Avoiding Divergent Oscillation of a Cascode GaN Device Under High-Current Turn-Off Condition

The cascode structure is widely used for high-voltage normally-on GaN devices. However, the capacitance mismatch between the high-voltage GaN device and the low-voltage normally-off Si MOSFET may induce several undesired features, such as Si MOSFET reaches avalanche during turn-off and the high-voltage GaN device loses zero-voltage switching turn-on condition internally during a soft-switching turn-on process in every switching cycle. This paper presents another issue associated with the capacitance mismatch in the cascode GaN devices. Divergent oscillation could occur at high-current turn-off condition and, eventually, destroys the device. The intrinsic reason of this phenomenon is analyzed in detail in this paper. A simple solution is proposed by adding an additional capacitor whose position is critical and should be optimized. Experimental results validate the theoretical analysis and show that the proposed method improves device performance significantly under high-current turn-off condition.