This thesis deals with an introduction to the theory of switching power supplies and with properties of magnetic materials. Special attention is paid to nanocrystalline materials. It also discusses the issues of simulating the behavior of ferromagnets. The properties of commonly used ferrite material and nanocrystalline materials was measured. Using the created program for the design of forward converter is designed a switching power supply to verify its proposal. Design optimization is based on using a nanocrystalline core, which allows to reduce the operating frequency.