A holistic design optimization method for LLC converters in light electric vehicle chargers

Abstract

Since the early 1990s, gasoline prices have been steadily increasing, making traditional transportation more expensive. Meanwhile, advancements in material science and battery manufacturing technology have led to gradually decreasing battery costs and increasing energy storage capacities each year. These changes are encouraging the shift from traditional gasoline engines to modern electric drive systems, leading to a growing focus on developing Electric Vehicle (EV) technologies. EVs are gaining attention due to their energy-saving capabilities, low carbon emissions, and long-term cost-effectiveness. The power source for EVs is a high-voltage DC rechargeable battery that can be fully charged by connecting to an external electrical source. EVs require an onboard charging system that converts alternating current (AC) from the grid to direct current (DC) to charge the vehicle's battery. This charging system can be connected to a household outlet or a fast-charging station. Therefore, a highly efficient and compact battery charging system plays a crucial role in the electrical system of EVs. A novel optimization method for the design of gapped and distributed core magnetics in LLC converters, suitable for applications ranging from battery chargers to light electric vehicle charging systems, is presented in this thesis. Through a sequence of integrated studies, a comprehensive framework for the design and optimization that addresses the challenges posed by varying loads and operational conditions is developed. Initially, an innovative optimization technique utilizing by sweeping the magnetic flux density is introduced. This technique enables the optimal magnetic flux density to be selected, minimizing the power loss, cost, and volume of magnetic components. Unlike traditional design algorithms that focus on singular core configurations, this method considers multiple distributed cores. The validity of this approach is demonstrated through co-simulation and experimental results on a high-power prototype, offering a significant advancement over conventional design methodologies. Further, the exploration extends to a holistic design optimization method. This method systematically considers the non-linear load profiles characteristic of battery charging applications and incorporates a multi-core configuration for transformer size reduction. The effectiveness of this approach is evidenced by the implementation in a light electric vehicle charger, achieving a peak efficiency of 98.2%. Additionally, a high-precision simulation model incorporating parasitic elements is developed to accurately represent LLC resonant converters. This model enhances the understanding of system behaviors under various operational conditions, highlighting the importance of considering transient phenomena. An extended describing function model is also examined, facilitating the control of LLC converters with variable switching frequencies. This model aids in the compensator design for constant-current and constant-voltage operations, ensuring stable performance across diverse load conditions. A specific focus is placed on the design and optimization of a three-phase LLC converter with a Δ−Δ winding configuration. This study proposes a systematic design framework that optimizes the resonant frequency to minimize power loss, volume, and cost, showcasing a substantial improvement in magnetic losses compared to traditional configurations. In this thesis, these contributions offer a significant advancement in the field of LLC converter optimization, presenting a robust foundation for future research and development in power conversion systems. In conclusion, this thesis presents a series of interconnected studies that collectively enhance the understanding and implementation of LLC converter technology for LEV charging and beyond. While each research component is valuable on its own, together they contribute to a broader narrative of innovation and optimization in power electronics design. The findings not only offer practical solutions to current design challenges but also pave the way for future advancements in this field. This study is supported in part by European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie under Grant 101031029, in part by the 2232 International Fellowship for Outstanding Researchers Program of TUBITAK under Grant 118C374 and in part by the Scientific Research Projects Unit of Istanbul Technical University under Grant MDK-2022-43680.