High-Frequency Wireless Power Transfer System for Future Electrification
2024-05
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High-Frequency Wireless Power Transfer System for Future Electrification
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2024-05
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Over the past century, power electronics have become an indispensable technology in a variety of applications, including consumer appliances, industrial applications, and electric transportation, evolving alongside the advent of electricity and semiconductors. The historical development of power electronics significantly highlighted their role in meeting the growing demand for environmental sustainability. This was particularly emphasized in global policies such as CE100/RE100, which targeted reducing CO2 emissions and promoting the use of alternative fuels. This shift towards electric systems from fossil fuels emphasized the importance of battery-powered applications, highlighting their role in future electrified solutions. These applications required the development of advanced charging systems that were not only characterized by efficient conversion, and compact size for integration but also offered the convenience of charging without the need for physical connection of wires. In order to implement such a versatile power transfer system, a high-frequency (HF) wireless power transfer (WPT) system was an excellent candidate due to its benefits, such as its lightweight, compact size, and convenient agile power distribution. However, implementing WPT systems at 10's of MHz frequency presented significant challenges, including complex circuit design, and controlling transmission power. Overcoming these obstacles was crucial for the successful implementation of HF WPT systems in future electrified applications. This dissertation described the design and control methodologies of an HF WPT system for efficient and compact future electrified applications. A significant portion of this work involved overcoming technical limitations by designing an HF WPT system operation, including coupling coils/inverter/rectifier topologies at 10's of MHz frequencies. Above all, a self-synchronized class E rectifier was proposed to address signal mismatch caused by propagation delay at 13.56 MHz. Propagation delay, an inevitable issue in HF operation due to detection circuits and gate drivers, not only reduces efficiency and leads to operational errors but also affects the zero-voltage switching (ZVS) condition, which is a key characteristic of fast-switching circuits. The proposed method utilized the voltage at the node between the Cs-Ls resonant filter to generate a precise gate signal by compensating the propagation delay. Experimental results demonstrated its success in offsetting a propagation delay of 8 ns with a class E rectifier, operating at an output power of 228 W and a switching frequency of 13.56 MHz. Consequently, this work showcased the feasibility of synchronization at 13.56 MHz frequencies. Subsequently, the class E2 DC-to-DC converter for the WPT system was designed utilizing the self-synchronized class E rectifier. To implement this, the design had to account for the inverter and coils, incorporating a self-synchronized class E rectifier that aligned with the overall specifications of the WPT system. To fulfill this requirement, we developed the design and tuning method of a finite class E inverter/rectifier capable of transitioning between inverter and rectifier modes. The proposed finite class E2 converter was designed to maintain ZVS under load variant conditions and was successfully confirmed to manage transitions between inverter and rectifier modes effectively. After implementing a class E2 converter for the HF WPT system, improving the coupling coil's performance became crucial to achieving enhanced power transmission efficiency across mid-range distances. Initially, the critical coupling coefficient was adjusted to extend the transmission distance. This design method allowed us to extend the transmission distance up to the level of the coil's outer diameter, without additional repeater coils. However, this increase in distance led to huge stress on the compensation capacitor. To address these issues, we designed self-resonant spiral coils that achieve series resonance without extra capacitors, by aligning two spiral coils face-to-face, thus inducing internal series capacitors. This design simplifies tuning and reduces the risk of component damage due to high voltages. In addition, we developed a coil optimization method using machine learning techniques (ML). Feed-forward neural networks (FNN) were employed to design and optimize spiral coils. This approach reduced the computation time by over 10,000 compared to traditional Finite Element Analysis methods such as Ansys-Maxwell and HFSS, while maintaining above 95% accuracy. Finally, controlling the transmission power from the transmitter to the receiver stood as a primary challenge in the design of HF WPT systems. This dissertation introduced a control methodology of self-synchronized class E rectifiers, considering the nonlinear capacitance of FETs. At the 13.56MHz operation, the nonlinear capacitance of FETs significantly impacted the performance of the rectifier. Therefore, we proposed an estimation method of equivalent Coss and analyzed the effect of nonlinear capacitance on the control performances. In particular, we established a control strategy using two optimal points, Pmax and Pmin, verifying that feedback control of the output power in the class E rectifier was effective. Based on this, the prototype of the HF WPT system was fabricated by integrating components such as the proposed inverter/rectifier design and tuning methods, coil optimization methods, and output power control methods. In experimental validation, conventional control methods exhibited an efficiency of 82% to 74% across the 160W to 130W operation range, while the proposed WPT system achieved a 2 to 3% higher conversion efficiency and a wider output power control range of 160W to 90W. Lastly, the closed-loop control of the entire WPT system was verified under dynamic operation from 160W to 100W, with a minimal power ripple (1.7%). In conclusion, this dissertation presents a comprehensive study focused on advancing the field of HF WPT systems for future electrified applications. The work includes key innovations such as the development of a self-synchronized class E rectifier capable of overcoming isolation issues, machine learning-based optimization of spiral coil designs for enhanced transmission efficiency, and a control strategy of class E rectifier for the WPT system. These technical achievements, validated at an operational level of 13.56MHz and 250W, merge to deliver a system with an impressive maximum efficiency of up to 82%. The findings herein not only resolve current challenges in the design and control of WPT systems but also provide a robust foundation for future research and development in the field of electrified and sustainable technologies.
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University of Minnesota Ph.D. dissertation.May 2024. Major: Electrical Engineering. Advisors: Jungwon Choi, Sairaj Dhople. 1 computer file (PDF); xi, 145 pages.
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Kim, Minki. (2024). High-Frequency Wireless Power Transfer System for Future Electrification. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/264321.
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