The static design of ac/dc and dc/dc switching-mode power supplies (SMPS) relies on a simple but repetitive process. Although specific spreadsheets, available in various computer-aided design (CAD) programs, are widely used, they are difficult to use in educational applications. In this paper, a graphic tool programmed in MATLAB is presented, which allows students to apply and to reflect upon the knowledge acquired in theoretical classes. This tool has been successfully employed in the course "Sistemas Electronicos de Alimentacion" in the Telecommunications Engineering degree as part of a project in new education methods (University of Oviedo project PB-08-019). During practical sessions, the students used the tool to design a SMPS, achieving better learning results than was the case without the tool. In addition, the tool was successfully used to improve the students' theoretical knowledge of the subject. (Contains 17 figures.)