Impact of Gallium Nitride Switching Devices on High-Frequency Power Electronics Converters

Defense is held on 21.9.2021 12:15 – 15:15 via remote technology, https://aalto.zoom.us/j/62622110601Power converters for photo-voltaic (PV) grid are one of the main applications of Gallium Nitride (GaN) switching devices. These converters in grid-connected applications are also required to supply reactive power. During the flow of reactive power, switching devices have to conduct in reverse direction. However, these GaN HEMTs suffer from high voltage drop during the reverse conduction. It is very critical to accurately model these additional losses to achieve the optimized performance. HERIC inverter from transformer-less solar inverter family is chosen for this work due to its high efficiency performance and low leakage current. Conduction losses for the inverter including reverse conduction losses are modelled for various existing uni-polar PWM schemes. Results showed that for loads with low power factor, reverse conduction losses constitute the majority of the total conduction losses. Based on these results, a modified uni-polar PWM scheme is introduced to reduce these losses and enhance the efficiency of the inverter. Fast switching of these GaN HEMTs have made it possible to reduce the size of magnetic components and increase the overall power density of the system. However, fast switching transitions in the converter, generate higher electro-magnetic-interference (EMI) noise. To reduce the noise emissions of the converter it is essential to reduce the parasitic elements in the converter. These parasitic elements facilitate the propagation of high-frequency noise, which will require bulkier EMI filters to comply with power quality standards. Hence, in this work a high voltage gain converter with multi-phase coupled inductor, for electric vehicle (EV) applications, is considered as a case study. A comprehensive model of the inductor structure, including parasitic capacitances is presented in this thesis. The values of those parasitic capacitances are computed with the aid of computer aided simulations and by performing impedance measurements on inductor prototype. EMI measurements of the converter verify that by optimizing the parasitic elements, noise emissions of the converter can be reduced. GaN devices are also suitable candidates for switching devices in power converters for modernized, more electric aircrafts (MEA). In addition to improving the performance of converters by the application of these switching devices, it is also very critical to review new converter topologies. Two power factor correction (PFC) topologies, Swiss rectifier and Delta rectifier, are considered for this study due to their low number of high-frequency switches. Detailed simulation models of these rectifiers are developed to choose an appropriate topology for their application as front-end rectifier in an isolated AC-DC power supply. After the selection of the rectifier topology, an isolated DC-DC converter is designed and optimized with the aid of an optimization tool. Practical performance of the converter is also enhanced by the application of GaN devices and by minimizing the parasitic elements in the transformer and printed circuit boards (PCBs)