Abstract
Wireless power transfer systems have been widely applied in the field of portable and implantable devices, featuring contact-free and reliable energy supply. Novel implant systems, such as brain-computer interfaces, impose the challenges of strong miniaturization and operation under loosely coupled conditions. Therefore, maximizing power transfer efficiency while decreasing the size of transmitter and receiver structures becomes a central research question. This paper presents a unified design strategy of modeling, analyzing and optimizing planar spiral coils with integrated capacitive elements, so-called capacitively segmented coils, for operation in wireless power transfer interfaces. It mathematically analyzes and experimentally verifies that the combination of capacitive coil segmentation, increased operational frequencies and geometrical coil optimization can be used to establish wireless power transfer links with comparatively high efficiency, small size and limited detuning effects in lossy dielectric environments. The paper embraces the formulation and verification of a broadband analytical link model based on partial element equivalent circuits, which is subsequently used to determine dominant coupling and loss mechanisms and to optimize the coils' geometries for high efficiency. Moreover, an extended analysis shows how the capacitive coil segmentation can effectively suppress dielectric losses and non-uniform current distributions by canceling the inductive contribution of every coil segment at the frequency of operation. Utilizing these methods, an exemplary 40.68MHz wireless power link with a 30mm primary and a 10mm secondary coil is designed and evaluated: With a maximum efficiency of up to 31% in biological tissue at 20mm separation distance, it features efficiency levels which are up to ten times higher and a specific absorption rate which is up to five times lower compared to non-segmented systems. When operated at 150MHz in air, efficiency levels are up to 1.5 times higher than in state-of-the-art systems of the same size.
Highlights
Since the end of the 19th century, engineers are seeking for concepts and mechanisms to enable wireless power transfer (WPT) systems which are both efficient and reliable
The measurement results with respect to maximum efficiency and real part of the coil impedances R11 are shown in Fig. 19: First of all, it can be noted that the capacitive segmentation is already increasing the efficiency level of the coil system in air, minimizing dielectric losses within the FR4 substrate
Attaching the unsegmented coil system to a stack of tissues strongly decreases the efficiency, while the coil impedances strongly deviate from the impedances in air: R11 is increased by a factor of 10, which would detune connected power conditioning circuits in practical applications
Summary
Since the end of the 19th century, engineers are seeking for concepts and mechanisms to enable wireless power transfer (WPT) systems which are both efficient and reliable. The increase of the operational frequency and the capacitive segmentation of inductors, these design strategies have not been combined to reach the optimal performance It is the aim of this paper to provide a unified and broadband model of the wireless link to capture all relevant near-field and self-resonance effects and to deduce an optimization strategy which allows to operate coils with high number of turns in the very high frequency range with very minor detuning effects due to dielectric and lossy material, facilitating the generation of high efficiency and miniaturized WPT systems. The technical part of this paper is organized as follows: A partial element equivalent circuit model including inductive and capacitive effects is deduced and evaluated within section II in order to obtain a broadband model of the wireless power transfer interface As various matrix inversions are included, the computation of Z will require suitable numerical tools
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