Medium-frequency power transformers are used in DC-DC converters topologies such as photovoltaic, wind turbines, fuel cells, electric vehicles and uninterruptible power supply systems. In the design of magnetic circuit components such as transformer and inductor, the choice of the core material is of great importance in terms of losses and magnetic induction. For determining the size of the medium-frequency transformer, frequency and magnetic flux density are also two key parameters. In this study, the core volume change with the frequency is mathematically analyzed and modeled by using finite element analysis method. Parametric analysis provides very useful information in order to determine the excitation voltage level, which does not cause the saturation in the core, for the switching frequency. Thus, parameters of the designed transformer equivalent circuit can be optimized. Then, medium-frequency transformer is operated in power electronic circuits and saturation effect and core losses according to the preferred magnetic materials are investigated by performing co-simulation.