On the Triggering Mechanism for Self-Sustained Oscillation in Wide Band-Gap Semiconductors
Wide bandgap (WBG) semiconductors are very attractive due to the outstanding characteristics that enable high-efficiency power electronics converters. However, due to the achievable fast transitions, these devices can suffer from unintended behavior such as under-damped ringing, voltage and current overshoot, half-bridge shoot-through, increased electromagnetic interference (EMI), and self-sustained oscillation (SSO).This thesis provides an analytical treatment of the triggering mechanism leading to SSO, which is an undesired phenomenon that may occur during turn-off transitions of WBG transistors due to their fast switching performance. For this analysis, a large signal model has been developed in state-space form to determine the likelihood of forced cycles in a simplified application circuit. Forced cycles are known to be a necessary but not sufficient condition for SSO to occur. In this sense, preventing the occurrence of forced cycles eliminates any possibility of destabilizing the circuit. Forced cycles occur when the gate-source voltage of the active switch rings back above threshold and causes channel conduction. The model presented in this thesis is capable of predicting the maximum gate-source voltage ring-back for any level of intrinsic parasitics and operating conditions. The model presented in this thesis is empirically validated with an application circuit utilizing GaN high electron mobility transistors (HEMTs). GaN HEMTs are known for very high switching speed, which also introduces susceptibility to SSO. The modeling framework introduced in this thesis is expected to be useful to application designers in creating application circuits that take maximum advantage of the attractive properties of WBG devices while ensuring immunity to the SSO phenomenon by some intentionally selected design margin.