Communication Channel Measurement and Modeling for Inductive Wireless Power Transfer

Abstract

Wireless Power Transfer (WPT) technology has experienced phenomenal success in the application of wireless charging of smartphones. With the Qi Specification becoming the dominant standard, wireless charging's convenience goes beyond cable-free charging to facilitate interoperability between chargers and devices from different vendors.

For wireless charging to operate correctly, the power transmitter (charger) and the receiver (user device) should communicate with each other. In the current release of the Qi Specification (Rel 1.2.4), the communication is performed in the same frequency band as the power transfer (in-band communication). Therefore, the properties of the in-band channel determine the quality of the communication link. However, since WPT has only become hugely popular in recent years, the in-band channel has not yet attracted a lot of attention.

In this thesis, the focus is to measure and characterize the in-band channel for scenarios of interest to wireless charging. To enable the channel measurement, a custom-designed setup was made, allowing simultaneous measurements of currents and voltages in the transmitter and receiver. Using this setup, the channel has been characterized as a two-port network using Z-parameters. Observations regarding the Z-parameters, as well as the power transfer efficiency, in various situations are presented and discussed. The first harmonic approximation (FHA) based channel model was applied and extended for the analysis of the results.

Major findings of the thesis include: 1) the amplitude of the imaginary part of the Z-parameters is larger than the real part by one to two orders of magnitude in the frequency of interest (80 to 500 kHz), setting a high accuracy requirement of the measurement system, 2) Power transfer ratio behavior is dependent heavily on WPT circuit parameters interacting with the channel parameters, pointing to opportunities for optimization, 3) The effects of ferrites and foreign objects can be included by extending the simple FHA based channel model with additional coil(s) to model eddy current induced inductance change and resistive losses.

Popular Abstract

In recent years, the use of wireless power transfer (WPT) in mobile devices, like cellphones, has become hugely popular. With companies like Google, Apple and Samsung making use of the technology, WPT is expected to see increased usage not only in cellphones, but in other areas as well, for example in the automotive industry. WPT is in a sense a recent technology. The groundwork was laid in the early 19th century, but practical applications have only become commercially available on large scale in the recent decade.

WPT in its most common form is based on electromagnetic induction between two coils. The magnetic field generated by a transmitting coil threads the receiving coil, which causes a voltage to appear in the receiving coil. This voltage can be used, for example, to charge a cellphone battery, and thus electrical power can be transferred without the use of wires. This thesis work investigates the characteristics of the coil-to-coil power transfer system by making measurements of the voltages and currents in the coils in various different situations. With the use of a custom made measurement setup centered around an oscilloscope, the system has been measured with various distances and alignments between the coils, at various levels of input power and frequency, as well as with nearby metallic objects, so called foreign objects (FOs) or friendly metals (FMs), that can potentially have huge effects on the power transfer.

The results are mainly in the form of Z-parameters, which describe how voltages and currents are affected by the system. The findings show how the Z-parameters, as well as the efficiency of the system, are affected by the environment in which the power transfer is taking place. Attempts have also been made to model the channel through which power is transferred, based on circuit theory. Some of the issues that have come up along the way and insights gained from them during the execution of the study are also presented. It is hoped that the investigation and its results will aid in future studies into the WPT channel.

• author: Rosén, Love
• supervisor: Buon Kiong Lau; Laurens Swaans; Kiavash Faraji
• organization: Department of Electrical and Information Technology
• course: EITM01 20191
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Wireless power transfer, WPT, Qi Specification, Inductive coupling, Wireless charging, Channel modeling
• report number: LU/LTH-EIT 2020-750
• language: English
• id: 9006801
• date added to LUP: 2020-03-18 11:36:51
• date last changed: 2020-03-18 11:36:51