Design and Test of a 6 kV Phase-Leg using Four Stacked 1.7 kV SiC MOSFET High-Current Modules
Closed-loop active gate drive control of eight series-connected1.7 kV SiC MOSFETs is validated experimentally in this work. To mitigate the unbalanced voltages between devices, a modular active dv/dt control approach is implemented. This closed-loop control method demonstrated voltage balancing within 30% mis- match in steady-state and switching losses below 100 mJ for both turn-on and turn-off transitions at 6 kV under 240 A. The results shown validate the series-connection of SiC MOSFETs as a possible alternative to the use of individual devices with higher blocking voltages.
The voltage balancing method was validated with a half-bridge circuit shown in Fig. 1. For the half-bridge circuit, eight 1.7 kV rated SiC MOSFETs (four modules) were connected in series (S1 through S8). To act as a freewheeling diode, four additional modules were connected in series (D1 through D8), with each device's gate-source voltage clamped to -4 V.
Fig. 2 shows the load current Iload with all eight VDS voltages under a 6 kV dc-bus. Due to the power limitation of the dc power supply used in this experiment, a multiple-pulse test was conducted with a dc power supply, using the energy stored in the dc-bus to provide the 240 A inductive load current. Thus, the load resistor Rload, duty cycle, and switching frequency were tuned together in order to achieve 240 A switching current at 6 kV dc input and minimize the voltage drop of the dc-bus during the 8 ms test. Fig. 2 shows the balancing results and zoomed-in waveforms.