Research On Magnetically-resonant Encrypted Wireless Power Transfer System

With the popularization of the practical application of magnetically-resonant wireless power transfer(WPT)technology,how to realize selective wireless power transfer in multiple power receiving devices,namely encrypted WPT,has become a key issue worthy of research.Magnetically-resonant encrypted WPT technology is based on the frequency selectivity of the system The transmitter randomly changes the system operating frequency to prevent unauthorized receivers from stealing power.The authorized receivers can communicate with transmitter to know the current operating frequency of the system(information encryption),and adjust the self-resonant frequency through variable capacitor to maintain frequency matching with transmitter,so electric energy can be obtained with high power and high efficiency(energy encryption);Unauthorized receivers cannot know the current frequency from the transmitter,and cannot achieve frequency matching,therefore no or only a small amount of power can be received.Compared with other types of variable capacitors,switch controlled capacitor(SCC)has the advantages of continuously variable capacitance,easy to achieve soft switching,and fewer components.The resonance-state key waveforms of the four SCCs with different structures are given.The four SCCs are analyzed and compared in terms of capacitance adjustment range,voltage and current stress,soft switching characteristics,etc.,which provides a reference for the topology selection of variable capacitor for encrypted WPT system.This paper presents the open-loop and closed-loop control methods of the dual-side tuning encrypted WPT system.Compared with the traditional method,the following improvements have been made:(1)A charge increment ratio method for the calculation of equivalent capacitance and control variable of SCC is proposed.The method does not approximately substitute the actual waveforms of SCC terminal voltage and other signals with their fundamental component in the derivation process.Therefore,more accurate calculation results can be obtained than the traditional fundamental wave approximation method,thereby improving the transmission efficiency of the system.In addition,the explicit solution of SCC control variable can be obtained without resorting to numerical calculation,which significantly simplifies the solution process,so it can be applied to online real-time calculation.(2)Aiming at the problem of voltage and current distortion caused by the nonlinearity of the SCC switch,a correction method for the measurement signal of the induced electromotive force of the receiving coil is proposed,which can more accurately match the resonant frequency of the main circuit of the receiver and the fundamental frequency of the power supply,thereby improving the transmission efficiency.(3)A closed-loop control method of encrypted WPT system with a feedforward link is proposed,which solves the possible control failure issues when the system operating frequency steps in a wide range.The method is beneficial to widen the variation range of the system operating frequency,thus making it more difficult for unauthorized receivers to steal power and enhancing the security of encrypted power transmission.The effectiveness of the open-loop and closed-loop control methods is verified by simulation and experiments.The system performance under open-loop and closed-loop control is tested,analyzed and compared from the aspects of steady-state tuning accuracy,dynamic response time,anti-parameter disturbance capability and power encryption transmission performance.The experimental results are basically consistent with the simulation results,and the following conclusions are drawn: The dynamic response of the open-loop system is faster.The steady-state tuning accuracy of the closed-loop system is higher and the robustness is stronger.Both the open-loop and closed-loop systems have good power encryption transmission performance.