This paper presents a pre-sizing approach of DC-DC converters. This approach can be carried out in two main steps. The first step consists on determining the most adapted architecture to answer the specifications and the appropriate technologies of the passive and the active components of the selected architecture by considering major constraints (volume, cost...). The technology choice is carried out using models developed from manufacturer datasheets. The second step consists on optimizing the selected architecture by considering the appropriate technologies resulting from the first step under multi-physic constraints: thermal, losses (or efficiency), volume .... The proposed approach allows reducing significantly the computing time which is suitable in a pre-sizing phase where several parameters must be optimized and multi-physic constraints must be respected.