Practical circuit of ringing switching power supply

The picture below is a practical circuit of a ringing switching power supply. The purpose of the diode VD, resistor R and capacitor C connected to the primary winding Np of the transformer is to discharge the energy accumulated in the leakage inductance of the transformer. In other words, during the conduction period of the switch tube, the current accumulates energy in the leakage inductance, but this part of the energy is not transmitted to the secondary of the transformer as electrical power. Therefore, a reverse electromotive force is generated at the moment when the switch tube is turned off, and a very high surge voltage appears, which overlaps the collector voltage of the switch tube and easily breaks down the switch tube. If a diode and a resistor and capacitor are added, this part of the energy will be stored in the capacitor C through the diode VD and become DC, which can be discharged through the resistor R. The main technical indicators of the power supply are as follows: Input voltage: AC 85~276V; Output voltage: 18V; Output current 2A; Working frequency: 20kHz; Efficiency: 75%.