Bidirectional DC-DC Converters With Wide Voltage-gain Range For Electric Vehicles And Their Coordinated Operation Control

Electric vehicles,which can effectively mitigate the energy crisis and the environmental pollution,are a new trend for the future transportation.The hybrid energy sources system of the electric vehicles can fulfill the bidirectional throughput requirements of the slow response energy and the fast response power for the high voltage DC-link of electric driving systems.However,high specific power supercapacitors still have characteristics such as a low output voltage and a large variance in terminal voltages during rapid charging or discharging processes,and the terminal voltage range of the high specific energy batteries is also large.It is necessary to interface with the low-voltage energy sources and the high voltage DC-link by a high ratio bidirectional DC-DC converter.Therefore,studying high conversion efficiency bidirectional DC-DC converters with wide voltage-gain range is of very important significance for developing electric vehicles,decreasing air pollution and reducing dependence on non-renewable energy sources.Firstly,an asymmetric H-bridge bidirectional DC-DC converter with high step-up/step-down voltage gain is proposed for low-voltage supercapacitors in this paper.The narrow output PWM voltage of the converter is generated from the voltage difference between two wider output PWM voltages from the asymmetric H-bridge with duty cycles close to 0.5.The equivalent switching frequency of the output PWM voltage is double the actual switching frequency.In addition,the synchronous rectifiers allow ZVS turn-on and turn-off,and a wide step-down/step-up ratio range is achieved.Finally,the experimental results verify the feasibility of the proposed bidirectional converter.Secondly,an interleaved switched-capacitor bidirectional DC-DC converter with wide voltage-gain range is proposed for low-voltage batteries in this paper.The interleaved structure is adopted in the low-voltage side of this converter to reduce the ripple of the current through the low-voltage side,and the series-connected structure is adopted in the high-voltage side to achieve the high step-up/step-down voltage gain.In addition,the bidirectional synchronous rectification operations are carried out without any extra hardware,and the efficiency of the converter is improved.Furthermore,the operating principles,voltage and current stresses,and current ripple characteristics of the converter are analyzed.Finally,the experimental results validate the feasibility of the proposed converter.Finally,this paper studies the converter's coordinated operation control.Using the proposed DC-DC converters as the power interface between the low-voltage energy sources and the high-voltage DC-link,the lithium-battery plus supercapacitors hybrid energy sources system of electric vehicle is constructed.The coordinated operation control strategy of the converters is discussed from three aspects: power distribution,lithium-battery current control and supercapacitors current control.Power distribution of two energy sources and bidirectional power flow control are achieved,and the hybrid energy sources system can meet both the energy and power requirements of electric vehicle.The simulation model of the system is built with MATLAB software to study its operating principles,and lastly the correctness of the theoretical analysis and the feasibility of the hybrid energy source system are verified by experimental results.