U 1 includes square wave generating circuit with adjustable pulse width, shaping circuit, square wave control circuit, driving circuit and square wave power amplifying circuit. The 1 pin is the output pin of the square wave power amplifier, which is used to drive the high-frequency transformer TR 1. Pin 5 is a square wave pulse width adjustment pin. Adjusting the voltage of pin 5 can control the pulse width of the square wave, thus controlling the on-off time of the output of pin/kloc-0, and changing the average DC output voltage of the rectifier filter at the output end of the high-frequency transformer. U2 is an optocoupler, which is used to isolate and feed back the voltage change at the load end.

Circuit working principle:

After the power supply is turned on, the 1 pin of U 1 outputs a square-wave switch control pulse signal, which is obtained at the output end through the high-frequency transformer TR 1.

High-frequency voltage, after high-frequency rectification and filtering by D3 C4, the average DC voltage is obtained. When the output average DC voltage is high, the voltage at pin 1 of U2 rises, the LED inside U2 lights up (brightens), and the phototransistor inside U2 turns on (deepens), so that the voltage at pin 5 of U 1 rises. As the voltage at pin 5 rises, the high-frequency square wave inside U 1 rises. The turn-on time of pin 1 output is shortened, which makes the average DC voltage of TR 1 output lower, and vice versa.