Design of protection circuit based on common DC switching power supply

Overview

With the development of science and technology, power electronic equipment and people's work, life, and the relationship between the increasingly close, and electronic equipment can not be separated from the reliable power supply, so the DC switching power supply began to play a more and more important role, and successively entered a variety of electronic, electrical equipment fields, program-controlled control equipment, such as power supply has been widely used in DC switching power supply [1-3]. At the same time, with many High-tech, including high-frequency switch technology, soft switch technology, power factor correction technology, synchronous rectification technology, intelligent technology, surface installation technology and other technology development, switching power supply technology is constantly innovating, which provides a wide range of development space for DC switching power supply. However, because of the complexity of the control circuit in switching power supply, transistors and integrated devices are less capable of withstanding electric and thermal shock, which brings great inconvenience to the users in the process of use. In order to protect the switching power supply itself and the load, according to the principle and characteristics of DC switching power supply, the design of overheat protection, overcurrent protection, over-voltage protection and soft start protection circuit.

Principle and characteristics of 2 switching power supply

2.1 Working principle

The DC switching power supply consists of the input part, the power conversion part, the output part and the control part. The Power conversion part is the core of the switching power supply, which is the high-frequency chopper of the unsteady DC and the transformation function required to complete the output. It is mainly composed of switch transistor and high-frequency transformer. Fig. 1 draws the schematic diagram of the DC switching power supply and the equivalent principle block diagram, which is composed of a full wave rectifier, a switch tube V, an excitation signal, a continuation diode VP, a 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 high intelligent components, especially by improving the loss of two rectifier devices, and in the Power Ferrite (MN-ZN) materials to increase scientific and technological innovation in order to improve the high frequency and greater magnetic flux density to obtain high magnetic performance, At the same time, the application of SMT technology makes the switching power supply made great progress, on both sides of the circuit board layout components to ensure that the switching power supply 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 there are more serious switch interferences, and the ability to adapt to bad environment and sudden failure is weak. As a result of the domestic microelectronics technology, blocking container parts production technology and magnetic materials technology and some advanced countries still have a certain gap, so the DC switching power supply technology difficult, maintenance trouble and high cost costs,

3 Protection of DC switching power supply

Based on the characteristics of DC switching power supply and the actual electrical condition, in order to make the DC switching power supply work safely and reliably under the condition of bad environment and sudden failure, this paper designs a variety of protection circuits according to different conditions.

3.1 Overcurrent protection circuit

In the DC switching power supply circuit, in order to protect the adjustment tube in the circuit short-circuit, the current increases not be burned. The basic method is that when the output current exceeds a certain value, the adjustment tube is in the reverse offset state, thus cut-off, automatically cut off the circuit current. As shown in Fig. 1, overcurrent protection circuit consists of transistor BG2 and partial voltage resistor R4 and R5. When the circuit is working normally, the pressure action of R4 and R5 makes the BG2 base electrode potential higher than the emitter potential, and the transmitting junction bears the reverse voltage. So the BG2 in the cut-off state (equivalent to open circuit), no effect on the voltage regulator. When the circuit short-circuit, the output voltage is zero, BG2 emission is equivalent to grounding, then the BG2 in the saturated conduction state (equivalent to short circuit), so that the adjustment of the BG1 base and launch very close to the short circuit, and in the cut-off state, cut off the circuit current, so as to achieve protection purposes.

3.2 Over voltage protection circuit

Overvoltage protection of switching regulators in DC switching power supply includes input overvoltage protection and output overvoltage protection. If the voltage of the DC power supply (such as batteries and rectifiers) used by the switching voltage regulator is too high, it will cause the switching voltage regulator to not work properly and even damage the internal device, so it is necessary to use the input overvoltage protection circuit in the switching power supply. Fig. 3 is a protective circuit consisting of a transistor and a relay, in this circuit, when the input DC power voltage than voltage regulator diode breakdown voltage, the voltage regulator breakdown, there is current through the resistance r, so that the transistor T-conduction, relay action, often closed contact disconnect, cut off the input. The polarity protection circuit of the input power supply can be combined with the input overvoltage protection, which constitutes the polarity protection identification and overvoltage protection circuit.