Forgive my electronics ineptitude if this is a stupid question, but I was thinking that if capacitors are used to store the energy supplied by the bridge rectifier in unregulated AC power supplies then what means they can't be used to store the energy in a boost converter (or buck converter) while the MOSFET (or whatever's switching) is turned off? I looked and couldn't find if this has been done before, so if it's not possible could someone explain to my why? Thanks in advance.