Design of wireless power transfer systems in the presence of living objects

Abstract

Many existing wireless power transfer (WPT) systems are designed for a free-space environment with efficiency maximisation as the primary objective. However, in the presence of living objects, the design of such systems needs to consider modifications to the magnetic link and field distributions due to the interaction between the system and biological tissue. Parameters that maximise link efficiency in air should be changed to regain optimal system operation. Furthermore, additional design constraints are required to minimise adverse health effects and meet safety requirements.

Firstly, this thesis presents an overview of the design of WPT systems in air, using link efficiency-led principles in conjunction with high-frequency soft-switching power electronics. This is followed by a summary of safety considerations applicable to WPT systems, and a simulation study to evaluate the compliance of a wirelessly-powered drone demonstrator to safety limits.

Wireless power for high-power implantable medical devices is then studied. A general link efficiency expression is derived for arbitrary tuning capacitance and receiver load. A coil model is devised, incorporating a frequency- and tissue type-dependent series resistance for losses due to a magnetic field. Varying frequency, number of coil turns, tuning method and receiver load can lead to improved link efficiency, lower tissue heating and reduced temperature rises.

Whilst WPT for high-power implantable medical devices is feasible only over short distances, the range can be extended for low-power medical implants and wearable devices. The development of a position-insensitive long-range WPT system is reported, in which an ultra-low power open-loop flyback impedance emulator maintains maximum power transfer, and transmitter power modulation using wireless feedback prevents receiver damage when moving close to the transmitter. For such systems, a method of detecting living objects in a coil's near field is found to be possible, which could be used in foreign object detection for safety purposes.