It is called SMPS, Switch mode power supply. It does DC to DC conversion by using PWM Pulse Width Modulation technic. Most computers and all TV (at HT) uses the same technology.
Can be used for Step-up or Step-down the voltages.

They use Ferrite core transformer as usually they work on high speed around 15KHz oscilation.

Reqiures feed-back error detection and correction as must. It is more complicated babar, why you need that?? Try to work-out at Peter's solution. I used the same method and it is working all right.

Rauf