Impact of PCB Parasitic Capacitance on Switching Transients in Chopper and Half-Bridge Configurations Utilizing TO-247 SiC Devices

Silicon Carbide (SiC) MOSFETs and Schottky diodes in the TO-247 package are economical options for chopper (buck/boost) and half-bridge configurations, which are fundamental building blocks for various power converter topologies. However, the fast switching of SiC implies high dv/dt and di/dt, imposing a constraint on the PCB portion of power loop inductance in minimizing voltage overshoot during the turn-OFF transient. Although the vertical PCB power loop layout effectively reduces the PCB loop inductance, it increases the PCB parasitic capacitance. Due to the considerable lead inductance of the TO-247 package, this PCB capacitance is paralleled to the device's output capacitance through the package lead inductance, altering the switching transient. This article analyzes the effect of PCB capacitance on turn-OFF switching transient and ringing in chopper and half-bridge configurations with SiC devices in the TO-247 package. Initially, small-signal models incorporating PCB capacitance are derived. Subsequently, these models are validated in the frequency domain, and the switching transients are compared through double pulse test (DPT) on two PCB prototypes with the same layout but different stack-ups, yielding different PCB capacitances. Further, a comparative study of the proposed models with direct parallel approximation of PCB and device output capacitance is presented. Finally, the proposed small-signal models are analyzed to establish criteria, in terms of TO-247 lead and PCB loop inductance, for minimizing the impact of PCB capacitance on switching transients.