A four-leg buck inverter for three-phase four-wire systems with the function of reducing DC-bus ripples

Three-phase four-wire inverters are usually used to feed unbalanced three-phase loads with neutral currents. The unbalanced three-phase loads also bring to second-order ripples in the DC bus, which should be mitigated by bulky DC-bus capacitors to improve the system performance. In this case, the DC capacitance is designed for the second-order ripple frequency instead of the switching frequency, so it can not be reduced even when SiC MOSFETs are adopted to achieve high switching frequency. Although various topologies of three-phase four-wire inverters has been proposed to provide the path for neutral currents, they cannot handle the second-order ripples. Also, some active power decoupling solutions can be adopted, but they require additional active swithes and components, which increases the cost of the system. In this paper, a four-leg buck inverter is proposed, which consists of four DC-DC buck converters. Each buck converter is independently controlled. This topology can not only provide neutral currents, but also reduce the second-order ripples in the DC bus with active power decoupling control. The proposed topology doesn't require any additional active switches comparing to the conventional topologies with neutral legs. The effectiveness of proposed topology is verified by the simulation in MATLAB/Simulink

Accurate characterisation and modelling of SiC MOSFETs for transient simulation

Silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) possess properties that are superior compared to their silicon counterparts, such as low conduction and switching losses, high thermal conductivity and operating temperatures, etc. SiC MOSFETs need to be evaluated either experimentally or through simulation to fully exploit or understand their benefits. Compared to experimentation, simulation is more time and cost-efficient so is preferred at the initial converter design stage. However, accurate and fast models of SiC MOS FETs need to be established for such simulation, which is challenging due to fast switching speed and frequency of SiC MOSFETs. This thesis focuses on improving the accuracy and speed of models of SiC MOSFETs and simplifying the modelling process at the same time. Firstly, the characteristics of SiC MOSFETs required for the modelling were analysed. It was found that the I-V and C-V characteristics in their dynamic state have a significant impact on the accuracy of the models. However, the existing methods to measure and extract these characteristics are complex due to the multiple measurement equipment configurations are required. This thesis proposes a simplified dynamic-state characterisation method. This method analyses the relationship between characteristics and the switching waveforms of SiC MOSFETs, and extracts these characteristics directly from the switching waveforms measured by a double pulse tester to simplify the measurement process. These measured dynamic-state characteristics, combined with the conventional static-state characteristics, were used to built the SiC MOSFET model.The relative root-mean-square (RMS) errors of the model can be reduced by at least a factor of 3, compared to the model that only considers the conventional static-state characteristics. Based on the extracted characteristics, a measurement-based hybrid data-driven modelling method is proposed. Conventional equation-based models have drawbacks such as a complex modelling process, poor adaptability, low accuracy and slow simulation speed. The proposed modelling method utilised a hybrid data-driven model based on artificial neural networks to simplify the modelling process and improve the adaptability. The switching waveforms simulated by the proposed model are 1.5 ∼ 3 times closer to the experimental waveforms, compared to the commercial equation-based Angelov model. At the same time, the proposed model is 30% faster than the Angelov model in terms of simulation speed. However, equipment required for the measurement-based modelling may not be available for some converter designers, therefore, a step-by-step datasheet-based modelling method is proposed, which is completely based on the datasheet without the use of any further data or equipment. Compared to the measurement-based modelling method, the datasheet-based modelling method results in 24% increase in RMS errors and cannot accurately match the gate driver resistor used in practical experiment. However, the datasheet-based modelling method features a simpler modelling process and 15% faster simulation speed so provides a more cost and time-efficient process for converter designers to quickly validate their converter design