Practical Design of DC Switching Power Supply Protection Circuit

1 Overview

With the development of science and technology, the relationship between power electronic equipment and people's work and life is becoming increasingly close, and electronic equipment cannot do without a reliable power supply. Therefore, DC switching power supplies have begun to play an increasingly important role and have successively entered various electronic and electrical equipment fields. Program-controlled switches, communications, electronic detection equipment power supplies, control equipment power supplies, etc. have widely used DC switching power supplies. At the same time, with the development of many high-tech technologies, including high-frequency switching technology, soft switching technology, power factor correction technology, synchronous rectification technology, intelligent technology, surface mounting technology, etc., switching power supply technology is constantly innovating, which provides a wide range of development space for DC switching power supplies. However, due to the relatively complex control circuit in the switching power supply, the transistors and integrated devices have poor ability to withstand electrical and thermal shocks, which brings great inconvenience to users during use. In order to protect the safety of the switching power supply itself and the load, overheat protection, overcurrent protection, overvoltage protection and soft start protection circuits are designed according to the principles and characteristics of the DC switching power supply.

2 Principles and characteristics of switching power supply

2.1 Working Principle

The DC switching power supply consists of an input part, a power conversion part, an output part, and a control part. The power conversion part is the core of the switching power supply, which performs high-frequency chopping on unstable DC and completes the conversion function required for the output. It is mainly composed of a switching transistor and a high-frequency transformer. Figure 1 shows the schematic diagram and equivalent schematic diagram of the DC switching power supply, which consists of a full-wave rectifier, a switching tube V, an excitation signal, a freewheeling diode Vp, an energy storage inductor, and a filter capacitor C. In fact, the core part of the DC switching power supply is a DC transformer.

2.2 Features

In order to meet the needs of users, major switching power supply manufacturers at home and abroad are committed to the simultaneous development of new and highly intelligent components, especially by improving the loss of secondary rectifiers and increasing technological innovation in power ferrite (Mn-Zn) materials to improve the magnetic properties at high frequencies and large magnetic flux densities. At the same time, the application of SMT technology has made great progress in switching power supplies, arranging components on both sides of the circuit board to ensure that the switching power supply is light, small and thin. Therefore, the development trend of DC switching power supplies is high frequency, high reliability, low consumption, low noise, anti-interference and modularization.

The disadvantages of DC switching power supplies are that they have serious switching interference and are less able to adapt to harsh environments and sudden failures. Since domestic microelectronics technology, resistor and capacitor device production technology, and magnetic material technology are still somewhat behind those of some technologically advanced countries, the production technology of DC switching power supplies is difficult, maintenance is troublesome, and the cost is high.

3 Protection of DC switching power supply

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.

3.1 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 being cut off and automatically cutting off the circuit current. As shown in Figure 1, the overcurrent protection circuit consists of a transistor BG2 and voltage divider resistors R4 and R5. When the circuit is working normally, the base potential of BG2 is higher than the emitter potential through the pressure 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) and 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.

3.2 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, 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. Figure 3 shows 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 actuates, 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.

3.3 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 will also damage the capacitor, shorten its life and damage it prematurely. For this reason, a current limiting resistor should be connected when the power is turned on, and the capacitor is 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, after the transient process of power on is over, a relay is used to automatically short it 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.

As shown in Figure 4(a), at the moment the power is turned 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 the circuit shown in Figure 4(b) instead of the RC delay circuit.

3.4 Overheat protection circuit

The high integration and light weight of the switching regulator in the DC switching power supply greatly improves its power density per unit volume. Therefore, if the components inside the power supply device do not have a correspondingly higher requirement for the working environment temperature, the circuit performance will inevitably deteriorate and the components will fail prematurely. Therefore, an overheat protection circuit should be set in the high-power DC switching power supply.

This paper uses a temperature relay to detect the temperature inside the power supply device. When the power supply device overheats, the temperature relay will be activated, so that the whole machine alarm circuit is in an alarm state, realizing overheat protection of the power supply. As shown in Figure 5(a), the P-type control gate thermal thyristor is placed near the power switch transistor in the protection circuit. According to the characteristics of TT102 (the conduction temperature of the device is determined by the Rr value, and the larger the Rr, the lower the conduction temperature), when the tube shell temperature of the power tube or the temperature inside the device exceeds the allowable value, the thermal thyristor will be turned on, making the light-emitting diode light up to alarm. If it is combined with an optocoupler, the whole machine alarm circuit can be activated to protect the switching power supply. The circuit can also be designed as shown in Figure 5(b), which is used as an overheat protection of the power transistor. The base current of the transistor switch tube is bypassed by the N-type control gate thermal thyristor TT201, the switch tube is cut off, and the collector current is cut off to prevent overheating.

4 Summary

This article mainly discusses various protection methods for the internal components of DC switching power supplies and introduces some specific circuits. For a given DC switching power supply, whether the protection circuit is complete and works according to the predetermined settings is crucial to the safety and reliability of the power supply device. Because the protection schemes and circuit structures of switching power supplies are diverse, a reasonable protection scheme and circuit structure should be selected for a specific power supply device. In practical applications, several protection methods are usually combined to form a complete protection system to ensure the normal operation of the DC switching power supply.