Linear SR Mode of Power MOSFETs and its Application in an EMI-Suppressing Rectifier Bridge

This work introduces the property of linear, reverse operation of a power MOSFET. In a typical synchronous rectifier (SR) application, in its conduction phase, a MOSFET is fully turned on to minimize loss. Yet another operating mode is found for SR MOSFETs: the drain voltage exhibits a linear relationship to the gate voltage where the gate voltage level is around the threshold. The body diode is not activated, and the dropout voltage is lower than the diode threshold. This linear relationship holds from DC to MHz range. Therefore, the reverse-operated MOSFET is employed to inject high frequency ripple signal to the power circuit. An active rectifier bridge with EMI mitigation function is built with SR MOSFETs to demonstrate an application of the introduced linear property with analogue control technique developed. Measurement of the third-quadrant linear SR characteristics for different MOSFET samples is exhibited. The active bridge prototype is used to replace the diode bridge in a PFC converter. Experiment shows EMI reduction, filter size reduction as well as loss reduction.