Current driven synchronous rectifier for LLC resonant converter with a novel integrated current transformer

This paper proposes a synchronous rectifier with a current transformer integrated with the main transformer. This Integrated Synchronous Rectifier is insensitive to parasitic inductance. It is completely self-contained and needs no external power supply. The integrated current transformer reduces circuit joints in the power path and makes the system more compact. A 0.5mΩ soldering joint in 25A current path produces 0.31W loss. A method to integrate the current transformer into the main transformer is explained. A model is suggested to explain the operation principle of the proposed Integrated Synchronous rectifier. An enhanced driving circuit is designed to provide accurate turn on and off timing for the synchronous rectifier MOSFET. A 300W 400V-12V converter is built to demonstrate the advantages of the proposed current driven synchronous rectifier. It can achieve 97.2% efficiency. There is 3.4% efficiency increase compared with schottky diode.