Research On Isolated High-Efficiency On-Board Charger Based On Two-Stage Circuit

On-Board Charger(OBC)is a key component of Electric Vehicle(EV).As on-board equipment,OBC must have the characteristics of high efficiency,high power-density and wider charging voltage range.Based on the above requirements,two-stage circuit is always used for the topology of OBC.However,two-stage circuit has the disadvantages of high cost,low efficiency and low power-density.In this paper,interleaved boost converter(IBC)cascaded LLC resonant converter is selected as the circuit topology of OBC.Both IBC and LLC resonant converter are well modeled.And the intrinsic parameters affacting the efficiency of OBC are mined and OBC is optimized.The efficiency of IBC is calculated by analytical method,which is the function of switching frequency,boost inductance and bus voltage.Based on the above discovery,combining with the limitation of maximum magnetic flux density and current ripple,a comprehensive optimization design method is proposed to optimize the three intrinsic parameters of IBC for increasing efficiency without adding the cost of hardware.The proposed optimization method provides theoretical support and practice guidance for the industry to develop low-cost,high efficiency and high power-density IBC.The efficiency of LLC resonant converter at the resonant point is calculated by time domain method,which is the function of resonant frequency and magnetizing inductor.Then resonant frequency and magnetizing inductor are optimized based on the requirement of efficiency.The above method provides an idea for the industry to develop high efficiency and high power-density LLC resonant converter.The curves of relationship between maximum output voltage and resonant inductor is figured out by fundamental harmonic approximation(FHA)method as well as the curves of relationship between minimum switching frequency and resonant inductor.It is found that the maximum output voltage and the minimum switching frequency limit the upper value and the lower value of resonant inductor,which is used to optimize resonant inductor.The above design method can help the industry to develop high power-density and wide-gain LLC resonant converter.The maximum magnetizing current and the maximum resonant current of magnetic components at the worst condition for LLC resonant converter are calculated by time domain method and used to accurately design magnetic components,which assures that all the magentic components can work safely in a wide frequency range.It is of positive significance to increase the reliability of OBC.The primary leakage of transformer is calucalted by energy method and considered in the design process of resonant inductor,which increases the design accuracy of resonant parameters.The main point of this paper is to make OBC,when the cost is limited,be able to charge battery with high-efficiency in a wide output voltage range by optimizing the intrinsic parameters.The main difficuty is that OBC must charge battery with high efficiency in a wide output voltage range.The proposed optimal design method in this paper can solve the above problems well,which is confirmed by the experiment of the developed prototype.