This paper is a contribution in the developments. of the steady state equations and dynamic representation of current fed switching structures (buck, boost) in continuous mode. The basic principle for the mathematical modelling is the first order expansion of the transition matrices. A comparison with classical voltage fed power cells shows an higher degree in the system transmittances as the consequence of the introduction of a new system variable the input capacitance voltage. Validations are realised on a breadboard PWM shunt regulator.