This paper investigates the effect of changing cell temperature and intensity of solar radiation on the choice and design of different topologies of DC-DC converter commonly used in PV systems. Under fluctuation of climatic conditions, the MPP of PV generator will change. The MPPT must adjust the converter duty cycle to track the new MPP. Thus the converter must be chosen to be able to match the MPP under different atmospheric conditions. In addition, when the duty cycle changes as a result of changed climatic conditions, the boundary of the converter design parameters will change. So, these parameters must be chosen to achieve the best performance at all. All these aspects are studied in this work and important results are obtained.