General block diagram models are proposed for PWM DC-DC convertersin continuous and discontinuous conduction modes withfixed switching frequency.Both current mode control and voltage mode control are addressed inthese models.Based on these models detailed nonlinear and linearized sampled-data dynamics are derived.Asymptotic orbital stability is analyzed.Audio-susceptibility and output impedance are derived. In this approach,discontinuous conduction mode andcurrent mode control can be analyzed systematicallywithout special effort.A companion paper (Part II) addresses the same issuesfor the power stage of a PWM DC-DC converter.