Many applications require current sources rather than voltage sources. When you need a high-current source, using a linear regulator is inadvisable, because of the high power dissipation in the series resistor.
http://www.edn.com/article/CA217668.html
Power supply (or power supply unit, PSU) is the device that transfers electric power from a source to a load using electronic circuits.
http://www.smps.us/power-supply.html
Of necessity this is only a broad overview of switched mode power supplies or SMPS. The switched mode power supplies or SMPS owes its origins back to the days when valve or tube car radios needed the large HT supply, e.g. 150V DC to be generated from an automobile power system of nominally 12V DC.
http://www.electronics-tutorials.com/basics/switched-mode-power-supplies.htm
The switching power supply provides 12 volts, at 10 amps, maximum, using a discrete transistor regulator with an op-amp functioning as a comparator in the feedback circuit.
http://members.tripod.com/michaelgellis/power4.html
In this small switching power supply, a Schmitt trigger oscillator is used to drive a switching transistor that supplies current to a small inductor. Energy is stored in the inductor while the transistor is on, and released into the load circuit when the transistor switches off.
http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#ps5.gif
http://www.steve-w.dircon.co.uk/fleadh/mphil/history.htm
http://www.howstuffworks.com/power-supply.htm
Control circuit full bridge
http://www.hills2.u-net.com/electron/smps.htm
This site offers a download area of Christophe BASSO's SMPS Spice models.
http://perso.wanadoo.fr/cbasso/Spice.htm
Uses SG3524 and BUZ11
http://members.tripod.com/valveaudio/images/schematics/switcher1.JPG