input overvoltage in the switching power supply. protect the circuit. Figure 3 is a protection circuit composed of a transistor and a relay. 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, causing The transistor T is turned on, the relay operates, 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 over-voltage protection to form a polarity protection identification and over-voltage protection circuit.
3. Soft start protection circuit
The switching regulator circuit is relatively complex. The input terminal of the switching regulator is generally connected to an input filter with a small inductor and a large capacitor. 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 can melt the contacts of a regular power switch or relay and cause the input fuse to blow. In addition, surge current will also damage the capacitor, shortening its life and causing premature failure. For this reason, a current-limiting resistor should be connected when starting up, and the capacitor will be charged 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-circuit it after the startup transient process, so that the DC power supply directly supplies power to the switching regulator. , this kind of circuit is called the "soft start" circuit of the DC switching power supply.
4. Overheat protection circuit
The high integration and light weight and small size of the switching regulator in the DC switching power supply have greatly increased the power density per unit volume. Therefore, if the requirements for the working environment temperature of the components inside the power supply device do not increase accordingly, there will inevitably be The circuit performance will deteriorate and components will fail prematurely. Therefore, an overheating protection circuit should be installed in the high-power DC switching power supply.
