SEPIC design- part II: Capacitive damping

This paper completes the discussion of issues related to models and methods for peak-current-controlled SEPIC (PCC-SEPIC) design aimed at achieving intrinsic stability of the current loop. In the companion paper (Part I) a new model of peak current control for SEPIC has been introduced. This model takes into account the role played by the coupling capacitor in identifying reliable stability boundary for SEPIC. This is obtained by introducing the difference between input voltage and coupling capacitor voltage in the control law. By making use of the control law model obtained in Part I, this paper presents a method to design an RC damping branch to be put in parallel to the coupling capacitor. Such RC damping branch is selected to guarantee the desired value of the damping factor of dominant complex poles of the closed current loop input-to-coupling capacitor voltage gain. A procedure to identify the optimal values for the damping branch is proposed. A converter has been mounted and its control-to output gain has been measured to verify the correctness of the results. The design approach presented in this paper can be used for all those applications where output inductor required to achieve the same damping factor would be much greater than input inductor, as discusses in companion paper Part I.