Research On Multi-level Converters With Hybrid Power Switches

In recent years,with the development of semiconductor industry,Wide-Band-Gap(WBG)materials,represented by Silicon Carbiede,have entered people’s eyes.They are favored by many scholars for better performance like high blocking voltage,high working temperature,low loss and fast switching speed.However,manufacturing technology and high price at present limit their application.As a result,it is of great significance to explore the hybrid applications of Si and SiC switching devices in converters.In this paper,the principle of hybrid applications of Si and SiC power diveces is analyzed firstly.The basic purpose is to make full use of fast switching speed and low switching losses of SiC devices and low cost of Si devices by concentrating switching losses on SiC devices and only on-state losses on Si devices.Therefore,the efficiency of the multi-level converter improves.Then,this paper introduces the working principles and modulation strategies of two hybrid application schemes based on the traditional three-level Active Neutral-Point Clamped Converter(ANPC),and compares the advantages and disadvantages of the two schemes from the perspectives of cost and loss.In order to further improve efficiency,the paper proposes an improved modulation strategy based on ANPC with hybrid switching devices in order to optimize on-state losses by increasing the current path in some modes to reduce the current in each path.Considering that the efficiency of three-level ANPC is limited after applying the improved modulation,it is extended to a five-level full bridge ANPC by introducing a bridge arm and the modulation is optimized according to the topology characteristics.Then,the optimization results of five-level ANPC through the theoretical calculation and analysis are given and the relationships between input voltage and optimization effects are analyzed.Finally,the prototypes of the three-level ANPC and five-level ANPC are built according to the power circuit hardware design.The experimental results show that the application of hybrid switching devices can significantly improve the efficiency of multi-level converters,and the improved modulation can also significantly decrease the on-state losses of five-level ANPC.