Switch-Mode Power Supplies (SMPS) generate distorted non-sinusoidal current waveforms flowing into conductors (wires, striplines, windings), which cause ohmic losses mainly ruled by the skin-effect. Skin effect is traditionally studied in the frequency domain; most studies on the effects of distorted SMPS current waveform are devoted to examine the so-called high-frequency effects, namely the behavior of components and systems at the switching frequency and its harmonics, considering a virtual sinusoidal steady-state operation at each frequency of the range under study. In this paper skin-effect has been studied in the time-domain by means of a formulation based on a magnetic vector potential A and on a scalar potential φ. The obtained results show that the equivalent per-unit length resistance of conductors carrying typical SMPS non sinusoidal current waveforms depend both on switching frequency and duty-cycle and is rather different from dc and ac sinusoidal equivalent resistances.