How this is calculated. In continuous conduction the duty cycle is just
D = Vout/Vin, and the inductor ripple follows from the volt-
seconds applied while the high side is on:
L = Vout(Vin − Vout) / (Vin · fsw · ΔIL).
Ripple is a design choice, not a given — 20–40 % of full load is the usual compromise, because
less ripple means a bigger inductor and more ripple means higher peak current and more core
loss. The peak matters twice over: it sets the inductor's saturation rating and it is what the
current limit trips on, so
size the inductor for the peak, not the average. Output
capacitance comes from charge balance,
C = ΔIL / (8·fsw·ΔV),
which assumes an ideal cap — with anything other than ceramics the ESR term dominates, so check
the ESR limit above too. Below half the ripple current the converter drops out of continuous
conduction and these relations no longer hold.
This duty cycle is very high or very low. Real converters have minimum on and off times, so check the controller datasheet — you may need a different switching frequency or a two-stage conversion.