Research On Hybrid Capacitive And Inductive Wireless Power Transfer Technology

Wireless power transfer technology uses different kinds of fields(e.g.magnetic field,electric field and ultrasound,etc.)as power transmission medium.It has unique advantages in applications like electric vehicles,consumer electronic devices,industrial field equipment and implantable medical devices.Thanks to its unique feature of transferring power without the need of physical contact,a wireless powered system is generally superior to the conventional wired power transfer system in terms of safety,neatness,aesthetic and ease-of-use.It is obvious that this technology has a broad application prospects and huge commercial value.The advantage of inductive power transfer is that it can easily realize high power and long distance transfer.However,the magnetic field is naturally devergent,which may generate eddy current in the nearby metal and other low resistance objects.The eddy current,on the one hand,will heat up these objects;on the other hand will seriously affect the wireless power-transfer-system efficiency.The electric field of capacitive power transfer is more concentrated between the primary and secondary capacitor plates,thus has less side effects to the surrounding environment.For example,it almost does not generate eddy current in the surrounding metal objects.Moreover,the metal capacitor plates is also easier to realize low cost and high flexibility as compared with the inductive power transfer.However,the capacitive power transfer also suffers from some disadvantages,like lower power density,sensitive to the coupling capacitance variation and high voltage stress around the coupling capacitors when transferring power increases.The hybrid capacitive and inductive wireless power transfer technology can utilize the advantages of both capacitive power transfer and inductive power transfer.It has unique advantages in terms of improving system efficiency,increasing system power density and reducing electric/magnetic field radiation.However,the state-of-art researches on hybrid capacitive and inductive wireless power transfer are still very limited.Many work need to be done especially in terms of the modeling and design of the hybrid coupler,the modeling and control of the voltage stress of the coupler,how to reduce the leakage electric/magnetic field,optimized design of the high frequency inverter for wireless power transfer application,the system efficiency optimization.This thesis focuses on the research of hybrid capacitive and inductive wireless power transfer technology.The main research contents of the thesis are as follows:(1)The modeling and optimized design of capacitive coupler and inductivecoupler.An optimized modeling and design method of the capacitive coupler and inductive coupler is proposed and analyzed.The parameters that affecting the coupling capacitance and voltage stress of the capacitive coupler as well as those affecting the self-inductance of the coil are evaluated using theoretical analysis,FEM simulation,circuit level simulation and experimental measurement.A general design method of capacitive coupler and inductive coupler is proposed.On top of that,an optimized calculation and design method of capacitive coupler considering fringing effect using conformal mapping is proposed.A voltage stress oriented capacitive coupler modeling and design method that can realize minimum voltage stress is proposed and verified with simulation and experiment.The proposed method can be used to find the minimum voltage stress point of a capacitive coupler.(2)The modeling and optimized design of megahertz inverter for hybrid wirelesspower transfer systemThe coupling capacitance of the hybrid system is normally in the pico-farad level,to increase the power transfer ability,the system switching frequency needs to be pushed to megahertz.By comprehensive utilization of the up-to-date gallium nitride technology,a design method of the high frequency power converter for hybrid wireless power transfer application is proposed.The key parameters that affecting the design of the high frequency converter are analyzed first,this including the junction capacitance of the rectifier,the output capacitance of the MOSFET,the gate charge of the MOSFET and the parasitic parameters of the PCB,etc.The high frequency model of the power semiconductor device is built,and the high frequency gate driver circuit design method is given.A circuit level high frequency simulation model is built based on the above device model and gate drive circuit.The different PCB layout method is compared regarding their parasitic inductance of the switching loop,and an optimized layout method is given based on the comparison results.Finally,several megahertz power converter prototypes are designed and implemented based on the design method investigated in this thesis.(3)Research on parallel type hybrid capacitive and inductive wireless powertransfer technology.Several problems exist in the current inductive and capacitive power transfer technology.For example,the output of a capacitive power transfer system is susceptible to the coupling status;the power density of a inductive power transfer system is relatively low and the leakage magnetic field of a inductive power transfer system is seriously high.A parallel hybrid capacitive and inductive wireless power transfer technology is proposed.An in-depth research on the design method of the hybrid coupler is conducted in connection with characteristics of the coupling capacitance and inductance in different states.The mathematical models of the capacitive branch,the inductive branch as well as the hybrid coupler are derived and applied to the design of the hybrid system.Aiming at the optimized gain characteristics,the system design method is proposed,and a prototype is designed to validate the theory.At 6.78 MHz switching frequency and 40 W output,the system efficiency reaches 80.02%.When the coupling capacitance varies from 70 p F to 480 p F,the open-loop output voltage variation is less than 8V,which is much lower than the 18 V variation of the conventional capacitive power transfer system.(4)Research on “receiver-controlled” hybrid capacitive and inductive wirelesspower transfer technology.To solve the problem that a single input multiple output(SIMO)wireless power transfer system suffers from serious leakage magnetic field and low system efficiency,a “receiver-controlled” hybrid capacitive and inductive wireless power transfer technology is proposed.Firstly,the “receiver-controlled” wireless power transfer concept is explained.Then the working principle of the proposed coupler based on “receiver-controlled” concept is explained,as well as its design method and influencing factors.A single input/ three outputs wireless power transfer prototype has been built based on the proposed “receiver-controlled” concept for experimental validation.The experiment result shows an obvious increase of system efficiency to about 81.42% with 6.78 MHz switching frequency and 120 W output power.The leakage magnetic field of the no-load transmitter coil is also reduced to the level that close to the background noise.