Studies of self-driven synchronous rectification in low voltage power conversion

In self-driven synchronous rectification circuit, transformer secondary winding is often used directly to drive synchronous MOSFETs. Transformer leakage inductance and other parasitic inductance in the rectification loop are found to contribute to rectification loss. The body diode of synchronous rectifiers is on during current commutation in two SRs because reflected secondary voltage falls on these parasitic inductances instead of the gate terminal of SR in this duration. Body diode turn on greatly degrades the performance of SR. The case is worse in high frequency, high current applications. Detailed analysis of this phenomenon is given in the paper. An equivalent model is established to analyze and evaluate the performance of SR with the existence of parasitic inductance. Simulation and experiment waveforms confirm the model and analysis. Additionally, optimal driving waveforms for SRs are discussed. Two improved gate drive methods are presented and compared.