Switching charge is a MOSFET parameter that is widely used in the estimation of switching losses for switch-mode power supplies. The definition which most often appears on datasheets relies on the triangular behavior of voltage and current during a switching event. However, reduced surface field devices cannot be approximated in this way, as their capacitance-voltage characteristics differ fundamentally. This paper reviews the traditional assumptions of voltage and current behavior, re-evaluates these assumptions in reduced surface field devices, and proposes a new method for defining the switching charge parameter, supported by measured switching losses.
