Investigation of Compact Auxiliary Components for Medium-Voltage Converters with Enhanced Insulation and Wide-Bandgap Device Utilization

Abstract

Medium-voltage (MV) power electronic systems demand compact, reliable auxiliary and protection hardware that must simultaneously satisfy high insulation voltage, low common-mode (CM) coupling capacitance, and stable operation under fast voltage transients driven by wide-bandgap (WBG) switching devices. This dissertation addresses these competing requirements through three interconnected research directions targeting magnetic isolation, capacitive isolation, and high-voltage solid-state switching for compact MV auxiliary power supplies and switching functions. The first contribution develops a current-transformer-based auxiliary power supply (APS) for distributed MV gate drivers and sensors. An insulation-oriented design methodology is established that co-optimizes transformer geometry, winding placement, conductor structure, and dielectric spacing based on electric-field (E-field) distribution and CM coupling paths. The hardware prototype delivers 20 W output power with 0.83 pF CM capacitance and partial-discharge-free (PD-free) operation up to 14 kV, demonstrating that magnetic isolation can satisfy strict low-coupling requirements when insulation and magnetic performance are co-designed. The second contribution investigates capacitive isolation as a compact, planar alternative for high-frequency MV auxiliary power transfer. A novel multi-dielectric terminal E-field control technique is proposed for PCB-based isolation capacitors to address the dominant insulation failure mode: terminal-edge E-field concentration. By separating the capacitance-forming region from the high-field terminal region and applying material-specific insulation design to each, the proposed extended and encapsulated terminal structure improves PD-free voltage from approximately 4.1 kV to above 38.9 kV—roughly a ten-times improvement in insulation capability. A comparative analysis with the transformer-based design shows that the capacitive structure becomes more volume-efficient beyond approximately 11 kV, with the key tradeoff being higher inherent CM coupling capacitance. Building on this capacitor, a MHz-frequency capacitive-isolated resonant SEPIC converter is developed and experimentally verified, achieving wide voltage-conversion operation, regulated output, over 100 W output power, and a peak efficiency of approximately 87.7%, demonstrating feasibility for practical compact MV auxiliary power delivery. The third contribution presents a SiC MOSFET-based super-cascode switch for MV power switching, capacitor discharge, dc-link chopping, and protection functions. A systematic balancing-network design method is established by separating and analyzing the distinct roles of capacitance offset (ΔC), absolute balancing capacitance (Cₙ), and drain-source snubber capacitance (Cd,ₙ) on turn-on synchronization, turn-off voltage distribution, oscillation suppression, switching speed, device-voltage utilization, and operating-voltage range. A complete turn-off voltage-distribution model is developed to predict final device voltages without assuming ideal sharing, enabling the design to be extended beyond turn-on-only discharge applications. Experimental validation across reduced-device tests, five-device double-pulse tests, and a ten-device hardware prototype demonstrates PD-free operation to 28.1 kV, high-speed switching up to 14 kV at approximately 90.3% device-voltage utilization, and controlled capacitor discharge and dc-link chopping up to a 22 kV dc link using stacked 3.3 kV SiC MOSFETs. Together, these three research directions provide design methods, analytical frameworks, and hardware demonstrations for compact, high-density MV auxiliary and protection hardware. The results quantify and clarify the fundamental tradeoffs among insulation voltage, CM coupling, capacitance density, switching speed, and device-voltage utilization, offering practical design guidance for next-generation medium-voltage power electronic systems.