Introduction to a protection scheme based on an LED switching power supply
LED switch power overcurrent protection circuit, LED switch power overvoltage protection circuit, LED switch power soft start protection circuit, LED switch power overheat protection circuit... Industry professionals have contributed several practical circuit diagrams to help you design protection for LED switch power supplies.
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 when the circuit is short circuited or the current increases. The basic method is that when the output current exceeds a certain value, the regulating tube is in a reverse bias state, thereby cutting off and automatically cutting off the circuit current. As shown in Figure 1, the overcurrent protection circuit consists of a transistor BG2 and voltage divider resistors R4 and R5.
When the circuit is working normally, the voltage applied by R4 and R5 causes the base potential of BG2 to be higher than the emitter potential, and the emitter junction is subjected to a reverse voltage. So BG2 is in an off state (equivalent to an open circuit), which has no effect on the voltage regulator circuit. When the circuit is short circuited, the output voltage is zero, and the emitter of BG2 is equivalent to ground. BG2 is in a saturated conducting state (equivalent to a short circuit), which makes the base and emitter of the regulating tube BG1 close to a short circuit and in a cut-off state, cutting off the circuit current and achieving the purpose of protection.
LED Switching Power Supply Overvoltage Protection Circuit
The overvoltage protection of the switching 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 switching regulator is too high, it will cause the switching regulator to malfunction and even damage internal components. Therefore, it is necessary to use an input overvoltage protection circuit in LED switching 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 diode breaks down, and a current flows through the resistor R, causing the transistor T to conduct and the relay to operate. 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 switching regulator power supply is relatively complex, and the input terminal of the switching regulator is generally connected to an input filter with small inductance and large capacitance. At the moment of startup, the filtering capacitor will experience 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 melt. In addition, surge currents can also damage capacitors, shorten their lifespan, and cause premature damage. For this reason, a current limiting resistor should be connected during startup to charge the capacitor. In order to prevent the current limiting resistor from consuming too much power and affecting the normal operation of the switching 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 switching regulator. This circuit is called the "soft start" circuit of the DC LED switching power supply.
