This paper proposes a PI-fuzzy controller to control the output voltage of a boost converter. The Mamdani PI-fuzzy controller operates from a set of rules emulating the behavior of a Classical-PI controller, without estimating the proportional and integral constants. The fuzzy control poses a set of nine basic rules. Performance of the proposed controller is validated by simulation and compared with a deadbeat controller. Controllers were implemented in Matlab/Simulink. The fuzzy-PI controller has zero steady state error and excellent dynamic response to different references, but it presents greater ripple in the inductor current. This research demonstrates that the fuzzy-PI controller is a good alternative for controlling power converters and non-linear systems.