Gate drive for synchronous rectifiers in resonant converters

摘要

A resonant converter, including a transformer for separating a high-voltage (primary) side from a relatively low-voltage (secondary) side, has at least one synchronous rectifier and an auxiliary sense winding coupled to the gate thereof. The input capacitances of the synchronous rectifiers are reflected to the primary side and the secondary side by the square of the ratio of the number of auxiliary sense winding turns to the number of primary and secondary winding turns, respectively, thereby reducing the required size of the discrete resonant capacitor. In one embodiment, a gate bias voltage approximately equal to the device threshold voltage is applied to the gate of the synchronous rectifiers. The auxiliary sense windings are etched into a conductive film pattern of the secondary windings. The auxiliary sense windings provide nearly identical secondary and gate drive voltages so that the synchronous rectifiers are gated substantially at the zero-voltage crossings of the secondary winding voltages. The result is a substantially lossless gate drive for synchronous rectifiers in high power density resonant converters.