Adaptive tuning of resonant inductive power and communication links

Abstract

Resonant inductor-capacitor (LC) circuits with high quality factors are required to efficiently generate large magnetic fields for transmitters in wirelessly-powered communication applications. The narrow bandwidth from such high quality factor circuits necessitates accurate tuning of the resonator, requiring multiple tuning capacitors and additional circuitry, incurring greater manufacturing costs due to the need for greater board and die area, as well as a greater number of high-voltage switches needed to withstand the high capacitor voltage caused by the high quality factor. Furthermore, the narrow bandwidth of the resonant circuit impedes fast data transmission to the receiver, resulting in a trade-off between power transfer efficiency and data bandwidth. This work explores a new topology and timings for tuning large-signal LC circuits, whereby only a single additional capacitor is needed to achieve accurate and wideband tuning. This method of zerovoltage switched fractional capacitance is low-loss and synchronous with the resonator oscillation. The need for only a single tuning capacitor significantly reduces system costs by reducing the amount of high-voltage circuitry required. The new tuning method also permits easy detection of the resonant condition, allowing for low system complexity for a self-tuning architecture. The tuning method also facilitates synchronous adjustment of the resonant and operating frequency, presenting the opportunity for frequency and phase modulation at data rates exceeding the classical limitations due to the antenna quality factor. This report consists of five primary sections: literature review; analysis, simulation and implementation of the new tuning method using LTSpice and discrete components; implementation of the tuning method in an integrated circuit; using the tuning method to achieve synchronous frequency/phase modulation without energy loss from the antenna; and implementation of the modulation method in an integrated circuit. The initial simulations and breadboarding provide insight into the system-level challenges, however the main benefits of the tuning method are best realised in integrated form, with all system functions on a single silicon die. Translating the basic tuning and modulation concepts into self-contained integrated systems involves considerable amounts of design effort, as well as additional challenges to overcome and trade-offs to make. The first integrated circuit explores the basic tuning method and resonance detection, and how to combine high-speed timing generation and tuning error detection with on chip antenna drivers and tuning switches. The second integrated circuit includes new system-level architecture to achieve frequency/phase shift modulation, with new architecture developed to achieve precise modulation.

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