• Electric Vehicle Chargers
• Magnetic integration
• LLC resonant converters
• DC-DC converters
• High frequency transformers

This work is a part of a research activity concerning electric vehicle charging conducted at the ABB research centre in Ladenburg, Germany. Battery chargers play a key role in the diffusion of electric vehicles (EV), which represent the best solution for dramatically reducing emissions in urban contexts and pollutants on a global scale. In fact, they are intended to provide controlled power for recharging the batteries, balancing at best different key aspects such as battery life preservation, recharge duration, low power losses, minimal distortion and high power factor at the mains size, while also ensuring electrical insulation from the grid for safety purposes. Cost effectiveness, small dimensions and light weight are also important features for such application, especially when on-board chargers are considered. A review of the most innovative EV charging techniques is conducted, highlighting the key role of resonant converters.
In Chapter 2 the LLC resonant converter is analysed with different variations of the First Harmonic Analyses (FHA). Zero voltage switching (ZVS) and a robust voltage control are the main features of this topology. The most important operative conditions are studied, focusing on which ones allow to achieve ZVS. A design methodology for the resonant tank is proposed, taking into account the battery voltage range and the micro-controller frequency resolution. As application example, a 10 kW LLC converter operating at 100 kHz rated frequency is considered as case study, assuming a secondary voltage range 320-420 Vac and a primary bus voltage of 800 Vdc. For the case study a resonant tanks design is proposed and a PLECS® model is implemented. Analysis and simulations show that the correct operation of the converter is related to the passive elements. As a result, in order to achieve high efficiency, an oriented design of the transformer and of the resonant inductors is needed.
The design of the transformer is addressed first, beginning with the explanation of some preliminary choices according to the converter specifications: such as the operative frequency and output power. Once the preliminary choices have been made, a step-by-step design procedure for the high-frequency transformer is proposed in Chapter 4. Differently from standard procedures, a set of libraries is supposed to be get ready, containing the most significant data related to soft magnetic materials, cores shape and size, copper and alloy conductor materials, and multi-strand Litz wires. In this way, the optimal design of the component consists in the most suited combination of library elements according to the design goals, with null customisation of the standard parts. After the parameter selection, analytic formulas, packed into a Matlab® function, are used to calculate total losses and efficiency. The analytic function also provides a raw estimation of the materials cost and the average temperature of the device, which the designer has to take into account as well. The most effective solution for each set of design specifications is found by varying the combination of library components and the windings arrangement. Finally, for the cited above case study four candidate designs are selected, featuring different combinations of magnetic materials, core shape and dimensions, and number of turns for the winding.
In Chapter 5, the four transformers are analysed with a finite element software, ANSYS- Maxwell®. In this way is possible to validate the analytical results by means of 3D FEM models. With such models, an accurate calculation of the inductances is also possible as well.
When the values of the equivalent leakage and magnetising inductances is not included as design specifications, as initially assumed, external inductors could be included in the converter design to adjust at best the converter behaviour. A design for the series and parallel inductors is proposed to estimate overall size, weight, cost and losses of the magnetic components.
In order to reduce costs and dimensions of the overall converter, the possibility of a magnetic integrated transformer is investigated. One can consider to adjust the transformer structure and design to meet also the inductance requirements, thus basically achieving the integration of the external components inside the transformer itself. To such purpose, several different possibilities are examined. Finally, a comparison between the “external inductors solution” and the “magnetic integrated solution” is performed.