Practical application of MOSFET synchronous rectifiers

The application of MOSFET synchronous rectifiers (SRs) to a forward power converter with resonant reset is analyzed. Under the assumptions of ideal gate-drive waveforms, the theoretical minimum losses are determined. The analysis shows that the gate charge and on-resistance of the current generation of power MOSFETs are low enough to yield a significant increase in rectifier efficiency. Two simple gate drive circuits are presented, and the effects of parasitic circuit inductances on the gate voltage waveforms are shown. The measured converter losses for these SR drive circuits are compared to the losses for the Schottky rectifiers, as well as to the theoretical minimum losses. For the 3.3 V output at load currents between 5 and 10 A, the losses approach the theoretical minimum. For higher load currents, the MOSFET body-drain diode causes losses during its reverse recovery, due to the nonideal timing of gate drive voltages