UBC Theses and Dissertations
Idle power loss suppression and spacing optimization in multi-coil wireless power transfer systems Badowich, Connor Brent
This thesis presents two optimization studies of multi-coil wireless power transfer (WPT) systems: suppressing idle power losses in WPT, and maximizing the efficiency with optimized spacing. In the first study, idle power losses are characterized and suppressed in a 4-coil WPT system. By deriving analytical expressions for efficiency and idle power loss from an equivalent circuit model, it is found that peak losses occur when a system is designed for maximum efficiency. A design of a 4-coil system is proposed which suppresses idle losses by mismatching the transmitter from the source. The receiver of the proposed design is then optimized to compensate for the mismatch and maintain high efficiency. Using this technique, idle power losses were reduced from 38% to 13%, while the efficiency only dropped from 85% to 76%. A 13.56 MHz WPT system is designed and tested to verify the effectiveness of idle power suppression. In the second study, the spacing within a cascaded chain of identical resonator coils is studied and optimized to maximize the wireless power transfer efficiency. Although the efficiency of multi-coil systems is in general not convex in terms of the spacing, we show that it is convex when the higher-order coupling between non-adjacent coils is neglected. Convex optimization is then applied to this approximated model and it is determined to be valid for loosely spaced systems. For tightly spaced systems the higher-order coupling is accounted for by treating the terms as a perturbation to the convex solution. A two-step optimization process is conducted which uses the convex solution as an initial point for a full-system optimization. This process is applied to chains of up to 30 coils transmitting power across distances up to 4.0 m, with variations in quality factors of the coils. From these optimizations we show that there exists an optimal number of coils to use at a fixed transmission distance for maximum power transfer. Furthermore, we show that placement of select high-Q coils provides the greatest benefit to efficiency when positioned across the largest spaces in the chain.
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