Research On Driving Technology For Synchronous Rectification Switch Converter

Synchronous rectification is a novel technology in which the fast recovery diode or the schottky barrier diode is substituted with a very low on-resistance MOSFET. Synchronous rectification technology has already been applied widely in low voltage and high current output switching converters. The key to synchronous rectification is the design of the driven circuits.In this thesis, the work principle, advantage and disadvantage of various driven modes of synchronous rectification are analyzed in detail. It is found that the voltage self-driven synchronous rectification converter has the best future for its simple structure, economy and high efficiency. The voltage self-driven synchronous rectification applied in two-transistor forward converter is analyzed.The losses of synchronous rectification driven circuit with secondary winding direct-driven circuit, gate charge retention voltage driven circuit and output filter inductance coupling voltage driven circuit in two-transistor forward converter are analyzed. It is pointed out that the efficiency reduces seriously because of the body diode losses due to its conduction during the dead-time in the secondary winding direct-driven circuit. The gate charge retention driven circuits can effectively solve the problem that the diode turns on during the dead time in Two-Transistor forward synchronous rectification converter, thus reduces the rectification loss and increases the efficiency. The output filter inductor coupling voltage driven circuit can also effectively solve the problem, but the resonance between the leakage inductor and parasitic capacitor of the synchronous transistors can not be avoided, thus the losses of synchronous transistors increases.Three methods of the synchronous rectification circuits are proposed based on the theory analysis. A prototype of Two-Transistor forward converter with synchronous rectification is designed. The design methods of its main circuit, control circuit, driving circuits of the main swith and auxiliary power supply are analyzed in detail. In order to compare their losses, simulation and experiments are made on the three synchronous rectification circuits. The validity of theoretical analysis and the feasibility of the proposed design methods are verified by the simulation and experimental results.