Doctoral Dissertation
On the design and optimization of RF power amplifiers using wide bandgap devices
Compact radio-frequency (RF) power amplifiers are essential building blocks in various applications, including radio transmission, medical imaging, wireless power transfer, industrial plasma generation, and micro-satellite propulsion. Over the past decades, new wide bandgap (WBG) semiconductor devices, particularly gallium nitride (GaN) and silicon carbide (SiC), were successfully commercialized. These devices, some specially designed and optimized for high-frequency operation, have theoretical benefits over silicon (Si) counterparts of multiple orders-of-magnitude. Consequently, they became the prime focus for those looking to further increase the power amplifier efficiency beyond what was previously possible. Unfortunately, while these WBG devices promise exceptional performances, recent studies have found that they possess additional undocumented loss mechanisms called dynamic on-resistance and junction capacitance Coss loss. To attain the maximum efficiency with these devices, additional design consideration and optimization is therefore needed. In this thesis, we address the challenges that are in the way of achieving a high-efficiency RF power amplifier system. The main goal is to improve and optimize how switched-mode amplifiers, specifically class-E amplifiers, are designed as much as possible so that high efficiency, high power, and fast control speed can be achieved simultaneously. First, to get the highest efficiency out of an amplifier with a WBG device, we present an analysis on how to select the optimal input voltage and the device size such that the two additional losses will be minimized. To enable the additional loss to be easily simulated, we also propose a distributed model for the loss, in analogy with the generalized Steinmetz equation. Second, to efficiently scale up the power, we present a unique design of a class-E amplifier called “power-combinable class-E,” which allows multiple amplifiers to be directly connected at the output combining the power. This eliminates the need for a separate power combiner circuit, a source of efficiency loss in a standard multi-amplifier system. Third, to adjust the output power of the proposed amplifier system, we develop a new power modulation scheme called the Modular On/Off and Phase-Shifting control. This control technique requires no additional component to be added to the circuit. In Modular On/Off, a different number of sub-circuits are turned on/off to crudely control the output power. In Phase-Shifting, one sub-circuit is phase-shifted away from the rest to finely adjust the output power. Finally, we examine a broader aspect of optimization. We look at a wireless power transfer (WPT) system employing a power amplifier as a single unit to be optimized. Specifically, we consider the hurdles that prevent the high-frequency WPT system’s adoption and present a method to improve the circuit’s efficiency as well as reduce its size by designing the WPT coils such that their leakage and magnetizing inductances can be used as resonating inductors for the class-E power amplifiers.