Power converters used in high reliability radiation hardened space applications trail their commercial counterparts in terms of power density and efficiency. This is due to the additional challenges that arise in the design of space rated power converters from the harsh environment they need to operate in, to the limited availability of space qualified components and field demonstrated power converter topologies. New radiation hardened Gallium Nitride (GaN) Field Effect Transistors (FETs) with their inherent radiation tolerance and superior performance over Silicon Power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are a promising alternative to improve power density and performance in space power converters. This thesis presents the considerations and design of a practical implementation of the Phase Shifted Full Bridge DC-DC Isolated converter with synchronous rectification for space applications. Recently released radiation hardened GaN FETs were used in the Full Bridge and synchronous rectifier power stages. A survey outlining the benefits of new radiation hardened GaN FETs for space power applications compared to current radiation hardened power MOSFETs is included. In addition, this work presents the overall design process followed to design the DC-DC converter power stage, as well as a comprehensive power loss analysis. Furthermore, this work includes details to implement a conventional hard-switched Full Bridge DC-DC converter for this application. An efficiency and component stress comparison was performed between the hard-switched Full Bridge design and the Phase Shifted Full Bridge DC-DC converter design. This comparison highlights the benefits of phase shift modulation (PSM) and zero voltage switching (ZVS) for GaN FET applications. Furthermore, different magnetic designs were characterized and compared for efficiency in both converters. The DC-DC converters implemented in this work regulate the output to a nominal 20 V, delivering 500 W from a nominal 100 V DC Bus input. Complete fault analysis and protection circuitry required for a space-qualified implementation is not addressed by this work. / MS / Recently released radiation-hardened Gallium Nitride (GaN) Field Effect Transistors (FETs) offer the opportunity to increase efficiency and power density of space DC-DC power converters. The current state of the art for space DC-DC power conversion trails their commercial counterparts in terms of power density and efficiency. This is mainly due to two factors. The first factor is related to the additional challenges that arise in the design of space rated power converters from the harsh environment they need to operate in, to the limited availability of space qualified components and field demonstrated converter topologies. The second factor lies in producing reliable radiation hardened power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). GaN FETs not only have better electrical performance than power MOSFETs, they have also demonstrated inherent tolerance to radiation. This results in less structural device changes needed to make GaN FETs operate reliably under high radiation compared to their MOSFETs counterparts. This work outlines the design implications of using newly released radiation hardened GaN FETs to implement a fixed frequency isolated Phase Shifted Full Bridge DC-DC converter while strictly abiding to the design constraints found in space-power converter applications. In addition, a one-to-one performance comparison was made between the soft-switched Phase Shift modulated Full Bridge and the conventional hard-switched Full Bridge DC-DC converter. Finally, different magnetic designs were evaluated in the laboratory to assess their impact on converter efficiency.

The battery charger plays an important role in the development of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs).This thesis focuses on the DC-DC converter for high voltage battery charger and is divided into four chapters. The background related to EV battery charger is introduced, and the topologies of isolated DC-DC converter possibly applied in battery charge are sketched in Chapter 1. Since the EV battery charger is high voltage high power, the phase-shifted full bridge and LLC converters, which are popularly used in high power applications, are discussed in detail in Chapter 2. They are generally considered as high efficiency, high power density and high reliability, but their prominent features are also limited in certain range of operation. To make full use of the advantages and to avoid the limitation of the phase-shifted full bridge and LLC converters, a novel hybrid resonant and PWM converter combining resonant LLC half-bridge and phase shifted full-bridge topology is proposed and is described in Chapter 3. The converter achieves high efficiency and true soft switching for the entire operation range, which is very important for high voltage EV battery charger application. A 3.4 kW hardware prototype has been designed, implemented and tested to verify that the proposed hybrid converter truly avoids the disadvantages of LLC and phase-shifted full bridge converters while maintaining their advantages. In this proposed hybrid converter, the utilization efficiency of the auxiliary transformer is not that ideal. When the duty cycle is large, LLC converter charges one of the capacitors but the energy stored in the capacitor has no chance to be transferred to the output, resulting in the low utilization efficiency of the auxiliary transformer. To utilize the auxiliary transformer fully while keeping all the prominent features of the previous hybrid converter in Chapter 3, an improved hybrid resonant and PWM converter is proposed in Chapter 4. The idea has been verified with simulations. The last chapter is the conclusion which summaries the key features and findings of the two proposed hybrid converters. / Master of Science

Bi-directional DC-DC converters are used for applications that require a flow of energy in two directions, while a wide range converter offer efficient operation over a wide range of input and output voltages. However, an efficient technology that is both bi-directional and Wide Input Wide Output (WIWO), currently, does not currently exist.   To find a suitable topology, the work began by surveying the existing literature and when a potentially suitable solution was identified, it was evaluated via simulation.   Using a wide range, unidirectional topology as the starting point, a converter topology was designed, capable of reconfiguring its transformer ratios by controlling the synchronization of its switches.   By aiming to use soft switching in simulation, this topology was improved to reach 92\% efficiency in the forward mode and 95\% in the reverse mode of operation. Furthermore, a prototype of this converter was developed that reached 82\% efficiency. While this prototype requires a better controller, hardware optimization and testing for optimal performance, the proposed technology was verified via simulation to work as a WIWO converter that is also bi-directional.