MOSFET flyback-diode conduction and dV/dt effects in power ICs in low-voltage motor control applications

In a power IC, the direct drive of a low-voltage motor by one or more MOSFET half-bridges results in inductive-flyback diode conduction during switching transitions. PISCES computer simulation reveals that this diode current in an integrated lateral MOSFET manifests itself as parasitic bipolar conduction where a substantial fraction of the carriers injected from the forward-biased drain-to-body junction is collected by the transistor´s source. The physics of parasitic bipolar turn-off, or recovery, is shown to depend on the application. The resulting driving-point conditions can be classified into one of three possible diode recovery scenarios. In either natural or synchronously clamped recovery, high dV/dt is prevented by the inductive time constant of the motor. In forced recovery operation, fast diode turn-off and high dV/dt results from shoot-through between the recovering diode and the MOSFET within a given half-bridge. analysis reveals that a high dV/dt leading to minority carrier injection from the source slows the forced reverse-recovery time without leading to destructive snapback