High-Frequency Design Consideration and EMI Mitigation in SiC-based Multilevel Converters

Medium Voltage (MV) power conversion systems are essential in high power applications to address the increasing demand of energy and the increasing penetration of renewable energy sources. MV power electronics converters are the key elements for power conversion in MV systems and are the focus of this study. Multilevel converter topologies are promising topologies in MV applications because of their reduced voltage stress on devices, excellent output quality, reduced semiconductor losses, lower common mode voltage among other advantages. However, they may suffer from the large number of switching devices and capacitors, as well as the need to regulate capacitor voltages. SiC MOSFETs can achieve higher switching speeds, higher switching frequencies, higher voltage ratings, higher operation temperatures compared to traditional Si devices. They have shown promise to increase the efficiency and power density of the converters, but may suffer from higher voltage overshoots, increased Electromagnetic Interference (EMI) emission and so on. In SiC-based multilevel converters, the features of multilevel topologies, and the features of SiC MOSFETs are coupled together. The benefits, challenges, and solutions of using SiC MOSFETs in multilevel converters are studied explicitly in this work. With the high switching speeds and high switching frequencies of SiC MOSFETs, and the large number of switches and capacitors in multilevel topologies, SiC-based multilevel converters need to be studied while considering high-frequency voltage and current behaviors and the interactions among them at different locations. Firstly, the use of SiC-based multilevel converter in the high-speed motor drive application is explored. A three-phase inverter is designed and built employing five-level Stacked Multicell Converter topology and SiC MOSFETs. The benefits and challenges of using multilevel converter topology and using SiC MOSFETs for this application are explored. A fitting topology is selected, and a prototype is designed, both with attentions paid to deal with the high switching speeds of SiC MOSFETs. The inverter is verified through experiments to meet all specifications with a high efficiency. Then a unique type of converter, converters with Integrated Capacitor Blocked Transistor (ICBT) cells are studied. Unlike the traditional methods, there are no fast-developing voltage unbalances, or high cell capacitor voltage ripples in ICBT-based converters. The ideal operation principle is analyzed and verified by the simulation results. Then the impacts of non-idealities on the operation are analyzed, and a control method is proposed for this type of converter. The operation and control of ICBT-based converters are verified by experimental results to achieve low cell capacitor voltage ripples and excellent voltage balance in Medium Voltage high power applications. Lastly, the conducted EMI emission in SiC-based multilevel converters are studied. Four SiC-based multilevel converters are studied, with the focus on the power circuit in one converter and the auxiliary circuits in the other three converters. The complexity of noise generation and propagation in multilevel converters is presented. The conducted EMI disturbances are experimentally evaluated, analyzed, and effectively mitigated in all four cases.