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.