Research On Multi-modes Controlled CLLLC Resonant Converter

With the improvement of manufacturing level of semiconductor device and the development of digital control technology,high-frequency isolated bidirectional DC/DC converter has been widely used in electric vehicles,distributed generation,rail transit and other fields.LLC resonant converter has excellent soft-switching feature.It has always been a hot research object.the CLLLC resonant converter with symmetrical primary and secondary structures is taken as the research object.Combined with its application in the energy feedback of electric locomotive,the working principle of the converter is analyzed,the soft-starting?power flow commutation and wide range voltage input are researched.Firstly,the working principle and working states of CLLLC converter are analyzed.The equivalent circuit of the converter is established by using the fundamental harmonic approximation method.The gain curves of the converter are obtained.The influence of parameter on converter is analyzed.The operating characteristics of CLLLC converter are analyzed,which provides the theoretical basis for multi-modes control of the converter.Secondly,the CLLLC converter soft-starting is studied,an improved control strategy combined with phase-shift control and frequency modulation control is proposed.Power flow direction switching detection algorithm based on voltage and current detection is proposed.A control strategy is proposed to improve the transient characteristic of the converter when power flow direction changing.The power flow direction of the CLLLC converter can be switched smoothly.A hybrid control strategy combined with topology switching + phase shift control + frequency modulation control is proposed to meet the regulation requrements of the converter under different voltage ranges.Thirdly,completed the power device selection,high-frequency transformer parameter design and the hardware design of current sampling,voltage sampling,drive circuit,communication circuit.Produced a laboratory prototype with input 99V-121V,output 600V and rated power 2kW.Finally,the experimental results show that the CLLLC converter can achieve a smooth soft-start by using the control strategy in this paper,the converter power flow can be switched smoothly.the voltage input range of the converter can be broadened to a certain extent.Experimental results verified that the control strategies researched in this paper on soft-start,power flow directon change,wide range voltage input is reliable and effective.