Several Practical Protection Circuit Designs for DC Switching Power Supplies

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 [1-3]. 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, overheating 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

DC switching power supply It 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. It performs high-frequency chopping on unstable DC and completes the conversion function required for output. It is mainly composed of switching transistors and high-frequency transformers. Figure 1 shows the schematic diagram and equivalent principle block diagram of the DC switching power supply, which is composed 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 rectifier devices and increasing technological innovation in power ferrite (Mn-Zn) materials to improve the high 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 disadvantage of DC switching power supplies is that there is a relatively serious switching interference and the ability to adapt to harsh environments and sudden failures is weak. Since there is still a certain gap between domestic microelectronics technology, resistor and capacitor device production technology, and magnetic material technology and some technologically advanced countries, the production technology of DC switching power supply 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 supply and actual electrical conditions, in order to make DC switching power supply work safely and reliably in harsh environment and sudden fault conditions, this paper 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, thus cut off, and automatically cuts off the circuit current. As shown in Figure 1, the overcurrent protection circuit consists of transistor BG2 and voltage divider resistors R4 and R5. When the circuit works normally, through the pressure effect of R4 and R5, the base potential of BG2 is higher than the emitter potential, 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 being grounded, so BG2 is in a saturated conduction state (equivalent to a short circuit), which makes the base and emitter of the adjustment tube BG1 close to a short circuit and in a cut-off state, cutting off the circuit current and achieving the protection purpose.

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.