Zikang Tong

PhD 2022

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Doctoral Dissertation

Semiconductor technology and circuit techniques for high-performance MHz-range power converters

Conventional power converters contain semiconductor devices switching in the tens to hundreds of kilohertz (kHz) range. Extending the switching frequency to the multi-MHz range brings opportunities to reduce the size and weight of power converters as the energy storage requirements decrease. Additionally, MHz-frequency power converters and amplifiers enable new applications such as plasma generators for semiconductor processing equipment, medical sanitation, and CO2 reforming. Despite these promises and opportunities, building efficient power converters at much higher frequencies still poses a significant challenge. In MHz-frequencies, wide bandgap (WBG) semiconductor devices, such as gallium nitride (GaN) and silicon carbide (SiC), have the potential to improve the performance of these systems as they have orders of magnitude lower specific on-resistance compared to silicon (Si) devices. One of the main issues is the soft-switching Coss losses in WBG devices, which have not been previously well-studied and modeled in the literature, and these losses significantly degrade the efficiency of power converters. We present the measurement results and techniques to characterize the Coss losses in wide bandgap devices, as well as discuss the physical root causes of these losses in SiC power devices. In addition to the Coss losses, effectively utilizing SiC MOSFETs poses a challenge, as designing fast transitioning and low loss gate drivers at MHz frequencies is difficult. As a solution, we develop resonant gate drivers that can drive SiC MOSFETs up to 30 MHz while conserving over five times as much gating power compared to available commercial counterparts. Lastly, we utilize these WBG devices in broadband power amplifier demonstrations suitable for radiofrequency (RF) plasma generation applications at 13.56 MHz. To achieve high performance across broadband, we employ various RF circuit techniques including reactance compensation, phase-switched impedance modulation, and power combining. As a result, these amplifiers showcase some of the highest efficiencies published in the literature, including over 90% across a 4 MHz bandwidth for a 300 W system and over 95% efficiency across 4 MHz for a 1 kW system.

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