Ph.D student, Stanford University
High-frequency, high-power resonant converter with Wideband gap devices for wireless power transfer systems
Demand for automated guided vehicles (AGVs), mobile robots that move materials in a manufacturing facility or warehouse, has increased substantially because they improve productivity and minimize operational costs. As AGVs become widely used, wireless power transfer (WPT) is drawing a lot of attention as a way to eliminate manual intervention during battery charging.
My research focuses on examining and implementing an efficient power conversion system to charge electric vehicles (EVs) and then extend the system to provide WPT. An innovative contribution of my work is the use of wide band gap (WBG) devices for EVs to reduce the size and weight of the entire WPT system and to increase efficiency. I have demonstrated a high-power resonant inverter using an enhancement mode gallium nitride (eGaN) device with magnetic resonant coupling (MRC) coils at 13.56 MHz for wireless power transfer (WPT). The power inverter driving the transmitting coils is based on a class Φ2 inverter, a single-switch topology with low switch-voltage stress and fast transient response. The implementation utilizes a recently available eGaN device in a low inductance package that is compatible with operation in the 10’s of MHz switching frequency. In this work, I present experimental measurements of the inverter in a WPT application and characterize the system performance over various distances and operating conditions.
Second, I have designed and implemented an impedance compression network (ICN) to correct distance variation or misalignment between coils in WPT systems. MRC coils provide high efficiency for charging mid-range WPT applications. However, a distance variation or misalignment between coils causes a coil-impedance change and significantly affects performances of WPT systems. In order to mitigate a coil-impedance variation, I proposed an ICN simultaneously compressing changes in magnitude and phase of impedance. The ICN consists of a resistance compression network (RCN) and phase compression network (PCN). While the RCN effectively compresses the variation in the magnitude of impedance by using one pair consisting of an inductor and a capacitor, the PCN is designed using a Smith chart to reduce a phase variation. It leads us to improve a WPT system performance in high-power applications.
Jungwon Choi is a Ph.D student in the Department of Electrical Engineering at Stanford University and is advised by Professor Juan Rivas-Davila. Her research interest is to design efficient RF resonant converters and matching networks in wireless power transfer (WPT) systems for electric vehicles (EVs), and to evaluate wide band gap devices to operate at high switching frequency. She have been collaborating with Daihen Corporation to develop WPT systems and they adopted her inverter design to implement their next generation product. She received her Master of Science degree in Electrical Engineering and Computer Science at University of Michigan, Ann Arbor in 2013 and B.S in Electrical Engineering from Korea University, Korea in 2009.