This study presents a non-linear design methodology for fuzzy controllers employed in switching DC-DC converters that operates in large-signal domain. Unlike conventional controller design approach, which addresses only the small-signal system stability, the proposed design methodology tackles the design using a non-linear control viewpoint. The proposed approach uses a Tagaki-Sugeno (T-S) fuzzy model to represent the switching converter. The Lyapunov theory is then adopted to derive the sufficient stability conditions in terms of linear matrix inequalities. These inequalities are then employed for the design of a large-signal stable fuzzy controller. Worth mentioning is that the consideration of the circuit parameter uncertainty and the mismatch of the fuzzy membership functions in the proposed approach, greatly complicate the stability analysis, which leads to a conservative result. To alleviate the conservativeness, a membership-function condition is proposed to guide the design of the membership functions of the fuzzy controller. Furthermore, free matrices can be introduced to further relax the conservativeness of stability analysis result. Simulation results on the regulation of a boost DC-DC converter subject to load and input voltage disturbances, using the proposed fuzzy controller are provided to illustrate the effectiveness of the proposed approach.