Research On Power Output Characteristic Analysis And Regulation Control Method Of Wireless Charging System

Wireless power transfer based on magnetic coupling has the advantages of isolation and harsh environment adaptability.In recent years,it has been applied in the fields of mobile phone and electric vehicle charging,medical electronics and orbital robots.Due to the wireless power transfer ability,the troubles caused by power supply cables can be eliminated.Since the power output characteristic of the wireless charging system is closely related to the equivalent input impedance of the rectifier bridge and the state of the resonant tank,in order to achieve high power and high efficiency transmission,on the one hand,the equivalent input impedance of the rectifier bridge at the secondary side is required to be as resistive as possible;on the other hand,the resonant tank is required to resonate at the system operating frequency.However,in practical applications,the discontinuous conduction of the rectifier bridge input current and the frequency deviating state of resonant tank will both affect the system power output characteristic,resulting in the reduction of the power output capability.At the same time,other system characteristics,such as load-independent output characteristic and the zero-phase angle(ZPA)operating characteristic of the inverter will also be deteriorated or even lost.This paper analyzes the characteristic of the double-side LCC compensation type wireless charging system,and proposes a steady-state characteristics analysis method based on numerical solution by utilizing its characteristic.Regardless of whether the rectifier bridge input current operates in continuous or discontinuous conduction mode,and the resonant tank operates in the resonant state or the frequency deviating state,the proposed method can calculate system steady-state characteristics accurately and quickly by comprehensively using time-domain analysis method,frequency-domain analysis method and numerical solution method.The calculated steady-state characteristic includes the power output characteristic,the current and voltage stress of resonant tank,the phase angle of inverter output,etc.In order to verify the rapidity and accuracy of the proposed method,the calculation results of the proposed method,the commercial software simulation results and the prototype experimental results are compared in these three aspects.For the double-side LCC compensation wireless charging system,when the rectifier bridge input current operates in the discontinuous conductiong mode,the actual power output capability of the system is significantly lower than the theoretical design value calculated by the fundamental harmonic analysis(FHA)method.In order to address this problem,this paper analyzes the reason of discontinuous conduction mode,derives the critical occurrence condition when discontinuous conduction phenomenon occurs,gives a calculation method about equivalent input impedance of the rectifier bridge,and proposes an equivalent input impedance corrector.This corrector can boost equivalent input resistance value of the rectifier bridge to the ideal approximate value used in the FHA method by recovering continuous conduction mode of the current.On the one hand,the corrector can boost system output power to the rated design value of the FHA method.On the other hand,the corrector can be used as a supplementary repair method when using the FHA method to design a double-side LCC compensation system,because the addition of this corrector can solve the problem of power output capability reduction and not affect other system characteristics designed by the FHA method.When using the FHA method to design resonant tank parameters,the designer does not need to consider the power output capability reduction and the discontinuous conduction mode of the rectifier bridge input current,therefore,the design process is simplied.Experimental results of the prototype show that the addition of the corrector can boost power output capability by about 12%.This improvement is consistent with the theoretical calculation and simulation,which proves effectiveness of the proposed method.For the problem of power output capability reduction caused by frequency deviating,this paper gives a theoretical analysis in the perspective of system output characteristic,and adopts the double-side self-tuning of resonant tank as a solution to solve this problem.By adjusting equivalent capacitance value of the switched capacitor,natural frequency of the resonant tank can be adjusted to the system operating frequency,thus solving the problem of power output capability reduction.Therefore,this paper proposes a double-side self-tuning method based on the parameter recognition of magnetic coupler for the application scenarios where the frequency deviating occurs before the wireless charging system operates,such as the parameters variation of the coupler due to the relative position uncertainty between the primary and secondary sides when the electric vehicle is parked at the charging position.Based on the parameter recognition results of the magnetic coupler,the proposed method can quickly and directly tune the primary and secondary natural frequencies of resonant tank to the system operating frequency.Experimental results of the prototype show that the system can always operate in the resonant state,the load-independent output characteristic is recoverd and the ZPA operation characteristic is realized in the inverter.Compared with the system under the frequency deviating state,prototype efficiency is improved by about 3%.For the application scenarios where frequency deviating occurs during the operational stage,such as the capacitance variation caused by the ambient temperature variation or the coupler parameters variation caused by the abrupt changes of the relative position between the primary and secondary sides of the coupler,this paper proposes a double-side self-tuning method based on the dynamic search.The proposed method monitors the system output and the phase angle between inverter output voltage and current,and performs closed-loop adjustment of the switched capacitors in the primary and secondary sides respectively;it can gradually adjust the natural frequencies of the primary and secondary sides to the system operating frequency.The simulation and experimental results show that the system can operate in the resonant state by the proposed method,thus solving the problem of power output capability reduction and recovering the load-independent output characteristic.