The literature proposes some thermal models needed for the electrothermal simulation of power electronic systems, This paper gives a useful analysis about the choice of the thermal model circuit networks, equivalent to a discretization of the heat equation by the finite difference method (FDM) and the finite-element method (FEM), and an analytic model developed by applying an internal approximation of the heat diffusion problem, The effect of the boundary condition representation and the introduced errors on temperature response at the heat source are studied. This study is advantageous, particularly for large surges of a short time duration.