Various protection circuits for internal components of DC power supply
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 reliable power sources. Therefore, DC switching power supplies have begun to play an increasingly important role and have entered various fields of electronic and electrical equipment. DC switching power supplies have been widely used in program controlled switches, communication, electronic detection equipment power supplies, control equipment power supplies, etc. 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 installation technology, etc., switching power supply technology is constantly innovating, providing a wide range of development space for DC switching power supply. However, due to the complexity of the control circuit in switching power supplies, transistors and integrated devices have poor resistance to 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, based on the principles and characteristics of DC switching power supply, overheating protection, overcurrent protection, overvoltage protection, and soft start protection circuits have been designed.
operational 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 output. It mainly consists of a switching transistor and a high-frequency transformer. Figure 1 shows the schematic diagram and equivalent schematic diagram of a DC switching power supply, which is composed of a full wave rectifier, switching transistor V, excitation signal, freewheeling diode Vp, energy storage inductor, and filtering capacitor C. In fact, the core part of a DC switching power supply is a DC transformer.
characteristic
In order to meet the needs of users, major switch power supply manufacturers both domestically and internationally are committed to synchronously developing new types of highly intelligent components, especially by improving the losses of secondary rectification devices and increasing technological innovation in power ferrite (Mn Zn) materials to improve the ability to obtain high magnetic properties at high frequencies and high magnetic flux densities. At the same time, the application of SMT technology has made significant progress in switch power supplies, Arrange 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 supply is high-frequency, high reliability, low consumption, low noise, anti-interference, and modularization.
The disadvantage of DC switching power supply is that it has severe switching interference and weak ability to adapt to harsh environments and sudden faults. Due to the gap between domestic microelectronics technology, production technology of resistive and capacitive devices, and magnetic material technology and some technologically advanced countries, the production technology of DC switching power supplies is difficult, maintenance is troublesome, and the cost is high.
Protection of DC switching power supply
Based on the characteristics and actual electrical conditions of DC switching power supplies, in order to ensure safe and reliable operation of DC switching power supplies in harsh environments and sudden faults, this article designs various protection circuits according to different situations.
Overcurrent protection circuit
In a DC switching power supply circuit, in order to protect the adjustment tube from being burned out when the circuit is short circuited or the current increases. The basic method is to adjust the transistor to a reverse bias state when the output current exceeds a certain value, thereby cutting off and automatically cutting off the circuit current. The overcurrent protection circuit consists of a transistor BG2 and a voltage divider resistor R4 and R5. When the circuit is working normally, the voltage action between R4 and R5 causes the base potential of BG2 to be higher than the emitter potential, and the emitter junction bears reverse voltage. So BG2 is in a cut-off state (equivalent to an open circuit), which 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 ground. Therefore, BG2 is in a saturated conduction state (equivalent to a short circuit), making the base and emitter of adjustment tube BG1 close to a short circuit and in a cut-off state, cutting off the circuit current and achieving protection purposes.
Overvoltage protection circuit
The circuit of a switch regulated power supply is relatively complex, and the input end of the switch regulated power supply is generally connected to an input filter with small inductance and large capacitance. At the moment of startup, the filter capacitor will flow a large surge current, which can be several times the normal input current. Such a large surge current will melt the contacts of ordinary power switches or relays, and cause the input fuse to blow. In addition, surge currents can also damage capacitors, shortening their lifespan and causing premature damage. For this reason, a current limiting resistor should be connected when starting up, and the capacitor should be charged through this current limiting resistor. In order to prevent the current limiting resistor from consuming too much power, which may affect the normal operation of the switch regulator, a relay is used to automatically short circuit it after the transient process of startup, so that the DC power supply directly supplies power to the switch regulator. This circuit is called the "soft start" circuit of the DC switch power supply.
