When switch SW1 turns OFF, both inductor L1-a and L1-b windings switch polarity in response to the declining current. Diode D1 becomes forward-biased and clamps both inductor windings to the output voltage. For this simplified explanation, the designer can ignore the voltage drop in the diode. The voltage on capacitor C1 is fixed at the input voltage, by nature of capacitor action to release or absorb current as the capacitor tries to maintain a constant voltage. If capacitor C1 is a relatively large value, its voltage will remain constant for all modes of the SEPIC converter’s operation. In practice, due to size and cost limitations, a smaller capacitance is chosen and an AC voltage is allowed on the capacitor. Even though the selected capacitor will not remain at a “fixed” voltage, the inductor will compensate for any voltage deviation because the voltage will be clamped to the output through diode D1. In summary, the energy stored while SW1 was ON is delivered to the load by L1-b and C1 during the time that SW1 is turned OFF.