Generally, buck converter controller is designed to control the output voltage of the converter. However, design of the controller in a photovoltaic power conditioning system is different from theoretical design guideline. The controller in a photovoltaic power conditioning system controls the input voltage of the converter (the output voltage of the solar cell) to meet a maximum power point tracking (MPPT) performance. In this study, a new model for buck converter used in a photovoltaic power conditioning system is proposed, which is linearized after state-space averaging in each period. Also, mathematical expression of the modeled buck converter is interpreted separately as small and large signals; therefore its appropriateness is measured to design linear voltage and current controllers.