Small-Signal Dynamics of Current-Mode Controlled Active Clamp Forward Converter with Main Switch Current Feedback

Active clamp forward converters (ACFCs) with current mode control are widely used for low voltage/high current applications. Current mode control for ACFCs usually employs the current feedback from the main switch at the primary side of the isolation transformer, which carries both the reflected inductor current and the transformer magnetizing current. With the additional magnetizing current feedback, the power stage dynamics with current loop closed are markedly altered from the case of the inductor-current only feedback. This paper analyzes the power stage dynamics and presents a factorized expression for the control-to-output transfer function with current loop closed, thus providing the new and exact information about the control design for the main switch current feedback. The accuracy of the proposed control-to-output transfer function is validated by both computer simulations and experimental measurements. The implication, significance, and utility of the new transfer function are addressed.