6.78 MHz Wireless Power Transfer Circuit Design Under Varying Operation Conditions

Wireless power transfer(WPT)is a promising technology for charging various electric and electronic devices,through which the electric power is transferred to the devices in a wireless manner.It eliminates the power cord and provides a convenient user experience.This technol-ogy has been attracting intensive attentions both in academia and industry.New demands also raise when increasingly applying the WPT into different applications.It is expected that the WPT systems are able to deliver a stable output power to the devices efficiently,and achieve large charging freedom,lower electromagnetic interference(EMI),compact size,and robustness against varying operation conditions.To address these requirements,6.78 megahertz(MHz)WPT systems based on magnetic resonance technique are developed and analyzed in this disser-tation.The 6.78 MHz WPT systems are advantageous in terms of a large transmission distance and smaller size,when comparing with the traditional WPT systems working at the frequency ranging from several dozens to hundreds kilohertz(kHz).Soft-switching based Class E power amplifier(PA)and rectifier are chosen in the 6.78 MHz WPT systems.The system efficiency could be very high under the nominal operation conditions.However,the WPT systems suffer from their high sensitivity to uncertainties in charging environments,such as varying coupling between transmitting and receiving coils,dc load,and number of receivers.So far,few work has simultaneously taken these uncertainties into consideration during the system design.This may make the final system impractical for the following real applications.In this dissertation,comprehensive analysis on the 6.78 MHz WPT systems are conducted under varying operation conditions,and design methodologies are developed to deal with the possible uncertainties.The purpose of this dissertation is to achieve high-performance 6.78 MHz WPT systems(including high efficiency,stable output power,low harmonic distortions,and robustness under varying operation conditions)through the efforts in circuit topology design and parameter optimization.The dissertation first begins with PA design.High-efficiency current-mode(CM)and voltage-mode(VM)Class E PAs are proposed.A design methodology is developed to en-able the CM/VM PA to deliver an increasing/reducing output power when the load increases.High efficiency is maintained over a wide variation of load.The certain operation mode and high efficiency are achieved through load-pull and impedance transformation techniques.The efficiency,output power,harmonic suppression,MOSFET endurance,and robustness are all taken into consideration during the design procedure.This effort makes the CM and VM Class E PAs practical and efficient in real applications.Next,a tunable Class E~2dc-dc converter for6.78 MHz WPT is proposed and analyzed,aiming at delivering a stable output power efficiently under varying operation conditions.The stable output power and high efficiency are achieved by adding a tunable impedance matching network(IMN)between the PA and transmitting coil.Thanks for the analysis on the target matching region and system optimization,tuning range of tunable capacitors and inductors are significantly reduced,and the system robustness against varying operation conditions is enhanced.Finally,the modeling,analysis,and optimization ap-proaches are extended to the single-to-multi(a single transmitting coil and multiple receiving coils)and multi-to-multi(multiple transmitting coils and multiple receiving coils)WPT sys-tems.These systems support concurrent charging to multiple receivers.Each receiver in these two types of systems is able to obtain wireless power independently and autonomously.High system efficiency is maintained when different receivers join into the system.The autonomous power distribution and high efficiency are achieved through system design and optimization.The geometries of transmitting and receiving coils are optimized to obtain uniform magnetic field and high inductance,respectively.The CM and VM Class E PAs and Class E rectifiers are employed for implementation of the entire WPT systems.Tracking and controlling circuits are not required,making the final systems high efficient and straightforward to be implemented.