Public defence in Electrical Power and Energy Engineering, M.Sc. Yining Liu

The title of the thesis: Inductive wireless power transfer systems with high positional freedom

Thesis defender: Yining Liu
Opponent: Prof. Paul Mitcheson, Imperial College, UK
Custos: Prof. Jorma Kyyrä, Aalto University School of Electrical Engineering, Department of Electrical Engineering and Automation

Wireless power transfer (WPT) technology has become a reality in our daily lives, from wireless charging pads for smartphones to the proposed contactless charging of electric vehicles. By transmitting power through electromagnetic fields, WPT offers a convenient and safe way to deliver power in various applications. However, currently available WPT solutions face challenges related to power transfer efficiency, and they usually require precise alignment between the transmitter and the receiver device. This thesis aims to address these challenges by exploring both the WPT link and the power converter stages, proposing several new solutions to improve end-to-end efficiency and enable two- or three-dimensional positional freedom for receiver charging. 

The thesis tackles the improvement of positional freedom from two perspectives. Firstly, increased freedom of movement is achieved through innovative coil designs that guide magnetic flux in desired directions. On the other hand, although plenty of coil structures have been proposed with increased tolerance of misalignment, specific blind spots in power transfer still exist in multi-channel WPT systems, which significantly degrades system overall performance. A new power-combining strategy has been introduced to eliminate blind spots, ensuring full positional freedom under simple control. 

To improve converter efficiency at higher frequencies, new converter topologies and parameter design methods are proposed in the thesis. A simplified version based on numerical equations allows for faster design and optimization across a wide range of operating conditions. Finally, the dissertation also presents system-level discussions and design strategies to mitigate non-ideal effects in real-world implementations. 

In summary, this research offers multiple solutions for building wireless power transfer systems that allow for more flexible charging positions while maintaining high efficiency, opening the door to wider applications in the wireless charging market.

Keywords: wireless power transfer, resonant converters, soft-switching techniques, high frequency operations, position freedom

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

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Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53