This article describes the design of amicro-transformer in monolithic technology for high frequencies comprisingplanar type coil and a magnetic circuit made of several layers of materials.This micro-transformer is integrated into a micro-converter of fly-back type.The Mohan method was used to determine the geometric parameters and theS-parameters were used to calculate technological parameters. The study ofelectromagnetic effects allowed us to show the role of ferrite, which is usedto confine the magnetic field lines and minimize disruption of the neighbor ship.To validate the dimensioning of the geometrical and technological parameters,with the help of the software PSIM6.0, we simulated the equivalent electricalcircuit of the converter containing the electrical circuit of the dimensionedplanar micro-transformer