Analysis and design of high efficiency Quasi-Resonant Buck converter

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, the switching loss is eliminated. This thesis analyses the resonant between the output inductor and the capacitor in detail, and find out the time when the voltage across the switch is zero. Then by using a simple zero current detective method and a properly delay circuit, to assure ZVS and ZCS turn-on of the switch. When the Buck converter works in BCM mode the output filter inductor is small, the reverse recovery loss of diode is eliminated, 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 3kw prototype is designed, the highest efficiency is 98.7%.