Research On Buck-Boost+CLLLC Cascaded Bidirectional DC Converter With Wide Input Range

In rail transit,DC micro-grid,electric vehicle and other applications,there is a situation that the power supply voltage for the equipment fluctuates in a wide range,which requires the DC/DC converter as the power supply device to have a wide DC gain to meet the requirements of a wide input voltage range.The article proposes a bi-directional DC / DC converter for the above applications.The converter is composed of four-switch Buck-Boost converter and CLLLC resonant converter.In order to reduce the number of switches,the two converters are bridge arm multiplexed,and a four-switch Buck-Boost + CLLLC cascaded bi-directional DC /DC converter with bridge arm multiplexed is obtained.The converter has a wide DC gain range and is suitable for wide input voltage range applications.At the same time,the converter can realize soft switching of all switches on the primary and secondary sides,so it is also suitable for high power applications.Firstly,the article analyzes the four-switch Buck-Boost circuit,including its working principle,soft-switch control method,and ways to improve the conversion efficiency.For CLLLC circuit,based on the introduction of its working process,an equivalent model is established to investigate the influence of resonance parameters on the resonance frequency and the influence of circuit parameters on the DC gain range.On this basis,the four-switch BuckBoost converter is cascaded with CLLLC resonant converter.Through analysis,it is found that the cascaded converter has a wider DC gain range than the above two converters,so it is more suitable for wide voltage input range situations.For the sake of reducing the number of switches in cascade converters,the four-switch Buck-Boost converter and the CLLLC resonant converter are multiplexed in the bridge arm to obtain the four-switch Buck-Boost+CLLLC cascaded bidirectional converter with bridge arm multiplexed.Aiming at the synchronous rectification control method of the converter,synchronous control technology is introduced.The working process,DC gain expression and control method are discussed in this article.In the resonant state,the working modes of the bridge-arm multiplexed cascade converter can be divided into eight types.Based on whether all the soft switches can be realized as criterion,eight working modes are identified and combined,and finally three working modes suitable for the converter are obtained.These three modes are analyzed,including power transmission characteristics,soft switch range and control methods.Finally,an experimental prototype with a voltage range of 175-350 V DC on the high voltage side and a voltage of 24 V DC on the low voltage side is built.In order to illustrate more clearly the design method of cascade converter with bridge-arm multiplexing proposed in this article,the design process of power level devices is given.The experimental results prove the feasibility of the control method and the correctness of the theoretical analysis.