Design Considerations on Insulation and Magnetics for Medium Voltage DC/DC Converters

Medium voltage (MV) SiC devices have drawn considerable attention thanks to their high voltage blocking and fast switching capabilities, enabling new generations of high-power and high-density MV power conversion. However, many challenges can be found in implementing MV converters with MV SiC devices, such as insulation, efficiency, density, and parasitics. In this dissertation, several techniques have been proposed to address those issues.

First, a low-loss leakage integration technique using ferrite bridges and 2-D laminated magnetic cores is proposed for shell-type transformer structures. Then, a 2-D lamination configuration for ribbon- and sheet-based magnetic core is proposed, which further reduces the leakage-induced eddy current loss.

Second, a ferrite shielding strategy is introduced to reduce eddy current losses in laminated-core inductors and transformers by covering critical surfaces and bracketing the core sides. The mechanism of the proposed ferrite is analyzed and experimentally validated.

Third, a dry-type insulation scheme is proposed for the MV inductor and transformer windings. Insulation materials are characterized with simple test sample designs. The winding structure, shielding, and field grading methods have been introduced. A simplified vacuum encapsulation process is also proposed with fewer chambers and less tubing while the quality of the encapsulation is maintained.

A common-mode (CM) switching loss reduction method is proposed for DC/DC feeding cascaded H-bridge (CHB) converters by studying the mechanism and applying a diode-clamped grounding circuit. The operating principle of the diode-clamped circuit for the CHB is also discussed.

A compact MV power stage design strategy is also developed. The proposed heatsink field shaping is implemented to reduce the insulation clearance requirement. With the sloping air duct design, additional creepage distance can also be provided. Simplified gate drive design with commercial solution and compact power stage design are also demonstrated.

Finally, a flux balancing scheme for MV dual-active-bridge (DAB) converter transformers is proposed utilizing the magnetizing harmonic current. Ferrite gaps generate harmonics for flux unbalance detection, modeled and verified with dedicated sensing and control in DAB prototypes.