8.2.2.2.5 Sepic Capacitor Selection

The selection of the SEPIC capacitor, CS, depends on the RMS current. The RMS current of the SEPIC capacitor is given by Equation 49.

Equation 49.

The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes the SEPIC much better suited to lower power applications where the RMS current through the capacitor is relatively small (relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than the maximum input voltage. There is an energy balance between CS and L1, which can be used to determine the value of the capacitor. Equation 50 shows the basic energy balance.

Equation 50.

where

Equation 51.

is the ripple voltage across the SEPIC capacitor, and

Equation 52.

is the ripple current through the inductor L1. The energy balance equation can be solved using Equation 53 to provide a minimum value for C_S.

Equation 53.