Planar transformers (PTs) are of particular interest in switching mode power supplies and, more generally, in power converter design. They provide enhanced performance, compactness, ruggedness, constant quality, and low cost. Their optimum performance can, however, only be obtained through an accurate design which accounts for both electromagnetic and thermal problems. For that reason, this paper proposes a new combined and accurate electrothermal model for PT design. First, the electrical model based on the eddy current equation is defined. A finite element method is applied to obtain the current density distributions. Second, a 1-D thermal model based on Green's function approach is developed to calculate the temperature distribution in the device. The results obtained using the proposed coupled model are compared with measurements. The comparison indicates a relatively good correlation between the results of our model and experimental data. An excellent agreement is however observed at early times.