Silicon Carbide JFET Super-Cascodes for Normally-On Current Source Inverter Switches in Medium Voltage Variable Speed Electrostatic Drives

This work focuses on the design and testing of silicon carbide JFET super-cascodes for medium voltage electrostatic machine drives. These drives require medium voltage (7kV peak) and low current (<; 2A) normally-on switches that are not available as monolithic semiconductors. Characterization of the super-cascodes (12 JFETs in series) demonstrates normally-on (1Ω, Vgs = 0) behavior and high voltage blocking capabilities (100uA at 10kV). Using a clamped inductive load, two gate-coupling methods, one adding a fixed vs a stack position scaled capacitance, were tested to determine the best solution for a low-loss high-switching frequency device. The fixed capacitance had the lowest loss of 5.6mJ when tested at 7kV and 2 A. Six super-cascodes were then combined in a current source inverter operating at 4.5 kVrms line - line, 400 Hz fundamental with a switching frequency of 20kHz driving a 2.1 kVA RC load to emulate the electrostatic machine.