Pushing the Limits of Efficiency and Power Density in High-Frequency Power Conversion Based on Wide-Band-Gap Technologies

The emergence of wide-band-gap (WBG) power transistors with low conduction losses and high-speed switching speeds has paved the way for more-than-ever efficient power electronics systems and huge energy saving potentials. Likewise, power density- the ratio of power to volume (or weight)-can be significantly increased in power converters based on gallium nitride (GaN) and silicon carbide (SiC) transistors. Converters with high efficiencies and large power densities are essential to forthcoming applications such as electric aircrafts, hyperloop transportation systems and DC grids. Nonetheless, major barriers for realizing such converters are:

€ Lack of important information in datasheets and models from WBG device manufacturers, and huge diversities in the performance of WBG transistors based on different GaN and SiC technologies. € Shortcomings of existing tools for accurate system-level and component-level performance analysis and loss measurement in efficient power converters at high-frequency (HF) and very-high-frequency (VHF) domains. € Necessity to enhance traditional converter topologies for compatibility with WBG devices, by designing high-quality magnetics and implementing novel control strategies to maximize their efficiency and power density, which are typically two opposing objectives.

Thesis in Chapter 2 focuses on accurate methods for voltage and current rise rate measurements of high-speed GaN and SiC transistors for pulsed-power applications, highlighting the effect of different parameters on the device switching speed. Next, we propose measurement methods for evaluation of gate loss, conduction and dynamic ON-resistance degradation loss and output capacitance loss in soft-switched WBG transistors for frequencies of up to 40 MHz, providing an insightful performance comparison between various SiC and GaN technologies for HF and VHF resonant and quasi-resonant power converters.

High-quality magnetic components, namely inductors and transformers, are inseparable building blocks of efficient power converters. Chapter 3 is dedicated to characterizing magnetics in the HF domain, with an overview of different loss evaluation methods.

Chapter 4 proposes advanced calorimetric techniques: First, a novel dual-chamber calorimeter with an unprecedented measurement accuracy and range is proposed for sensitive loss measurements in power electronics building blocks (e.g., HF inductors and transformers) and efficiency evaluations in highly-efficient converters where electrical measurements are prone to large errors. Next, a thermal method based on temperature mapping is presented, suitable for assessment of losses and their distribution in HF and VHF power circuits.

Improved DC-DC topologies for efficient operation at HF are the subjects of Chapter 5. An enhanced dual-active-bridge (E-DAB) topology is proposed for efficiency preservation over wide voltage gains, achieving a peak efficiency of 97.4% and a power density of 10 kW/l. By applying a new operation mode based on impulse rectification, traditional boost converters can achieve zero-voltage switching. Thanks to several optimizations in magnetics design, device selection, layout and control, a converter with an outstanding peak efficiency of 98.6% and a power density of 52 kW/l is realized.

The thesis provides insights for power electronics designers and device engineers to push the limits of conversion efficiency and power density to the maximum using WBG technologies.