Fully Integrated Wireless Power Transfer System For Biomedical Applications
Wireless power transfer (WPT) is commonly realized by means of near-field inductive coupling and is critical to many applications. It is highly desirable to fully integrate the receiver coil (Rx) and power shaping circuits on a single chip in a standard CMOS process with no additional post-processing steps or external components. This thesis investigates the feasibility of a fully integrated WPT system that is capable of wirelessly providing milliwatts of power at centimeter distances. We present a closed form analytical solution for the optimum load that achieves the maximum possible power efficiency under arbitrary input impedance conditions based on the general two-port parameters of the network. Our results generalize several well-known special cases. The formulation allows the design of an optimized WPT link through biological media using readily available electromagnetic (EM) simulation software and effectively decouples the design of the inductive coupling two-port from the problem of loading and power amplifier design. As a proof of concept, we present a 2x2.18mm2 on-chip coil with proper power shaping blocks such as a rectifier, and a regulator in addition to an adaptive matching network, which are co-integrated on a single die in a 0.13um CMOS process. The receiver is separated from the transmitter (Tx) by 10mm of biological media. The on-chip coil demonstrates a peak WPT efficiency of 1.42% in air, 0.97% efficiency while receiving power through a combined 7mm of 0.2 molar NaCl (with similar conductivity and permittivity as blood) and 3mm of air, and 0.8% of efficiency through a combined 7.5mm of bovine muscle and 2.5mm of air. Fully integrating the receiver coil introduces new challenges and opportunities that led to new circuit innovations in each power shaping block. The adaptation block employs a synchronous sampling scheme and provides the optimum susceptance on the basis of the mathematical derivations for the optimum load. The rectifier demonstrates an efficiency of 80% and uses a positive feedback structure to avoid plasma-induced gate-oxide damage caused by the on-chip coil. Finally the integrated regulator employs an auxiliary rectifier to provide better than 24dB of power supply rejection ratio (PSRR) over all frequencies with a low quiescent current of 6uA and no off-chip components.
energy harvesting, Inductive coupling, integrated CMOS coil, near-field, Regulator, Wireless power transfer
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