High-Efficiency Electronic Power Converters for Extreme Fast Charging (XFC) Stations

The transportation sector significantly contributes to global greenhouse gas emissions, accounting for approximately 23% of worldwide carbon dioxide (CO2) emissions. In response to environmental concerns and the need to contrast climate change, governments are supporting the phase-out of internal combustion engine vehicles (ICEVs) no longer meeting the stringent emission standards and, at the same time, promoting the adoption of electric vehicles (EVs). This transition is supported by several advantages, including absence of local pollution, reduced maintenance costs, lower overall ownership expenses, charging using locally available renewable energy, and limited acoustic emissions. Nonetheless, several challenges hinder the widespread adoption of EVs. Potential EV buyers are often deterred by the higher initial purchase price than equivalent ICEVs, limited driving range (typically 250 to 500 km), longer charging times, often exceeding 30 minutes for a full charge, and the lack of charging infrastructure. Expanding the charging infrastructure and enhancing the related technologies is crucial for a broader adoption of EVs. Notably, battery chargers of extraordinary performance are required to meet expectations and allow reduced charging times. EV extreme fast chargers (XFCs) are being developed, featuring charging powers of 350 kW or more. At the core of such systems, advanced high-power dc-dc converters–having efficiency, flexible interconnection, and safety as key design features–perform the involved power processing. This dissertation focuses on the development of a dc-dc converter module for EV XFC systems facing the challenge of obtaining galvanic isolation and high conversion efficiency over the wide range of operating conditions occurring in battery charging. The dissertation’s key contributions are divided into several parts. Initially, it provides an overview of existing dc-dc converter topologies suitable for EV XFC systems, exploring multi-stage configurations to enhance efficiency beyond the limitations of resonant converters. Differently, multi-stage topologies can accommodate wider operating voltage ranges with very high efficiencies. Novel multi-stage topologies are introduced and then compared. The dissertation also presents loss models for these converters, enabling comprehensive simulations to estimate losses and pinpoint areas for improvement. A simulation-based comparison of multi-stage dc-dc converter topologies is performed to identify optimal topologies for efficient EV charging, which are then investigated experimentally. The central part of the work presents the analysis and experimental results of the proposed buck-boost LLC (BB-LLC) and twin-bus buck (TBB) converters, contributing to the development of efficient two-stage EV charging solutions. It covers modeling, design considerations, and modulation techniques. The dissertation’s final section introduces an extremum seeking control (ESC) technique to determine optimal operating points for the TBB converter. This technique offers a model-free on-line optimization method that is robust against parameter uncertainties and changes in operating point. Overall, it is shown that the solution features high conversion efficiency over a wide range of output voltages. In conclusion, the research efforts described in this dissertation yielded several contributions, particularly in the domains of converter analysis and modeling, converter design and prototyping, and converter on-line optimization. Experimental results are reported considering a converter module prototype rated 10 kW, input voltage 800 V, and output range 250 V to 500 V, employing Silicon Carbide (SiC) and Gallium Nitride (GaN) semiconductors. These contributions are instrumental in advancing the state-of-the-art in high-power dc-dc converters for EV charging applications, contributing to a more sustainable and environmentally friendly transportation landscape.