The power stage of the PWM DC-DC converter is modeledand analyzed using the sampled-data approach. This workcomplements the companion paper TR 98-54 cite{jbrief} which develops sampled-datamodels and analysis for the closed-loop converter. Separatemodels for the power stage are essential for controller design.The work addressescontinuous and discontinuous conduction mode under voltage mode control,and continuous conduction mode under current mode control.For each configuration, nonlinear and linearized sampled-data models, andcontrol-to-output transfer function are derived. Use of these modelsfor controller design is illustrated. In addition, a newcontinuous-time switching frequency-dependent model of the power stage isderived from the sampled-data model. Examples illustrate the increasedaccuracy of control design using the new power stage models.