Design and control development of a buck-boost converter for electric vehicles (EVs) is presented in this paper. The proposed controller includes an outer closed-loop online voltage autotuner where parameters are tuning based on the online frequencyresponse measurement to maximise the bandwidth of the feedback control system. A conventional PI regulator is considered as an inner inductor current loop. The controller not only maintain the excellent dynamic characteristic, but it also holds superior robustness to the variations of system parameters, in particular, the input voltage and load disturbance. Also, no further tuning is required due to the real-time auto-tuning capability of the controller. The controller algorithm exhibits relatively simple implementation features, being able to be deployed independently of the design approximations and parasitic values. The feasibility of the proposed controller is demonstrated by modelling in Simulink, and the experimental validation based on the realised prototype has been carried out in the laboratory.