This article focuses on the application of two nonlinear predictive control strategies to fixed frequency switch-mode dc-dc converters. The first approach uses an averaged nonlinear model for predictions and provides an a priori robust stability guarantee for the resulting closed-loop system. The implementation of this scheme requires solving a single linear program on-line. The second approach employs a piecewise affine (PWA) approximation of the converter equations as prediction model and yields an explicit controller off-line, which allows for an a posteriori stability check. An assessment of the synthesis procedures and methodological characteristics is given and simulation results are shown to substantiate the effectiveness of the proposed approaches