A Critical Investigation of Coupled Inductors SEPIC Design Issues

This paper discusses design issues of single-ended primary-inductor converter (SEPIC) with coupled inductors (CIs). The main investigation is focused on the correlation existing among the size of coupling capacitor, the magnetic coupling factor of CIs, the voltage conversion ratio, and the amplitude of peak-to-peak (pk-pk) ripple current in the input and output ports of the CIs. It is shown how the above elements must be properly correlated to prevent large input and output ripple currents, particularly if tightly coupled inductors are used. Analytical design formulas for the coupling capacitor are applied to the design of a low-power SEPIC. Simulations and experimental measurements prove the validity of the model predictions and the reliability of related design rules.