A Protection Design Scheme Based on an LED Switching Power Supply
LED switch power supply overcurrent protection circuit
In the DC LED switch power supply circuit, in order to protect the adjustment tube from being burned out in case of circuit short circuit and current increase. 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.
LED switch power supply overvoltage protection circuit
The overvoltage protection of the switch regulator in DC LED 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 in the switch regulator is too high, it will cause the switch regulator to malfunction and even damage internal components. Therefore, it is necessary to use input overvoltage protection circuits in LED switch power supplies.
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 voltage regulator diode, the voltage regulator tube breaks down, and a current flows through the resistor R, causing transistor T to conduct. The relay acts, and the normally closed contact opens, cutting 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 discrimination and overvoltage protection circuit.
LED switch power supply soft start 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 LED switch power source.
