This thesis is an ongoing project of Alstom Power Sweden AB, Växjö with a collaboration of Blekinge Institute of Technology [BTH] to automatically measure the losses of a transformer. So far the company performed the measurements manually to get this loss, and they decided to automatize the measurement process. This paper presents the state space modelling approach of DC-DC Buck converter and simulation of the whole measurement circuit incorporated with the PID controller. The PID controller will control the outputs voltage of the buck converter and the output current of the resonant circuit (secondary current of the transformer) with PI controller and those will be displayed in graph. The switching frequency of the dc-dc converter (buck) is set to 50 kHz. This system is implemented in MATLAB Simulink software.