Using MOSFET Coss and Eoss in Switching-Loss Estimates

MOSFET output capacitance is strongly nonlinear, so multiplying a single Coss value by one-half voltage squared can misrepresent the energy moved during each switching event. Data sheets often provide both capacitance curves and stored-energy curves. The energy curve is usually the more direct input for loss estimates because it already integrates capacitance over drain voltage.

In a hard-switched node, stored output-capacitance energy is normally dissipated during turn-on unless the topology recovers part of it. Multiply the appropriate Eoss value by switching frequency, then consider the same mechanism in the companion device and the effect of diode recovery. When evaluating candidates in a power MOSFET catalog, compare Eoss at the real bus voltage rather than comparing Coss at an arbitrary test voltage.

Soft-switching converters need a different analysis. The available inductor or magnetizing current must move charge between switch-node capacitances within the available dead time. Here, charge-related curves and measured node transition time may be more useful than a simple energy-to-loss calculation. The required current changes with input voltage, load, temperature, and the capacitance of both power devices plus layout and transformer parasitics.

Measurement should confirm the model. Probe the switch node with a low-inductance connection, record the inductor current during dead time, and verify that the node reaches the required rail before the gate command. If it does not, the MOSFET may turn on with residual drain voltage and lose the expected zero-voltage-switching benefit.

For converter applications, the clean-energy semiconductor overview is a useful route to relevant device families. Final qualification should sweep bus voltage, load, dead time, and temperature while checking switching energy, overshoot, and electromagnetic noise. Nonlinear capacitance is a circuit behavior, not a single fixed component value.