Analysis and implementation of a quasi-resonant buck converter operating in a constant output power mode

This paper examines a constant frequency quasi-resonant buck converter controlled by PWM. Power converter components are selected such that it operates inherently (without any control) in a constant, maximum power mode over a wide range of the load current. This mode keeps the conduction loss low and in combination with the ZCS for the active devices provides high power conversion efficiency. A complete analysis of the circuit with a finite output inductance value is done. Produced graphical results show that constant power mode can be provided by an appropriate design of the output inductor. Less than 6% variation of the output power is experimentally achieved in the noncontrolled converter for more than 50% variation of its output current. The constant power mode is practically implemented in a 600 W, 130 kHz, phase-shift controlled bridge arrangement of the quasi-resonant buck converter. This converter circuit is very robust, operates in a ZVS mode for the MOSFETs and ZCS for the output rectifiers and features more than 95% efficiency.