| In recent years,Wireless Power Transfer(WPT)has attracted wide attention due to its flexible transmission mode and the power safety performance provided under severe conditions.The introduction of this technology will make the production,distribution,and use of electrical energy more extensive and more diversified.Wireless power transfer can be implemented based on a variety of conversion principles,of which Inductive Power Transfer(IPT)is the most widely used.The IPT system is very similar to the traditional resonant converter in terms of power conversion architecture,but the loose coupling nature of the transformer introduces many problems on the magnetic flux and circuit level.Based on this,this paper starts with the particularity of loosely-coupled resonant converters and studies its multi-scale modeling and multi-objective optimization problems.Firstly,using a multi-objective optimization viewpoint and finite element analysis,a set of simple and universal transformer evaluation and construction theory are proposed to solve the problem of comprehensive optimization of the mechanical structure,magnetic flux leakage and efficiency of loosely coupled transformers.Secondly,the performance of conventional compensation topology in different dimensions under a wide range of operating conditions is compared.A multi-objective evaluation mechanism for compensation topology is proposed,and a general design principle is refined.Based on this,the time domain and frequency domain of LCL-type loosely coupled resonant converter are derived.Analytic modeling is performed to determine the behavior of the converter at multiple scales.Finally,the common problem of the coupling sensitivity of the IPT system is solved,which extends the scope of application of the system.Coupling coils are the actuators of wireless power transfer and are the core of the system's performance.This article uses the efficiency index as a starting point for analysis,combined with finite element numerical simulation and theoretical calculation to establish an accurate coupling loss model.On this basis,quantitative descriptions are made of the linkage between design freedom and efficiency,power density,EMI and other key indicators.From the practical point of view,the constraints between the various key indicators of the coupled coil are systematically discussed.The proposed method constructs a holistic evaluation method for the power transmission capacity,efficiency,magnetic flux leakage level and power density of the coupled coils,and achieves the optimal balance among the various indicators of the coil under multivariable constraints.Finally,a circular symmetric coil with a diameter of 38 cm is built in the experiment.With an air gap distance of 15 cm,the efficiency reached 95.54% and the leakage flux did not exceed 19.2 uT.Experimental results agree well with the loss model and finite element numerical model,which verified the effectiveness of the method.Compensation topology is the basic structure of loosely coupled resonant converters,and resonant converters with different compensation structures exhibit large differences in performance.Therefore,for a typical application scenario,constructing a comprehensive evaluation index of the system is the basis for a reasonable choice of compensation topology,and is also the key to determine the research object.In chapter 3,the common compensation topology of loosely coupled resonant converters is summarized,and the design principles of different compensations are elaborated.Detailed comparisons are made on the compensation topologies from reactive power compensation capability,inherent power transfer efficiency and gain stability.After synthesizing the comparison results,the performance differences of the different compensation topologies in a wide operating range are described and the applicable range is defined.Through comparison,the excellent characteristics of the LCL compensation network are extracted.Based on the previous paper,this paper discusses the application of the reduced-order model in LCL LCRC,and builds a general LCL resonant converter time/frequency domain model.In the time domain model,considering the influence of higher harmonics,the system order under the effect of higher order harmonics is reduced,and a unified analytical expression of harmonic impedance is established to assist in solving the steady-state time domain waveform of the resonator.Combined with the steady-state time-domain analysis,the resonant parameter design can be instructed in detail to ensure the wide-coupling and ZVS conditions within a wide load section and a high power factor.In the frequency domain model,the extended describing function method is used to decouple the AC and DC variables in the direct-AC-DC system.The large-signal model is established at a time scale well below the switching frequency and further extracted.The low-frequency characteristics of the small-signal model prove that the frequency-domain behavior of the 6-order LCL-type LCRC can be essentially replaced by the 2nd-order underdamped equivalent system without loss of transient and steady-state description accuracy.The proposed equivalent reduced-order model greatly simplifies the analysis of the system frequency domain model and can effectively guide the design of the controller.The equivalent reduced-order model was used to predict the stability boundary and characteristic frequency ripple,and the limitations of the traditional PI controller in suppressing the output ripple and maintaining the system bandwidth is also demonstrated.To solve this problem,by combining the characteristics of the PI controller and the trap,it is possible to obtain a faster system response while suppressing the output ripple.Finally,the accuracy of the model and the effectiveness of the proposed solution are verified.Aiming at the problem of the influence of easy coupling coefficient on the power transmission characteristics of LCL loosely coupled resonant converter,an improved topology based on LCL is proposed to adapt to the change of the coupling coefficient over a wide range.From the perspective of smooth output power,this chapter quantitatively analyzes the relationship between voltage gain and output impedance angle and coupling coefficient and passive component parameters.Through analysis,it is found that the power characteristics appear as a single-peak curve with coupling fluctuations,and the change around the peak is relatively gentle.Therefore,the coupling section can be positioned near the peak point by properly designing parameter,so as to obtain more stable power characteristics.In the specific implementation,a three-element compensated LCC topology is fabricated by adding a first-order degree of freedom to the LCL resonant cavity,and the output power is approximately constant under large fluctuations of the coupling coefficient.Compared with the conventional topology,this compensation topology greatly reduces the sensitivity of the coupling coefficient.Finally,the effectiveness of the design method is verified by experiments. |
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