Investigation of a Soft-Switching Flyback Converter With Full Secondary Side-Based Control

A conventional Flyback converter's controller is based on the primary side of the coupled inductor-requiring isolated coupling for both the output voltage measurement and for any synchronous rectification gate signal. A novel communication approach is proposed here, whereby the need for such signal coupling is eliminated by moving the controller to the secondary side and communicating back to the primary switch through the coupled inductor itself. This simultaneously allows direct access to the output voltage, reduced component count, synchronous rectification, and soft primary switch turn-on without optoisolator propagation delay or additional magnetic coupling. This paper presents a novel drain-source voltage sensing method that provides reliable turn-on signals for the primary switch along with the principles of the secondary-side control. Concept feasibility is proven using a 65 W demonstrator with a simple constant-on-time control scheme. The benefits, limitations, and suggested improvements are presented.