Modelling and Analysis of Wireless Power Supply based on Electric Field Coupling

Abstract

In recent years, there is a growing trend of using wireless power suppliers as an alternative to traditional fixed contact power supplies for driving moveable devices with improved convenience and safety. Inductive Power Transfer (IPT) based on magnetic field coupling is the most popular wireless power transfer (WPT) solution, which has found many commercial, industrial, and biomedical applications. However, IPT cannot transfer power through metal barriers, and it has many practical limitations with regards to Electromagnetic Interference (EMI), physical size, and power losses. Capacitive Power Transfer (CPT) based on electric field coupling has been proposed as a new wireless power transfer solution that has the ability to cope with metal barriers, and potentially has lower EMI and power losses with a simpler coupling configuration. However, CPT is still in its early stage of development, and has not been widely used in WPT applications due to its low coupling capacitances in air and sensitivity to coupling misalignments. In this Ph.D. research, a single wire/pair CPT system is proposed to reduce the sensitivity of coupling misalignment in comparison with traditional two pairs coupling capacitor CPT systems. An accurate model for such a single wire CPT system is established to understand its power transfer mechanism and analyze the system performance by treating the earth as a quasi-conductor having an equivalent resistance. Furthermore, a practical contactless single wire CPT power supply for driving a Variable Message Sign (VMS) is designed, which demonstrated that about 5 watts of power can be transferred across its metallic back cover. Another significant contribution of this research is the study of power transfer mechanism of CPT system based on Poynting vector analysis. This fundamental research reveals how power is transferred across the coupling plates, and how it is distributed from the field point of view in a conventional series tuned CPT system. The average power flow to the load across the air gap between and around the coupling plates is determined by analytical and numerical analysis of the electric field, magnetic field, and Poynting vector. Compared to the traditional lumped circuit analysis, a clear advantage of Poynting vector analysis is that there is no need to consider the cross coupling separately because the whole coupling setup is considered all together from the field distribution point of view. This research is useful to gain a deeper understanding of capacitive power transfer mechanism and power flow distribution beyond the traditional circuit theory. The results obtained from this research demonstrate that the single wire CPT system has a great potential for developing simple wireless power supplies with larger distance and coupling tolerance, which may be used for charging low power compact devices such as biomedical sensors, Bluetooth headsets, and mobile phones. Moreover, the proposed Poynting vector analysis method shows the real and reactive power distribution around the coupling plates in relation to their physical setup (size, airgap distance, etc.), which would be useful for visualizing the power transfer channel and help practical EMC and safety design in the future.