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In this paper, a flatness-based control method is used to control the dc/dc converters of an electrical hybrid system. This system is composed of an ultracapacitor, which is connected in parallel to a fuel cell through a bidirectional converter. This association supplies a load through another dc/dc converter. To control these converters, the mathematical model of the studied system is first presented, and then, it is proven that the system is flat. Considering the electrostatic energy stored in the dc-bus capacitors as the system output, the state variables and control variables are extracted as functions of the system output and its derivative. The system is controlled by planning the desired reference trajectories on the flat output components, and forcing them to follow their own references. The fuel-cell-dynamics control is also studied to observe the criterion of (di/dt)max. Based on the used control strategy, a method is developed to calculate the minimum values of the dc-bus capacitors in the proposed parallel hybrid system. The simulation and implementation results are presented to validate operation of the proposed method in the hybrid system.