Author

Yibing Zhang

Date of Award

Summer 2020

Document Type

Open Access Thesis

Department

Electrical Engineering

First Advisor

Guoan Wang

Abstract

Wireless power transfer technology (WPT) has been rapidly developed in recent years. The primary benefit of WPT is that it replaces the traditional wire charging with a cordless charging method. WPT technology has been applied in many fields, such as bio-implants, electric vehicles, and wirelessly charging systems. According to the different energy transmission mechanism, WPT technology can be divided into magnetic field coupling (includes magnetically coupled inductive and magnetically-coupled resonant), microwave radiation, laser emission, electrical-field coupling, and ultrasonic transmission type. Among these technologies, the magnetic resonance coupling method has a better promise because of its long transfer distance and high efficiency. However, there are some questions that need to be resolved, among which the most prominent is that the technology has a low tolerance to the variations of the coupling factor because of the frequency splitting phenomenon, which would lead to transmission efficiency degradation of magnetic resonance coupling WPT systems. Hence, based on reviewing the research status and trend of WPT technology, this paper analyses the frequency splitting phenomenon of the wireless power transfer system, discusses the duffing resonator circuit and its properties, and designs a kind of high-efficiency wireless power transfer inductive system with both non-linear inductors and non-linear capacitors. The main research works of this paper are as follows:

Firstly, aiming at the frequency splitting problem during magnetic coupled resonance wireless power transmission, the frequency splitting phenomenon for the wireless power transfer system is studied by an electric circuit model method. The expression of the relationship between the load voltage, transmission efficiency, and coupling factor was derived, and the law of frequency splitting is discussed. Furtherly, an analysis of frequency splitting based on simulation also presented. Finally, the frequency splitting suppression method is proposed. The above research work provides a theoretical basis for solving the problem of frequency splitting and designing a kind of high-efficiency WPT system.

Subsequently, a duffing resonator circuit with a nonlinear capacitor, which can eliminate the frequency splitting and keep the high transmission efficiency and power delivered to the load is developed. With the help MATLAB software, the properties of the duffing resonance circuit are discussed furtherly. The results show that the duffing resonance circuit has significantly wider bandwidth than the conventional linear resonance circuit while achieving a similar amplitude level.

Finally, the high efficiency non-linear wireless power transfer system based on non-linear inductors with ferromagnetic thin film core and non-linear capacitors with ferroelectric thin film dielectrics is designed. Moreover, the system's performance is improved, the range of coupling factors significantly extended while both load power and high PTE were maintained. The reason for the high efficiency of the system is furtherly discussed, and the research result shows that non-linear inductor with ferromagnetic thin film core has variable inductance which can be synchronously changed along with the current through the inductor in the circuit. The non-linear capacitor with ferroelectric thin film dielectrics can also have variable capacitance, which can be synchronously changed along with the voltage applied to the capacitor. However, the voltage across the capacitor and current through the inductors are different initially, high power transmission efficiency can be achieved by self-tuning capability of inductance and capacitance from the film based non-linear resonators.

Research results of this paper can lay the solid foundations for the application of WPT technology in the fields of bio-implants, electric vehicles, wirelessly charging systems, etc.

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