In the pulse modulation circuit, R and L resonant circuits are added, so that the current flowing through the switch and the voltage drop at both ends of the pipe are the sine wave. This switching power supply becomes a resonant switching power supply. Using certain control technology, the switch tube can be switched when the current or voltage waveform is over zero, so as to reduce the volume of the power supply, increase the power supply control capacity, improve the switching speed, improve ripple has great benefits. So resonant switching power supply is the main technology of switching power supply.Also divided into: 1. ZCS -- zero current switch.The switch switches off at zero current. 2. ZVS -- zero voltage switch.The switch is turned off at zero voltage.