Research On 7.7kW Wireless Charger With Two Compensation Methods Considering EMF Limit

The technology of wireless power transfer(WPT)used in electric vehicle(EV)helps reduce the requirement of battery capacity,provides a safe and convenient user experience and gives energy assurance for automatic driving.The wireless charger has a large working gap and a open magnetic field.The dissipated high-frequency electromagnetic field can pollute the electromagnetic environment,which is potentially harmful to living organisms.This paper focuses on the design method of wireless chargers for EV with comprehensive consideration of efficiency,stress and electromagnetic exposure safety limits under parameters variation.Firstly,two normal compensation topologies for EV: series/series(S/S)and bilateral LCL are analyzed.Under the condition of constant current outputs,the analysis of transformer efficiency and stress characteristics is carried out.The optimal efficiency and optimal load conditions are derived.It indicates that bilateral LCL owns higher design freedom for its high-order compensation structure.Ensuring the stress constant,the optimal load conditions can be adjusted.At the same time,compared to S/S structure,the overall efficiency of bilateral LCL is more flat under parameters variation.Secondly,this paper proposed a new idea of using the transformer's primary ampere-turns and secondary ampere-turns as quantitative reference of the EMF design.The analysis shows that the product of primary ampere-turns and secondary ampere-turns is the same under S/S compensation and bilateral LCL compensation.Based on the analysis,a EMF cancel method by adjusting the allocation of primary and secondary side amperes-turns is proposed.The simulation has been done to verify its effectiveness,the overall design process is given under consideration of efficiency,stress,and EMF.The model of transformer,human body and EV are bulit under different working conditions and different postures.The EMFcancel effect of the metal shielding plate is studied,and its influence on the inductance,coupling coefficient and eddy current loss of the transformer is analyzed and verified by simulation.Finally,7.7kW wireless charging prototypes based on the S/S and bilateral LCL compensation are built.The efficiency characteristics of the two compensations are compared under misalignment,and the bilateral LCL is improved according to the optimal load point.The shown EMF test results of the ELT-400 meet the magnetic field strength limit given by SAE J2954 at full misalignment and full air gap.The optimized bilateral LCL compensation has a minimum efficiency of 90.7% and a maximum efficiency of 91.4%.