In the DC LED switching power supply circuit, in order to protect the adjusting tube from being short-circuited and the current is increased, it is not burned. The basic method is that when the output current exceeds a certain value, the adjustment tube is in a reverse bias state, thereby being turned off, and the circuit current is automatically cut off. As shown in Figure 1, the overcurrent protection circuit consists of a triode BG2 and voltage divider resistors R4, R5. When the circuit works normally, the base potential of BG2 is higher than the emitter potential through the pressure of R4 and R5, and the emitter junction is subjected to the reverse voltage. Therefore, BG2 is in the off state (equivalent to an open circuit) and 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. Then BG2 is in a saturated conduction state (corresponding to a short circuit), so that the base and emitter of the adjusting tube BG1 are nearly short-circuited, and are in an off state. Cut off the circuit current for protection purposes. 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 an unregulated DC power source (such as a battery and a rectifier) ​​used in a switching regulator is too high, it will cause the switching regulator to malfunction or even damage the internal components. Therefore, it is necessary to use the input in the LED switching power supply. Voltage protection circuit. Figure 3 is a protection circuit composed of a transistor and a relay. When the voltage of the input DC power supply is higher than the breakdown voltage of the Zener diode, the Zener diode breaks down and a current flows through the resistor R. Transistor T is turned on, the relay operates, 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. LED switching power supply soft start protection circuit The circuit of the switching regulator power supply is complicated, and the input end of the switching regulator is generally connected with an input filter of small inductance and large capacitance. At the instant of power-on, the filter capacitor will flow through a large inrush current, which can be several times the normal input current. Such large inrush currents can melt the contacts of the common power switch contacts or relays and cause the input fuse to blow. In addition, the inrush current also damages the capacitor, shortening its life and prematurely damaging it. To this end, a current limiting resistor should be connected to the power supply to charge the capacitor through this current limiting resistor. In order not to consume too much power of the current limiting resistor, the normal operation of the switching regulator is affected, and after the startup transient process is finished, a relay is automatically shorted to make the DC power supply directly supply the switching regulator. This circuit is called the "soft start" circuit of the DC LED switching power supply. 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 increase the power density per unit volume. Therefore, if the internal components of the power supply device do not have corresponding requirements for the working environment temperature, it is inevitable. Will deteriorate the performance of the circuit and the component will fail prematurely. Therefore, an overheat protection circuit should be provided in the high-power DC LED switching power supply. In this paper, the temperature relay is used to detect the temperature inside the power supply unit. When the power supply unit generates overheating, the temperature relay will act, so that the alarm circuit of the whole machine is in an alarm state to achieve overheat protection of the power supply. As shown in Fig. 4(a), a P-type control gate thermal thyristor is placed in the vicinity of the power switch transistor in the protection circuit. According to the characteristics of the TT102 (the on-state temperature of the device is determined by the Rr value, the larger the Rr, the on-temperature The lower the), when the case temperature of the power tube or the temperature inside the device exceeds the allowable value, the thermal thyristor is turned on, causing the LED to illuminate. If the optocoupler is used, the whole alarm circuit can be operated to protect the LED switching power supply. The circuit can also be designed as the overheat protection of the power transistor as shown in FIG. 4(b). The base current of the crystal 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. Prevent overheating.
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