The Design And Optimization Of 3kW WPT Charging System With Asymmetric Loosely Coupled Coil

Wireless power transfer(WPT)realizes the power transfer between the source and load through an air gap,which has the irreplaceable advantages compared with traditional cable-type power transmission,such as safety,reliability,and flexibility.According to the energy transmission mechanism,WPT is clarified into the radiation,the capacitive coupling,the inductive coupling and ultrasonic wave,and so on.Among several wireless power transfer methods,inductive power transfer(IPT)has apparent superiorities in the power transfer ability and efficiency.However,the conventional IPT system has the following drawbacks:(1)Due to the larger air gap between the primary coil and the secondary coil,the magnetic flux cannot be strongly coupled,which leads to the excessive reactive power loss,and decreases the conversion efficiency.(2)For the battery charging applications,the variable charging voltage and current,and the equivalent load,during the entire charging process increase the difficulty of system parameter design.(3)In traditional IPT system,the loosely coupled coils is mostly implemented by a symmetrical structure,i.e.the primary and secondary coils are with the approximate size.However,in practical applications,the secondary coil size tends to be smaller due to the installation space limit.Therefore,in order to realize a high efficiency IPT wireless charging system,on basis of studying inductive power transfer mechanism with Series-Series(SS)compensation topology,the relationship among inductive coil parameters,system efficiency,resonant voltage and current are derived.Furthermore,under the size limitation of the secondary coil,the design and optimization flow of the loosely coupled coil is proposed with considering the resonant voltage and current stress and the transmission efficiency.Firstly,the inductive transmission mechanism is derived.the parameters relationship between the mutual inductance model and the transformer model is analyzed..Moreover,this thesis point out the transformer model is easy to be employed in system design based on the desired voltage gain.After that,aiming at the project specification,by comparing the characteristics of the different compensation topologies,the SS compensation is selected in my design.Meanwhile,the characteristics of SS compensation are analyzed based on the transformer equivalent model.The theoretical analysis result shows that when the equivalent leakage inductance is fully compensated,the system can operate at the point where the voltage gain is independent of the load.Besides,the input impedance is inductive,which means zero-voltage switching(ZVS)of all primary MOSFETs is realized.Next,according to the characteristics of the battery load and the typical two-stage charging curve,the maximum charging power point of the charging curve is chosen to calculate the coil and compensation parameters.Combined with the project requirement,the asymmetric coil structure(primary coil diameter is 260 mm,secondary coil diameter is 110 mm)is adopted.Under the constrain of small size of secondary coil,the optimization process of loosely coupled coil parameters design are proposed by considering the resonant voltage stress,current stress and conversion efficiency.The influence of overlapping coil structure on parameters and transmission performance is also analyzed.The implementation principle and feasibility of pulse frequency modulation(PFM)strategy are discussed.Finally,based on the SS compensation topology and the above proposed design flow,A 3kW wireless charging system with peak efficiency of 96% has been designed,fabricated and tested in the lab.The IPT prototype has the 360~400V input,200~300V output,and the maximum charging current of 10 A..Based on the frequency control,the two-stage charging process of the battery is carried out,and the ZVS of the primary inverter power transistors are achieved during the entire charging process.The experimental results show the designed prototype not only can achieve the desired transfer power but also greatly reduces the resonant voltage and current stress as compared to the traditional design approach.