Improved Synchronous Rectifier Driving Strategy for Primary-Side Regulated (PSR) Flyback Converter in Light-Load Mode

Based on the concept derivation, this paper proposes an improved synchronous rectifier (SR) driving strategy focusing on the quasi-resonant primary-side regulation (QR-PSR) Flyback converter, for the applications of dc–dc front-end converter and ac–dc PFC converter both, to prevent reverse energy flowing in light-load/standby mode. Three different implementation candidates are exhibited and analyzed. This proposed driving strategy is programmable for the practical design with input voltage feedforward effect and helpful for achieving the rigorous target of low power loss under light output load condition presented by the U.S. Department of Energy (DoE). Two 60 W (12 V/5 A) lab-made prototypes of synchronous rectified QR-PSR Flyback ac–dc PFC converter and dc–dc front-end converter are built up, respectively, to verify the feasibility of this proposed driving strategy. In the lab-made prototype of ac–dc PFC converter, it improves the overall conversion efficiency more than 3% in light-load mode; in the lab-made prototype of dc–dc front-end converter, it saves the light load power loss up to 0.5 W, especially with 110 
$V_{\rm ac}$
input. Reliable operation is achieved when the proposed SR driving strategy is practically designed with enough margin; however, too large design margin could decrease the average conversion efficiency.