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Employing fuel cell (FC) as main source requires increasing and regulating its output voltage. In this paper, nonisolated dc-dc converter with high voltage ratio is proposed to interface between the FC and high-voltage dc bus. To take into account the low-voltage-high-density characteristics of power sources, a cascaded structure composed of two subconverters in cascade has been chosen and allows obtaining high voltage ratio. The choice of each subconverter is based on source requirements and its performances. Consequently, in this paper, a converter consisting of two-interleaved boost converter is chosen as first subconverter and a three-level boost converter is chosen as second subconverter. Control of the whole system is realized by energy trajectory planning based on flatness properties of the system. The design of trajectories is explained and allows respecting the fuel-cell constraints as main power source. To ensure correct design of the energy trajectories, a noninteger power-law function is used to model the static characteristic of the FC. This law allows investigating the effect of humidity and temperature on the dynamics of the proposed system. The control of both current and voltage balance across the output serial capacitors of the three-level boost converter is ensured by nonlinear controllers based on a new nonlinear model.