Design of MHz power amplifiers using wide bandgap devices
Keywords: Electrical Engineering.

Power amplifiers are essential building blocks in many applications, including radio transmission, wireless power transfer, medical devices, and plasma generation. Conventional linear power amplifiers, such as Class A, Class AB, Class B, and Class C, have good linearity but low efficiencies. Switched-mode power amplifiers, such as Class D, Class E, and Class F2, can achieve a theoretical efficiency of 100%. However, these power amplifiers are designed to operate only at a fixed operating point, and changes in frequency or loading conditions can result in a significant degradation of their efficiencies and output power. Wireless power transfer systems and plasma generators are among the increasing number of applications that use high-frequency power converters. Increasing switching frequency can reduce the energy storage requirements of the passive elements that can lead to higher power densities or even the elimination of magnetic cores. However, operating at higher frequencies requires faster switching devices in packages with low-parasitics. Wide bandgap (WBG) power devices like gallium nitride (GaN) and silicon carbide (SiC) devices, have high critical fields and high thermal conductivity that make them good candidates for efficient high-voltage and high-frequency operations. Commercially available GaN and SiC devices have ratings targeting different applications. Lateral GaN devices dominate in lower-voltage (< 650 V) and high-frequency applications as they have relatively small device capacitances (Coss, Ciss), which make them easy to drive at high frequencies. On the other hand, vertical SiC devices are often used in higher-voltage and low-frequency applications since they have higher blocking voltages and larger gate charge than their GaN counterparts. As a result, SiC devices usually require high-power and complicated gate drive circuitry. Recent work shows that in both GaN and SiC devices, losses in device Coss can exceed the conduction losses at high switching frequencies and relatively high voltages under zero-voltage-switching (ZVS) conditions. Moreover, the Coss energy loss (Eoss) per switching cycle increases with frequency in GaN devices but remains roughly independent of frequency in SiC devices. This means that at high frequencies, SiC devices can be preferable due to their smaller Coss energy loss even when taking into consideration the complexity of the gate drive circuit. This thesis addresses the challenges of designing efficient MHz switched-mode power amplifiers using wide bandgap power devices, which include power device selection and optimization, efficient gate drive design at MHz frequencies, bandwidth extension of switched-mode power amplifiers, and the use of MHz power amplifiers in emerging applications. First, I will present a cascode GaN/SiC power device using an enhancement-mode GaN HEMT and a depletion-mode SiC JFET. This cascode device has the combined advantages of both GaN and SiC devices, which include simple gate drive requirements, low Coss losses at high frequencies, and high-voltage-blocking capability. Experimental results show that the inverter using the cascode GaN/SiC device has higher efficiency and simpler auxiliary gate drive circuitry compared to the SiC MOSFET and SiC JFET of similar voltage ratings and Rds, ON. Analysis of the switching sequence of the cascode device indicates that it is possible to reduce the gate loss of the SiC JFET in the cascode structure by minimizing its gate resistance. Second, we turn our attention to address the limitations in the switched-mode power amplifiers. Their narrow-band operation limits their use in many applications requiring frequency tuning or multi-frequency operations. To extend the bandwidth, we designed a wideband Class E power amplifier using the reactance compensation technique along with a custom gate driver for SiC MOSFETs at MHz frequencies. The reactance compensation technique allows the bandwidth extension without adding any active switches or control complexity, which makes it suitable for high-frequency applications such as in plasma etching systems. Using the same idea of the reactance compensation, we designed an efficient wideband gate driver for SiC MOSFETs at 13.56 MHz. The designed 1 kW wideband Class E power amplifier achieves 93% efficiency at 13.56 MHz with a bandwidth of ±1 MHz. Third, expanding from the idea of the wideband design, we added the frequency-selective feature to achieve selective power delivery. The added frequency-selective LC networks resonate at different frequencies, and all of these resonant frequencies are within the bandwidth of the designed wideband power amplifier. By operating at different frequencies, the power amplifier is able to selectively deliver power only to the target load. The designed power amplifier can be used in plasma-assisted nitrogen fixation for clean and decentralized fertilizer production