Research On High Reliability Ga N-based Flying Capacitor Multilevel Converter

With the transformation of the economic structure and the improvement of science and technology,emerging fields represented by rail transit,data centers,electric vehicles and communication base stations are booming,which puts forward higher requirements for energy demand and energy structure.Advances in power electronics technology and the performance of core power devices have pro vided a powerful driving force in the development of energy technology towards greater capacity,higher efficiency,and more reliability.With the continuous iteration of silicon(Si)-based device fabrication technology,its electrical properties have approached the theoretical limit determined by material properties.The wide-bandgap semiconductor gallium nitride(GaN)is expected to bring about great changes in power converters due to its excellent material properties.GaN high electron mobility transistors have the advantages of small size,high power density,high switching frequency,and no reverse recovery process.However,due to the constraints of its own lateral structure,the withstand voltage and reliability of GaN transistors are not sufficient to ensure that they can be successfully adapted to medium and high voltage applications.This thesis aims to improve the system efficiency,energy density,cost competitiveness,and reliability of mid-to-high voltage level converters by using GaN transistor-based flying capacitor structures,and ultimately achieve miniaturization and high-efficiency of high-reliability power electronic converters.In order to promote this process,some theoretical and engineering methodological obstacles need to be overcome,including standardization and optimal control of GaN-based flying capacitor structure,fast fault location strategy,redundant structure and fault-tolerant operation mode design.In response to these topics,the following researches are carried out in this thesis:(1)The modular design criteria of the GaN-based flying capacitor structure are determined,so that the structure has higher scalability and portability.Two outstanding features of the GaN-based flying capacitor structure,no reverse recovery loss and frequency superposition characteristics,are demonstrated in the power factor correction converter.Prototype tests confirmed that after adopting the GaN-based flying capacitor structure,the reverse recovery loss in the converter was reduced to zero,and the volume of the main inductor was reduced by 78% compared with the traditional two-level structure.(2)For the flying capacitor converter,a universal fault detection strategy based on the detection of high-frequency harmonic components is desi gned.A prominent feature of the GaN flying capacitor structure is that the passive energy storage element is small,and the system will collapse quickly after a fault occurs.This puts forward extremely high requirements on the real-time performance of fault detection.The detection method based on harmonic components has been proved in various converter fault experiments that it can locate the faulty unit within one switching cycle after the fault occurs.(3)Fixed frequency model predictive control with out cost function is designed for multilevel structures.The steady-state performance of the converter is the same as that of the carrier phase-shift modulation while retaining the response speed advantage of the finite-set model predictive control.It is confirmed in the prototype that the new control method reduces the total harmonic distortion of the output current by more than 60% compared with the finite set model predictive control.The proposed new model predictive control can make the flying capacit or work in the unbalanced mode,which is beneficial to improve the waveform quality.The fault detection strategy in unbalanced mode is also given to ensure that the faulty unit can be located one cycle after the fault occurs.(4)Two redundant structures suitable for flying capacitor multilevel converters are proposed.The focus of the research is the determination of the optimal redundancy margin.The reliability of converters with different redundant structures is analyzed by Markov model.The obtained reliability model has passed the Monte Carlo test.Combined with the utility theory,it can provide theoretical support for the selection of the redundancy margin of the converter under different working conditions.At the same time,the fault-tolerant operation mode of the converter is determined.Through the proposed flying capacitor voltage rebalancing strategy,the voltage stress on the flying capacitor in the five-level converter after a fault is reduced by 35% compared with the non-fault-tolerant operation.This work demonstrates the great potential of combining low-voltage GaN transistors with flying capacitor structures.A systematic solution for the design,construction,optimal control,fault detection and fault-tolerant operation of the GaN-based flying capacitor multilevel converter is given.This enables GaN devices to promote the further development of medium and high voltage power electronic converters towards high system efficiency,high power density and high reliability.