Research On High Efficiency Wireless Charging Systems For Electric Vehicles And Its Wide Range Soft-switching Techniques

The wireless charging technology of electric vehicles can charge the vehicle conveniently and flexibly.It has many advantages such as safety,no mechanical wear and easy maintenance.It is a very promising technology.However,it also has some disadvantages such as high cost,heavy equipment weight and low efficiency,which limit its wide commercial application.This paper will focus on the research on the efficiency improvement technologies of electric vehicle wireless charging system.The specific research includes system optimization design,soft switching modulation,system model and control,and maximum efficiency tracking strategy.This paper firstly researches the multi-objective optimization method for electric vehicle wireless charging system.By analyzing the input and output characteristics and efficiency characteristics of electrical vehicle wireless charging systems,and the design method of the circuit parameters of the coupl er is proposed.By analyzing the cause and calculation method of the loss of the coupling mechanism in the electric vehicle wireless charging system,an overcompensation design method is proposed,which enables the system to achieve soft switching at the resonant frequency.The multi-objective optimization for the wireless charging systems is carried out on the three indicators of efficiency,power density and cost to provide guiding principles for system design.At present,soft-switching modulations have problems such as large output power fluctuation and complicated algorithm.By analyzing the cause of its power fluctuation,it is found that the output waveform is not optimally arranged.Based on this principle,an optimal sorting criterion is proposed to solve this problem,and the existing soft-switching modulation strategy is improved by using this criterion.However,the newly proposed modulation strategy brings an increase in the complexity of the algorithm.In this paper,the soft-switching modulation strategy is improved again based on the principle of the sigma-delta modulator.The new modulation strategy can be realized under the premise of less output fluctuation a nd faster program running speed to achieve soft-switching,thereby increasing the efficiency of wireless charging system inverters.There is a maximum efficiency load in the wireless charging system.When the load or coupling parameters change,the couplper will deviate from the maximum efficiency point.By comparing the similarities and differences of existing soft-switching modulation strategies,this paper proposes a new modulation strategy based on the principle of step density averaging,and uses the multi-level output characteristics of full-bridge inverters to further reduce output fluctuations.The new modulation strategy can be applied in the secondary side controlla ble rectification to replace the traditional DC-DC converter for load impedance matching.Secondary-side controlled rectification using this modulation strategy can achieve the same soft switching as the primary-side inverter.Therefore,both the primary and secondary side power electronic converters of the wireless charging system can achieve soft switching,and the coupler can also achieve impedance matching through secondary side controllable rectification to achieve maximum efficiency,thereby maximizing the efficiency of the entire wireless system.The final step in enabling the successful operation of a wireless charging system is the closed-loop control design,which needs to be supported by a dynamic model of the system.Although there are some dynamic models to describe the wireless charging system,such as the generalized state average model and the extended function method,the order of these models will double the original order of the system.Due to the existence of more inductance and capacitanc e in the system,the order of the system is already very high.The higher order of these models makes the system model more complex.Although there are some methods to reduce the order of the system model,these methods inevitably reduce the accuracy.In this paper,a sampling data modeling method that does not increase the order of the system and can ensure the accuracy of the model will be proposed,and a closed-loop controller will be designed with this model as the system.