Hi,
Often an approximation is used that is really pretty good...
The output voltage is considered constant with the input switching from positive to ground, and this allows us to compute the ripple current.
The ripple current is then applied to the output capacitor and that allows us to computer the ripple voltage.
The ripple current calculation also allows us to be able to specify the capacitor rating, and the value to get some decent ripple voltage spec.
The ripple voltage calculation is often needed to be able to tell if the power supply output is smooth enough for a given application, however a secondary filter improves this much when possible.
The ripple current calculation is based on V=L*di/dt and the ripple voltage is based on dv=i*dt/C, both of which are approximated by their delta equivalent forms.