Analytical Modeling of Fault Transient in a GaN HEMT Half Bridge and Its Overcurrent Protection With PCB Embedded Rogowski Coils

The low Short Circuit Withstand Time (SCWT) offered by GaN HEMTs imposes challenges on their reliability, especially in medium to high power applications such as electric vehicle chargers. This article models the short circuit fault transient in a GaN HEMT half-bridge configuration including parasitic elements, based on the state-variables in the circuit and the datasheet parameters. Faults at higher bus voltages result in increased instantaneous power loss, which in turn lead to higher junction temperatures. This results in reduced SCWT for the device. The model is further used to estimate the fault clearing time for different dc bus voltages. A scheme for ultrafast over-current protection of GaN HEMTs is presented. The protection scheme employs a non-intrusive PCB-embedded Rogowski Coil (PCB-RC) based current sensing technique for sensing the device current. Experimental results with the dc bus voltage set to 400 V, demonstrating the protection of the device subjected to a hard switched fault (HSF) are presented. The protection response time is reported to be 38 ns, which is less than the estimated fault clearing time.