| In recent years,the technology of pure electric vehicles has developed rapidly to reduce the environmental pollution caused by the burning of fossil fuels,and their share of the automobile market has been increasing.The 800 V high-voltage power battery pack not only realizes the ultra-fast charging,high-power transmission wiring harness and the compact control module of pure electric vehicles,but also puts forward the requirements of wide range,high frequency,high power density and flattening for the DC/DC converter in the auxiliary power system.The third-generation wide-gap semiconductor GaN devices are very suitable for high frequency and high power density of power electronic converters under 1000 V due to their characteristics of low conduction resistance and high switching frequency.Although the planar magnetic element can realize the flattening of the power electronic converter,its parasitic parameters and the high-frequency effect of winding current bring many new problems and challenges to its design.In this paper,the design of wide range DC/DC converter based on high voltage GaN device and the problems caused by high frequency operation of planar magnetic element are studied and the optimal control strategy is proposed.Firstly,the stacked bridge three-level LLC topology was selected for research based on the design specification requirements and GaN device specifications.The basic working principle of the topology in the half-bridge LLC mode,the multi-frequency mode and the PWM three-level mode is introduced in detail.Then the mechanism of voltage imbalance of input divider capacitor caused by the delay of switch tube driving signal in each working mode is analyzed.Based on this,a strategy to realize the voltage balance control of the input voltage-dividing capacitor is proposed.The planar magnetic element is a key component of the high-frequency resonant DC/DC converter.In this paper,we found that the high-frequency ripple current generated between various parasitic parameters when the planar magnetic element operates at high frequency.This causes distortion of the resonance current waveform and reduces the overall performance of the converter.The parasitic capacitance is reduced from 80.19 p F to 46.92 p F,the amplitude of high-frequency ripple current is reduced to 1/4 before optimization,by optimizing the material and structure parameters of PCB winding.Then this paper analyzes the high-frequency winding loss through Maxwell finite element simulation,and obtains that the fringing loss of the winding around the air gap accounts for about 80% of the total winding loss when working at high frequency.This paper proposes an optimization strategy that combines winding avoidance and distributed air gap by analyzing the effects of operating frequency,horizontal and vertical distance between the PCB winding and the air gap on the winding fringing loss.The temperature of the PCB winding near the air gap is reduced from 110℃ to 35℃ under 300 k Hz/1k W operating conditions,which greatly reduces the winding fringing loss.Finally,the parameter design and device selection of the resonant parameters and other hardware circuits of the stacked bridge three-level LLC resonant converter are performed.An experimental prototype with input voltage range of 400V～800V,output voltage range of 25V～40V and output power of 2k W was built.The feasibility of theoretical analysis and optimization strategy is verified by the hardware experiment platform. |
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