In this brief, the dynamics of a DC-DC buck converter is modeled using the limit cycle oscillations (LCO) induced by relay feedback test. The mathematical modeling of the converter is done based on sustained oscillatory output conditions and measurements made on LCO. The method is capable of identifying the model in presence of parameter variations. With the precise information of losses of the components available in the LCO, it is possible to get an accurate dynamic model of the converter. The identified dynamic model is validated with the step response. Also, the performance of the model is tested under different parameter perturbations. The proposed method is verified by simulation and experimental results. © 2004-2012 IEEE.