Various protection circuit designs for internal devices of switching power supplies
Protection of DC switching power supply
Based on the characteristics of DC switching power supply and the actual electrical situation, in order to make the DC switching power supply work safely and reliably in the harsh environment and sudden failure, this paper designs a variety of protection circuits according to different situations.
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 to be burned. The basic method is that when the output current exceeds a certain value, the adjustment tube is in reverse bias state, thus cutting off, automatically cut off the circuit current. As shown in Figure 1, the overcurrent protection circuit consists of transistor BG2 and voltage divider resistors R4 and R5. Circuit normal operation, through the pressure of R4 and R5, making the base of BG2 potential than the emitter potential is high, the emitter junction to withstand the reverse voltage. So BG2 is in the cut-off state (equivalent to open circuit), no effect on the voltage regulator circuit. When the circuit is short-circuited, the output voltage is zero, the emitter of BG2 is equivalent to ground, then BG2 is saturated conduction state (equivalent to a short circuit), so that the regulator BG1 base and emitter is close to short-circuit, and is in the cut-off state, cut off the circuit current, so as to achieve the purpose of protection.
Overvoltage protection circuit
DC switching power supply switching regulator overvoltage protection including input overvoltage protection and output overvoltage protection. If the switching regulator used by the unregulated DC power supply (such as batteries and rectifiers) if the voltage is too high, will lead to the switching regulator can not work properly, or even damage to the internal devices, so the switching power supply there is a need to use the input overvoltage protection circuit. Figure 3 for the transistor and relay protection circuit, in the circuit, when the input DC power supply voltage is higher than the breakdown voltage of the regulator diode value, the regulator breakdown, there is a current flow through the resistor R, so that the transistor T conductive, the relay action, normally closed contact disconnect, cut off the input. 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.
Soft-start protection circuit
Switching regulator power supply circuit is more complex, the input of the switching regulator is generally connected to the input filter with small inductance and large capacitance. At the moment of switching on, the filter capacitor will flow a large inrush current, which can be several times of the normal input current. Such a large inrush current can melt the contacts of a normal power switch or relay and blow the input fuse. Also, inrush currents can damage capacitors, shortening their life and damaging them prematurely. For this reason, a current limiting resistor should be connected at power-up, through which the capacitor is charged. In order not to make the current limiting resistor consumes too much power, so as to affect the normal operation of the switching regulator, and in the end of the power-on transient process, with a relay to automatically short it, so that the DC power supply directly to the switching regulator power supply, this circuit is known as the DC switching power supply "soft start" circuit .
