Self turn-on loss of MOSFET as synchronous rectifier in DC/DC buck converter - in case of a low driving impedance -

Abstract

In this paper we present a kind of losses on MOSFET as a synchronous rectifier which is very difficult to remove in compare with the other losses. For high performance, downsizing and lightweight of digital equipment as a personal computer, digital ICs are developed with high speed and densely integrated. The supply voltage for these ICs becomes lower and the supply current becomes higher. A buck converter is widely used as a DC/DC converter for this purpose. In the buck converter, a synchronous rectifier of MOSFET with ultra low on-resistance is widely used as a low-side switch for higher conversion efficiency instead of a Schottky barrier diode as a free wheel diode. When the high-side switch is turned on as the low-side switch is off, the drain-source voltage of the low-side switch rises rapidly and the gate-source voltage rises simultaneously through the drain-gate capacity. As a result, the gate-source voltage becomes up to the gate threshold voltage and the low-side switch becomes active state and the drain current flows. We named this phenomenon dasiaself turn-onpsila where the loss occurs in the low-side switch.(Murata et al., 2003) in this paper a precise analysis of the self turn-on phenomenon is presented and the experiments for verification are also presented.