Determination of the parameters of the transformer circuit is a very important task, as there are a huge number of them (transformers), of different power, different designs, performing different tasks, working in different conditions at voltages and currents of different frequencies and form. This article deals with the determination of the parameters of greatest interest, the parameters that take into account the phenomena in the magnetic core, and the active resistances of the windings, which are calculated for the increased frequency (up to 30 MHz), implemented in secondary power supplies and other semiconductor devices.