Design and control of DC-DC converter for ultrafast battery charger for electrical vehicles

The aim of this master thesis is to analyse and design the DC/DC stage of conversion for a 60 kW ultra-fast off-board battery charger for electric vehicles. The chosen topology is the LLC circuit of which is given a clear and detailed design method and a circuit control technique. In the last decade, resonant converters have become more and more relevant thanks to the high efficiency achieved, low EMI emissions and high power density. Compared to the traditional Pulse Width Modulation (PWM) converters, they operate at a higher switching frequency. This allows to reduce both size and weight of the passive components such as filters and transformers, on the other hand it becomes more challenging to design and control the converter. The only way to increase the switching frequency is the softly commutation of the switches in the resonant converters. As a matter of fact, they are able to achieve Zero Voltage Switching (ZVS), without which switching losses would be prohibitive. The studied converter has a reconfigurable output and a modular structure. Therefore, the project starts by the design of a single module characterised by a lower transfer power. The final converter will be realized by connecting multiple modules of the same type. The thesis is divided into the following chapters: - Chapter 1: Introduction, in which the structure features of the LLC DC/DC stage of conversion are illustrated. In addition, the main characteristics of a battery charge profile are presented. Moreover, scientific papers are explored to understand the state-of-art of LLC circuit solution. - Chapter 2: LLC structure and design. First of all, the main components and relevant waveforms are analysed to figure out the LLC operating principles. Then, an iterative step design procedure method is delineated to optimize its characteristics. - Chapter 3: LLC resonant converter dual-loop control. It is tested with PLECS simulations to verify the proposed theoretical model. - Chapter 4: Conclusion, in which the output results are summarized and commented. The developing of the battery charger has required synergy between people of a wide research team. My personal contribute in the battery charger project is concentrated on the implementation of the DC/DC converter. First of all, I focused on the scientific papers in order to build a theoretical analysis of the LLC circuit. Then, I implemented a design method to realize an optimized converter. Furthermore, I developed an innovative dual-loop control digitally implemented which differs to the analogue single loop control usually adopted in literature. Therefore, I proceed with a simulation firstly of a single module, then of the complete converter, in order to test the proposed control technique.