Five practical protection circuit diagrams for LED switching power supply

　　LED switching power supply overcurrent protection circuit, LED switching power supply overvoltage protection circuit, LED switching power supply soft start protection circuit, LED switching power supply overheat protection circuit... Industry insiders contribute five practical circuit diagrams to help you design the protection of LED switching power supply.

　　LED switching power supply overcurrent protection circuit

　　In the DC LED switching power supply circuit, in order to protect the adjustment tube from being burned when the circuit is short-circuited and the current increases. The basic method is that when the output current exceeds a certain value, the adjustment tube is in a reverse bias state, thus cut off, and automatically cuts 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 base potential of BG2 is higher than the emitter potential through the pressure effect of R4 and R5, and the emitter junction is subjected to reverse voltage. Therefore, BG2 is in a cut-off state (equivalent to an open circuit) and has no effect on the voltage stabilization circuit. When the circuit is short-circuited, the output voltage is zero, and the emitter of BG2 is equivalent to grounding, then BG2 is in a saturated conduction state (equivalent to a short circuit), so that the base and emitter of the adjustment tube BG1 are close to a short circuit, and in a cut-off state, cutting off the circuit current, thereby achieving the purpose of protection.

Figure 1 DC switching power management

Figure 2 LED switching power supply input overcurrent protection circuit

　　LED switching power supply overvoltage protection circuit

　　The overvoltage protection of the switching regulator in the 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 by the switching regulator is too high, the switching regulator will not work properly and even damage the internal components. Therefore, it is necessary to use an input overvoltage protection circuit in the LED switching power supply. Figure 3 shows 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 Zener diode, the Zener diode breaks down, and current flows through the resistor R, turning on the transistor T, the relay actuates, the normally closed contact is disconnected, and the input is cut off. 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.

Figure 3 LED switching power supply input overvoltage protection circuit 　　LED switching power supply soft start protection circuit

　　The circuit of the switching power supply is relatively complex. The input end of the switching regulator is generally connected to an input filter with a small inductance and a large capacitance. At the moment of power on, a large surge current will flow through the filter capacitor, which can be several times the normal input current. Such a large surge current will melt the contacts of the ordinary power switch or the contacts of the relay and blow the input fuse. In addition, the surge current will also damage the capacitor, shorten its life and damage it prematurely. For this reason, a current limiting resistor should be connected when the power is turned on, and the capacitor is charged through this current limiting resistor. In order to prevent the current limiting resistor from consuming too much power and affecting the normal operation of the switching regulator, after the transient process of power on is over, a relay is used to automatically short it 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.

Figure 4 LED switching power supply soft start protection circuit

　　LED switching power supply overheat protection circuit

　　The high integration and light weight of the switching regulator in the DC LED switching power supply greatly improves its power density per unit volume. Therefore, if the components inside the power supply device do not have a correspondingly higher requirement for the working environment temperature, the circuit performance will inevitably deteriorate and the components will fail prematurely. Therefore, an overheat protection circuit should be set in the high-power DC LED switching power supply.

Figure 5 LED switching power supply overheat protection circuit

　　This paper uses a temperature relay to detect the temperature inside the power supply device. When the power supply device overheats, the temperature relay will be activated, so that the whole machine alarm circuit is in an alarm state, realizing overheat protection of the power supply. As shown in Figure 4 (a), the P-type control gate thermal thyristor is placed near the power switch transistor in the protection circuit. According to the characteristics of TT102 (the conduction temperature of the device is determined by the Rr value, and the larger the Rr, the lower the conduction temperature), when the tube shell temperature of the power tube or the temperature inside the device exceeds the allowable value, the thermal thyristor will be turned on, making the light-emitting diode light up to alarm. If it is combined with an optocoupler, the whole machine alarm circuit can be activated to protect the LED switching power supply. The circuit can also be designed as shown in Figure 4 (b), which is used as an overheat protection of the power transistor. The base current of the transistor switch tube is bypassed by the N-type control gate thermal thyristor TT201, the switch tube is cut off, and the collector current is cut off to prevent overheating.