Quasi-clamped Inductive Switching behaviour of power Mosfets

Turn off behaviour of power MOSFETS with quasi-clamped inductive switching (QIS) loads is analysed. In most real applications the main inductive load is clamped during turn off but there is enough unclamped inductance in the circuit to cause voltage spikes and avalanche breakdown. This is the QIS situation and occurs in almost all practical circuits from motor drives to inverters to VRMs. Analytical expressions are derived for turn off losses under QIS which take into account the unclamped inductance in the circuit, gate drive parameters and more importantly, the source inductance of the package. The equations also predict the peak voltage during turn off and the conditions that would drive the device into avalanche. A common solution for avalanche prevention during inductive turn off is the protected Mosfet with an integrated drain to gate zener diode. The zener diode forces the Mosfet to turn on, thereby clamping the drain voltage. The trade offs in the design of such protected Mosfets are analysed. The findings are verified in the practical case of a power tool where the motor speed is controlled by PWM operation of a single switch. The circuit has large unclamped inductances arising from wiring and very high peak currents under locked rotor conditions. It is shown that a well designed Mosfet with avalanche protection can survive the locked rotor condition for longer durations, thereby increasing the overall product reliability.