Safe-Operating-Area of Snubberless Series Connected Silicon and SiC power devices

As power devices are series connected for voltage sharing, loss of gate drive synchronization and/or variation in device switching time constant can cause voltage imbalance. Capacitors (in snubbers) are usually added to maintain series voltage balance, however, in snubberless designs, where active gate drivers are used for voltage balancing during transients, it is necessary to evaluate the limits of the power device under transient unsynchronized switching. In series devices, desynchronization of the gate drivers in series connected devices will cause the faster switching device into avalanche during turn-OFF. Power device failure from BJT latch-up in MOSFETs and thyristor latch-up in IGBTs can result in potentially destructive consequences for the entire converter. Failure of the power device under avalanche is exacerbated by the (i) device commutation rate (ii) device junction temperature (iii) magnitude of gate drive switching mismatch and (iv) ratio of the DC bus voltage to intrinsic breakdown voltage of the device. This paper uses experimental measurements and finite element models to investigate the limits of power device failure under transient unsynchronized switching of series connected SiC trench MOSFET and silicon IGBT devices.