Common mode and differential mode impedances are crucial for EMC networks modeling and filter design in particular. In this paper, we propose an original analytical model of common mode and differential mode input impedances of a DC-DC converter, including major parasitical elements. This model has been synthesized using basic laws of electrical circuits. The analysis of the converter circuit is achieved through a switching function modeling its nonlinear behavior. To assess the model performance, PSpice online simulations of common mode and differential mode input impedances are achieved while the devices are operating. The results obtained agree with the analytical model in a wide EMC regulation frequency range (CISPR 22). Furthermore, we have carried out an online experimental test bench by means of a capacitive injection technique. The results obtained confirm the validity of the analytical model. To highlight the impedance switching effect, an offline measurement has also been conducted, using the impedance analyzer (Agilent 4294A).