In this paper, the implementation issues of physics-based power device models in the SABER circuit simulator through the MAST High Description Language (HDL) are discussed. The possibility to use dynamical if-statements in the system matrix (equations section) of a MAST template is presented. By means of this implementation method, systems of differential equations for boundary value problems can be efficiently implemented and solved in SABER. To illustrate the method, the implementation of a physics-based power diode model is taken as an example. Simulation results using this model are presented to demonstrate the efficiency and numerical stability of the method.