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The boost converter operates either in discontinuous conduction mode (DCM) or in continuous conduction mode (CCM). The operation mode is determined by the duty ratio, load, and parameters of the boost converter. The plant models in DCM and CCM are different in the frequency domain. Therefore, it will be difficult to design a controller with stable operation and fast transient response for both modes. Moreover, if the boost converter operates in CCM with the DCM control gain or vice versa, it will be unstable. In this paper, the proposed control strategy can make mode transitions between DCM and CCM seamlessly by adding a mode tracker, and then the boost converter can autonomously operate by selecting the appropriate control loop in both operation modes. The proposed controller still has a voltage control loop in DCM and current/voltage control loops in CCM. The proposed mode tracker will be explained with a frequency-domain analysis. In the case of a portable fuel cell, the boost converter is required to operate from very light load (DCM) to regular load (CCM) con- ditions. Because of the wide range operation of the portable fuel cell, the strategy of the proposed smooth mode transition will be suitable. In addition, smooth operation of the converter will also be beneficial to the reliability of the fuel cell stack. Furthermore, the proposed principle will be applicable to other mode transient mechanisms such as grid mode transitions, master-and-slave mode transitions, and so on. A 20-W boost converter prototype will be used to verify the performance of the proposed control scheme.