Power modules are one of the least reliable components in electrical systems functioning in harsh environments. Lifetime estimation of the modules has become a critical issue. The study case under focus is of a power supply for a particle accelerator system. The mission profile has an application specific pattern and it is irregular. An electro-thermal model using PLECS toolbox has been implemented to determine the junction temperature of the chips. Rainflow analysis has been employed to identify the mean and temperature swings of each cycle. An analytical lifetime estimation model based on the Coffin-Manson law considering both the average and temperature ranges is used. With Palmgren Miner rule the damage produced on the transistor is quantified. Lifetime predictions are made for three scenarios on a part of the mission profile.