Research On Control Strategy Of Flying Capacitor Type Five-Level Buck/Boost Converter

With the wide application of high-voltage renewable energy power generation and energy storage systems,multilevel DC/DC converter has received widespread attention due to its high efficiency,high power density and rapid dynamic performance.For the excellent performance and high reliability of the converter,this paper has carried out in-depth researches focusing on the operating characteristics,precharging method,capacitor voltage balancing control,converter control strategy.Firstly,the working operation of flying capacitor type five-level Buck/Boost converter is analyzed.The current paths under each working modes are analyzed in detail,and the converter is controlled based on the phase-shifted carrier PWM method.Based on the two working conditions of high-voltage side and low-voltage side start-up process of converter,the precharging methods are proposed respectively.The relationship between iuput and output voltage,the expression of inducor current ripple,capacitor voltage ripple,the voltage stress of switch and the current stress of switch are derived,which provides a foundation for the construction of the control model and the seletion of device parameters.Secondly,in order to maintain the stability of the flying capacitor voltage besides the neutral point voltage,the principle of the capacitor voltage adjustment is analyzed and the mechanism of instability based on the variable duty ratio control method under light load is presented.An improved phase-shifting control method is proposed to stabilize the capacitor voltage under light load,and the mathematical model of the capacitor voltage fluctuation is obtained.The dynamic response and stability of the capacitor voltage balance system is immune to the inductor current based on phase-shifting control method,which enhance the reliability of the converter covering the full load range.Thirdly,taking the output current as the disturbance,the mathematical model of the converter is established based on the state space equation for improving the response ability of the converter during load step.An output voltage and inductor current feedback dual loop control system is designed,and the output voltage is controlled accurately and rapidly,based on the parameters designed by the pole assignment.In addition,the output current feedforward control strategy is used to further improve the dynamic performance of the converter.Finally,the loss of five-level Buck/Boost converter is analyzed,and it is compared to the losses of two-level and three-level converters.The switching loss is reduced in five-level Buck/Boost converter significantly,which improve the power density.In this paper,a 1.57 kW experimental prototype was built to verify the efficiency,precharging method,capacitor voltage balancing control,converter control strategy.The control method introduced in this paper for five-level DC/DC converter presents excellent performance and high reliability.The research results provide a systematic theoretical basis for the design of converter with high efficiency,high power density and rapid dynamic performance,and it is of great significance to the engineering application of high-voltage renewable energy power generation and energy storage systems.