Research On Compensations Under Condition Of Parameters Variations In Inductive Power Transfer Systems

Wireless power transfer(WPT)technology has some irreplaceable advantages compared to plug-in solutions in applications such as biomedical implants,household electronics,electronic vehicles,mining applications and so on.Inductively coupled WPT(IPT)technology is based on magnetic coupling.Since IPT is relatively mature,the IPT technology has been widely used in a variety of applications.However,due to the relative position between the primary and secondary windings isn't fixed,the values of inductances and coupling coefficient of the loosely coupled transformer are changeable during operation.How to keep the output of the IPT system stable under the condition of wide variations of load and transformer's parameters is an urgent issue to be investigated.This thesis attempts to make an in-depth study of the output characteristics of the compensation used in IPT systems,aiming to find suitable compensation whose output is insensitive to parameters variations and output fluctuation versus parameters variation is small so as to improve the performance of the system.The main achievements of this thesis include: 1.Proposing two novel compensations which are suitable for the condition of wide variation of parameters.The output and input characteristics of the existing compensations in IPT systems under parameters variations are studied.And then,the optimization goals to achieve not only small output fluctuation,but also high efficiency,are presented.To realize the optimization goals,two novel compensation topologies,named series/series-parallel(S/SP)and parallel-series/series(PS/S),are proposed.These two compensations can achieve the optimization goals of:(1)output independent of not only load but also transformer's varying parameters such as coupling coefficient and mutual inductance,(2)zero input phase angle(ZPA).2.Proving the output characteristics of the proposed compensations are less sensitive to parameters variations.By comparing expressions of the transformer parameters under different relation positions,it is found that all transformer parameters can be expressed as functions of coupling coefficient.Therefore,the sensitivity of the output to coupling variation can be derived by partial differentiating the output with respect to coupling coefficient.By comparing the output sensitivities of the converters adopting S/SP and PS/S compensations with the converters adopting existing compensations,it is demonstrated that the output of the S/SP and PS/S compensations are less sensitive to the parameters variations.The effectiveness of the proposed compensations and the correctness of the theoretical analysis are verified according to simulation and experimental results.3.Presenting the parameter design method of the proposed S/SP compensation.The bifurcation phenomena of the S/SP compensated converter is studied.It is found that during practical operation the S/SP compensation will have more than one ZPA frequencies.Thus it is better to adopt fixed-frequency control.After analyzing characteristics of output gain,input impedance,and efficiency,the design criteria to achieve output-insensitivity,soft-switching,and high efficiency,as well as the selection of the designed operating point and operating frequency are given.Then,a systematical design methodology for an efficient IPT system is formulated.A 1.5 kW S/SP compensated converter with fixed-frequency control is built to achieve stable output and high efficiency under wide range of parameters variations.The efficiency of the prototype reaches 95.2%.The output changes by less than 5% at whole load range even though the coupling coefficient changes by 200%.4.Proposing a novel time-domain analysis method for resonant converters.A S/SP compensated resonant converter has been taken as an example to study the analysis method of resonant converters when serious waveform distortion occurred and the error resulting from the use of fundamental harmonic approximation(FHA)is significant.A generalized equivalent circuit of the S/SP resonant tank is derived,the accurate expressions for the key variables of the resonant tank and output voltage gain of the S/SP converter are given,and the error and cause of error resulting from the use of FHA are revealed.A simpler but more accurate analysis method for the high-order resonant converters is presented.Experimental results are in good agreement with the calculated results.5.Revealing the essential reason of the smaller output fluctuation of S/SP and PS/S compensations.A new classification of the compensations resulting in load-independent output are presented.The output characteristics for each compensation type are summarized,analyzed and compared with each other.It is found that the essential reason why the output fluctuations of the proposed S/SP and PS/S compensations against variations of load and coupling are small is due to their outputs at the designed operating point are load-and coupling-independent and their outputs exhibit a unimodal relationship with coupling.This finding provides a new research idea for the optimization of the compensation under the condition of wide variation of parameters.