Efficiency improvement of synchronous rectifier in a ZVS-PWM controlled series-resonant converter with active clamp

A novel ZVS-PWM controlled current-mode resonant converter previously proposed includes an active-damp circuit and a center-tap type synchronous rectifying circuit. This resonant converter has a high efficiency for a specific input voltage. However, as the input voltage varies from the specific value, the efficiency goes down severely. This paper clarifies the cause of this efficiency drop through the state transition analysis. To overcome the efficiency drop, the authors propose the usage of current-doubler type synchronous rectifying circuits with separate inductors and with a coupled inductor, and examine their steady-state characteristics. Furthermore, the output noise voltage characteristics are measured and compared with the center-tap type rectifying circuit. As a consequence, it is clarified that the proposed rectifying circuits have achieved a high efficiency over 85% for a wide variation of input voltage under the load condition of 3.3 V and 5 A, and that they also have extremely low noise characteristics