This paper proposes a unified modelling framework for model-free predictive control (MFPC) to enhance the robustness of model predictive control (MPC) against model uncertainties in power electronic (PE) systems. While previous simulations and experimental results have shown improved robustness of MFPC over MPC, the literature lacks a theoretical framework to support this improvement. Additionally, the design of MFPC controller design typically relies on trial and error, which is time-consuming and does not guarantee optimal performance. This paper aims to address these limitations by presenting a unified modelling framework for MFPC. The framework serves as a useful tool for evaluating the performance of MFPC and can help expedite the controller design process. To validate the effectiveness of the proposed framework, simulations and experiments are conducted on a buck converter prototype.