In this paper a new toolbox, PLECS, for the fast simulation of power electronic circuits under Simulink is presented. This toolbox provides the means for modeling large power electronic systems containing both electrical circuits and controllers. The program is based on a state-space formulation for circuits that consist of linear elements (RLC), transformers, sources, meters, and ideal switches. Diodes, thyristors, IGBTs and other nonlinear elements such as saturable inductors can be assembled by combining linear elements and switches. Thus, a piece-wise linear model is attained which leads to a stable and fast simulation. An attached benchmark simulation demonstrates the capability of PLECS.