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Title: A near-field wireless power transfer system with planar split-ring loops for medical implants
Author: Wang, Jingchen
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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With the continuous progress in science and technology, a myriad of implantable medical devices (IMDs) have been invented aimed at improving public health and wellbeing. One of the main problems with these devices is their limited battery lifetime. This results in otherwise unnecessary surgeries to replace depleted batteries leading to excessive medical expenses. Wireless power transfer (WPT), as a promising technology, could be used to remedy this. Wireless power technologies, both through the transfer of transmitted radio frequency (RF) power or the harvesting of RF energy from the ambient environment and its subsequent conversion to useable electrical energy, are emerging as important features for the future of electronic devices in general and have attracted an upsurge in research interest. Unfortunately, the path to realising this wire free charging dream is paved with many thorns and there still exist critical challenges to be addressed. This thesis aims to deal with some of these challenges, developing an efficient WPT system for IMDs. The work begins with a comprehensive study of currently applied methods of WPT, which broadly fall into two categories: far-field (radiative) WPT and near field (nonradiative) WPT. The review includes a brief history of WPT, comparisons between current methodologies applied and a comprehensive literature review. Magnetic resonance coupling (MRC) WPT is emphasised due to its advantages for the desired application making it the technology of choice for system development. Design of an MRC-WPT system requires an understanding of the performance of the four basic topologies available for the MRC method. Following an investigation of these, it is found that series primary circuits are generally most suitable for WPT and that the choice of a series or parallel secondary circuit is dependent on the relative size of the load impedance. Importantly, design parameters must be optimised to avoid the phenomena of frequency splitting to simultaneously obtain maximum power transfer efficiency (PTE) and load power. The use of printed spiral coils (PSCs) as inductors in the construction of WPT circuits for IMDs, which can save space and be integrated with other circuit boards, is then investigated. The challenges and issues of PSCs present for WPT mainly relate to maintaining an inductive characteristic at frequencies in the Medical Implant Communication Service (MICS) band and to maximising the PTE between primary and secondary circuits. Investigations of PSC design parameters are performed to obtain inductive characteristics at high frequencies and the split-ring loop is proposed to increase the Quality factor relative to that offered by the PSC, which is shown to enhance WPT performance. To simplify the necessary resonating circuit configuration for MRC-WPT, a self-resonating split-ring loop with a series inductor-capacitor characteristic has been developed. A pair of these self-resonators has been adopted into a series primary-series secondary WPT system operating at high frequency. This is different to traditional planar self-resonators, which offer parallel self-resonance characteristics that are less desirable due to their reduced system power insertion as a parallel primary resonator. Finally, a system for implantable devices is developed using the split-ring loop in consideration of the effects of body tissues, whose dielectric characteristics have a significant influence on WPT performance. Due concern is also paid to human safety from radiated RF power. A series resonating split-ring loop for transmitting power is formed at the desired frequency through the addition of a lumped element capacitor. A single loop as a receiving resonator with a low Specific Absorption Rate (SAR), is designed to allow greater transmit power to be used in comparison to previous work, whilst satisfying the relevant standards relating to human safety. A rectifier circuit is also designed to convert the received RF energy into useable electrical energy allowing the realisation of the proposed WPT system. In a nutshell, this thesis places emphasis on solutions to overcome challenges relating to the use of MRC-WPT for IMDs. An efficient near-field WPT system for such devices is successfully demonstrated and should have profound significance to pushing forward the future development of this topic.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral