Electromagnetic transformers have become obstacles to progress power supply miniaturization. Thus, piezoelectric transformers, which are more compact, immune to magnetic field and that present a high galvanic insulation, become more and more an alternative to electromagnetic transformers. In this paper, two different piezoelectric transformers have been studied and compared: a Rosen type and a radial mode type. The first approach consists in performing an analytical model. This approach is inspired by the technique used in electroacoustics and allows to build an electric equivalent circuit of the vibrating structure. The second approach deals with numerical finite element method using ANSYS software. A three-dimensional finite element model of each transformer has been then simulated to find out the resonance frequency but also to determine the transformation ratio as a function of the frequency for several values of the load resistance. In order to validate the models, experiments have been performed. Measurements of the voltage gain, output power and efficiency of both piezoelectric transformer types have been obtained and compared taking into account their characteristics.