MODELLING AND EFFICIENCY OPTIMIZATION OF WIRELESS POWER TRANSFER SYSTEMS HAVING ONE OR TWO RECEIVERS

Abstract

This thesis presents a study on the modelling of wireless power transfer systems and techniques to improve system efficiency. Using time domain modelling, useful modes of operation are identified, and performance graphs are plotted which help in designing the system for various consumer specifications, like power delivery and efficiency. The modelling is useful in finding curves for voltage gain, rms current, turn-on current of switches, ZVS angle, peak stresses across capacitors and switches, etc., as a function of load, operating frequency and coupling coefficient among receivers and transmitters. The regions of operation where zero voltage switching (ZVS) is obtained are also found as a function of load, coupling coefficient and system gain. Modelling of wireless power transfer systems is done predominantly using frequency domain modelling which approximates the currents and voltages in the resonant tank to be sinusoidal waveforms. Hence it provides an approximate analysis which requires overdesign of components to account for inaccuracies in the modelling. Time domain analysis as presented in this work gives exact solutions to designers. Experimental results on a wireless power transfer system prototype with one and two 5V, 5W receivers show the accuracy of the time domain modelling. This thesis also provides a comprehensive analysis of efficiency optimization for systems involving one or more receivers. For systems with voltage regulation on the output side, a study in this thesis demonstrates the required gain of the power conditioning unit to get a better system efficiency. This study is verified with experimental results on a system with half bridge inverter and rectifier on transmitter and receiver, respectively. For receivers with cross-coupling among them, the resonant frequency of the system changes. This work shows that the presence of cross-coupling between the receivers may or may not be beneficial to efficiency, so the focus is on the improvement of efficiency in its presence rather than the elimination of cross-coupling. To improve the efficiency, a switched capacitor network has been proposed that varies the resonant frequency of the receivers in the presence of cross-coupling to improve the total system efficiency using a perturb-and-observe algorithm. Experimental verification of a WPT system with two 5V, 5W receivers (having cross-coupling) show the validity of the analysis and proposed technique to improve efficiency.