A BARE-DIE SIC-BASED ELECTRO-THERMALLY CO-DESIGNED WIRE-BONDLESS HIGH-FREQUENCY DC-DC CONVERTER

Abstract

The current technological status of switch-mode power converters requires a paradigm shift to enable a substantial enhancement in power density. The emergence of wide-bandgap (WBG) devices such as Silicon Carbide (SiC) MOSFETs o↵ers the possibility to achieve high power-density by enabling higher switching frequency and higher temperature operation. This dissertation addresses the following shortcomings of conventional designs: 1) High values of commutation loop inductances and parasitic capacitances which prohibit fast, reliable and efficient switching performance, 2) inadequate thermal design capable of handling very high heat-fluxes (exceeding hundreds of W/cm2), which naturally stem from highly compact design and high allowable losses of the SiC devices, and 3) decoupled and sequential electrical and thermal designs, which leads to sub-optimal electro-thermal performance.

As a solution to these challenges, this dissertation investigates two design strategies: 1) a novel switch module structure with low parasitics, and 2) a novel planar transformer structure with integrated leakage inductance and cooling system. Both approaches result in enhanced thermal performance, optimized through simultaneous electro-thermal co-design. A common key highlight of the proposed solutions is the high degree of integration realized by use of sub-components that integrate both electrical and thermal performance. This will save real estate by reducing component count and by lessening electrical and thermal burdens.

In the first part of this dissertation, a detailed electrical characterization of a novel, wire-bondless, three-dimensional (3D), half-bridge switch module using bare-die SiC MOSFETs is presented. The switch assembly features the use of electro- thermally multi-functional components, simultaneously serving as bus-bars and heat sinks. A highly compact composition with embedded decoupling capacitors and gate driver components is realized with vertical loop structures for both power and gate drive circuits. Besides, the wire-bondless structure enables double-sided cooling, which significantly improves the thermal performance. 3D finite element analysis simulations and experiments demonstrate that the proposed switch module can achieve extremely low values of parasitic loop inductances (Lloop,power = 1.35 nH, Lloop,gate = 5.1 nH at a parasitic oscillation frequency of 100 MHz) as well as high thermal performance without entailing significant layout capacitances and resistances.

The second part of this dissertation proposes an electro-thermal design optimization method of a high-frequency planar transformer with an integrated leakage inductance and thermal management system. Aiming at the use in a high-frequency (>500 kHz) dual-active-bridge (DAB) converter, an optimal leakage inductance selection process is explored based on highly accurate analyses of the DAB converter operation for maximizing the efficiency. Effect of design variables like the number of turns of the transformer and cooler height on the transformer’s electrical parameters such as leakage inductance, ac resistance and parasitic capacitance is further analyzed in detail. The dependence of converter efficiency on these parameters is estimated using realistic simulations and analyses, and potential trade-o↵s of the design are investigated. Thermal modeling is used to evaluate the thermal performance of different designs. Based on a combination of the analyses, optimal designs are identified, which simultaneously ensure good electrical and thermal performance.

Finally, a compact DAB converter is designed based on the investigated components, operating at a switching frequency of 500 kHz. Robust gate-driver circuitry and auxiliary parts are also developed to tolerate such high switching frequency as well as high dv/dt. The optimal design processes, operation strategies and analytical models are validated through diverse experiments on 3.3 kW dc-dc converter operation. As a result of the investigations, the converter achieves zero-voltage-switching over various load conditions with satisfactory high-frequency waveforms and a peak efficiency of 98%. The converter’s operation at high power is validated through a designed loss-emulation test corresponding to 8.4 kW operation.