New insight into the mechanism of the spurious triggering pulse in the bridge-leg configuration

Abstract

The spurious triggering pulse is an intrinsic feature in the gate-source voltage of the synchronous switch of the bridge-leg configuration. If it exceeds the threshold voltage of the switch, the spurious turn-on will occur and consequently incur excessive switching losses, self-oscillation and even shoot-through. This paper intends to establish a circuit-level analytical model of the synchronous buck converter considering all important parasitic elements and the reverse recovery characteristics of the body diode of the synchronous MOSFET to explore the underlying mechanism of the spurious triggering pulse. It is discovered that apart from the generally acknowledged Cdv/dt induced increase in the gate voltage, the Ldi/dt induced decease in the source voltage contributes to the pulse as well. Besides, the impact of the gate impedance of the synchronous MOSFET on this pulse depends on the dominance of these two factors. The analytical results will be verified by the experimental results of the prototype of a 40V/6A synchronous buck converter. Keywords-MOSFET, spurious turn-on, Cdv/dt induced turn-on, bridge-leg configuration.Nomenclature C gs gate-source capacitance C gd gate-drain capacitance C ds drain-source capacitance L d drain inductance L s source inductance L g gate inductance L ds Total drain and source inductances R g gate drive resistance g fs transconductance V th threshold voltage Q RR reverse recovery charge S snappiness factor M CtrlFET M 2 SyncFET v ds drain-source voltage v gs gate-source voltage v gd gate-drain voltage i ch channel current i d drain current i g gate current i f body diode current I L load current V DD input voltage V GG high gate voltage v Lds voltage across L ds V ds(on) on-state voltage of the MOSFET V Db(on) on-state voltage of the body diode