Soft start protection circuit
The circuit of the switch regulator is complex, and the input end of the switch regulator is generally connected with the input filter with small inductance and large capacitance. At the moment of starting up, the filter capacitor flows through a large surge current, which can be several times the normal input current. Such a large surge current would melt the contacts of a conventional power switch or relay and cause the input fuse to blow out. In addition, the surge current will also damage the capacitor, which will shorten its life and cause premature damage. For this purpose, a current limiting resistor should be connected at startup to charge the capacitor. In order not to make the current-limiting resistance consume too much power, so as to affect the normal work of the switching voltage regulator, after the power on transient process is over, a relay is used to automatically short-connect it, so that the DC power supply directly supplies power to the switching voltage regulator. This circuit is called the "soft start" circuit of the DC LED switching power supply.
At the moment of power on, the input voltage is charged by the rectifier bridge (D1 ~ D4) and the current-limiting resistor R1 to limit the surge current. The inverter works normally when the capacitor C is charged to about 80% of the rated voltage. The thyristor trigger signal is generated through the auxiliary winding of the main transformer to make the thyristor conduct and short circuit the current limiting resistor R1, and the LED switching power supply is in normal operation state. In order to improve the accuracy of delay time and prevent the jitter and oscillation of relay operation, RC delay circuit can be replaced by a delay circuit.

Overheat protection circuit
The high integration and light weight and small volume of the switching voltage regulator in the DC LED switching power supply greatly improve the power density within the unit volume. Therefore, if the internal components of the power supply do not meet the requirements of the working environment temperature, the circuit performance will inevitably deteriorate and the components will fail prematurely. Therefore, overheating protection circuit should be set 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 device. When the power supply device is overheated, the temperature relay will act to make the alarm circuit of the whole machine in the state of alarm, so as to realize the overheating protection of the power supply. P type in protection circuit control gate hot thyristor is placed near the power switch triode, according to the characteristics of TT102 (by the Rr values determine the conduction temperature of the device, the greater the Rr, conducting the lower temperature), when the power tube shell temperature or the temperature inside the unit exceeds allowable values, thermal thyristor conduction, making leds light alarm. If coupled with the photoelectric coupler, the whole machine can make the alarm circuit action, protect the LED switching power supply. The circuit can also be designed with the crystal tube power overheat protection, the base current of the crystal switch tube by the N-type control gate thermal thyristor TT201 bypass, switch tube cut-off, cut off collector current, prevent overheating.

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