Research On Half Bridge Three-level LLC Resonant Converter And Its Parallel System Control

With the increasing awareness of energy conservation and environmental protection at home and abroad,higher efficient DC / DC converters are urgently needed.LLC resonant converter has high power density,low electromagnetic interference and high efficiency,which is widely used in vehicle OBC,high-power charging pile,new energy generation and other fields.In the application of vehicle OBC and high-power charging pile,the LLC resonant converter needs to achieve wide range output due to the change of load characteristics with charging,but the existing general LLC resonant converter has poor wide range output characteristics,especially it is difficult to achieve efficient and stable output under light load.Meanwhile,the DC / DC of high-power charging pile and new energy generation is usually high-voltage input and high-power output,while the existing full bridge LLC is not suitable for high-voltage input and single module LLC converter is difficult to achieve high-power output.In view of the problems of high voltage input,wide range and high power output of LLC resonant converter,this paper proposes a topology structure of half bridge three-level LLC resonant converter as the basic module and multi-module input parallel output parallel,and studies the half bridge three-level LLC resonant converter and its module parallel connection system.This paper first introduces the basic principle of half bridge three-level LLC resonant converter under the mixed control strategy,and then establishes the equivalent circuit under the frequency modulation and phase-shift mode of the converter,and analyzes and derives the ideal DC gain characteristics under the mixed control strategy of the converter.In view of the problem that the gain of the converter under light load is no longer in accordance with the ideal analysis characteristics,this paper analyzes the influence of parasitic parameters on the operation of the converter,establishes the equivalent working circuit model of the converter under the consideration of parasitic parameters,and then analyzes the reasons why the resonant current oscillates under normal load and the output gain cannot be adjusted under light load.In addition,a voltage regulation intermittent control strategy of the light load is proposed to realize the reliable regulation of the output voltage of the half bridge three-level LLC under the light load condition.In order to solve the problem that single module converter is difficult to achieve high power output,this paper proposes a power expansion method of multi module inputoutput parallel(OPOP),but there is a serious problem of uneven current between modules in practical application.In this paper,the parallel operation model of two module half bridge three-level LLC converter is established,the cause of uneven current caused by the parameter difference between LLC modules is analyzed,and the simulation verification is carried out.According to different applications,two parallel operation schemes based on droop current sharing method and master-slave current sharing method are designed,and the simulation verification is carried out.At last,this paper introduces two single module 7.5k W half bridge three-level LLC resonant converter test platform,including the main circuit parameter design,hardware circuit design and DSP based system software design,and experimental verification of the control strategy proposed in this paper.