The Boost Regulator Switching

In a forward (or buck) regulator power is continuously supplied to the outlet\filter capacitor. In a boost regulator, however, energy is pumped in a cyclic manner. The filter capacitor therefore has to be of a higher value.

The boost regulator, like the flyback regulator, pumps energy into the outlet\filter capacitor in a cyclic manner, and it is therefore desirable to operate in the discontinuous mode with a fixed peak current through the inductor.

The diode conduction time in a boost regulator, unlike the flyback regulator, is not fixed, but varies with the input voltage.

Ipk = 2 x Iout,max x (Vout / Vin,min)
Tdon = (L x Ipk) / (Vout - Vin)

Output voltage is regulated by controlling the duty cycle.

Vout = ((Ton / Tdon) + 1) x Vin

Ripple voltage is directly proportional to diode conduction time.

Tdon max = (L x Ipk) / (Vout - Vin,max)

Source : http://www.hills2.u-net.com/electron/smps.htm