When the inductor current of boundary conduction mode (BCM) Buck converter falls to zero, the output filter inductor will resonate with the capacitor across the switch, called quasi-resonant (QR). If the switch is turned on when the voltage of resonant capacitor is zero, ZVS and ZCS turn-on can be achieved, and the switching loss is eliminated. This thesis analyses the resonant between the output inductor and the capacitor in detail, and finds out the time when the voltage across the switch is zero. Then, by using a simple zero current detective method and a properly delayed circuit, ZVS and ZCS turn-on of the switch is assured. When the Buck converter works in BCM mode, the output filter inductor is small, the reverse recovery loss of diode is eliminated, and dynamic response time can be reduced. And there is a capacitor across the switch; when it is turned off, ZVS turn-off is achieved. At last, a 3 kW prototype is designed, and the highest efficiency is 98.7%.