This thesis is part of an ongoing project of Ericsson together with Blekinge Institute of Technology, Växjö University and Linneaus University to develop Signal Processing methods to increase the functionality, performance and reliability of dc-dc converters using signal processing methods. The aim of this work is to model the buck converter system of Ericssons BMR450 and develop a time-domain based system identication method to identify the capacitive load. The model is implemented in Matlab Simulink and is composed of simple circuits. The system is analyzed through excitation from the command input using binary signal, which include all frequencies of it. Two types of identication method, Black-box and Grey-box, used to analyze the system and hence identify the load parameter. The sampled data are analyzed and preprocessed to estimate a mathematical model and validate it. The proposed method is able to identify the load parameter with good accuracy.