POWER ELECTRONICS / SMPS POWER SUPPLY
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There are various methods of transforming electrical energy. Power electronics deals with conversion of electric energy by electronic circuits. A unit that perform such conversion is often called power supply unit, or PSU. Technically speaking, of course, a PSU does not "supply" power. It just transforms it from one form (such as AC) into another one (such as DC) and in the process changes its characteristics as required. Power electronics originated about a century ago. Originally it started with the application of grid-controlled gas-filled tubes for power control. The subsequent development of SCR, bipolar transistors and especially high-voltage MOSFETs really revolutionized the industry and resulted in rapid expansion of various power electronics applications.
There are two main classes of PSU depending on the mode of operation of their semiconductors: linear and switching. Most of modern power supplies except for linear amplifiers are of switching type. A PSU of such type is referred to as SMPS, which stands for switch mode power supply. The control of energy flow in such a device is performed with power semiconductors that are continuously switching on and off with frequencies that are much higher than the input AC line frequency. These switches are embedded in a network of inductors and capacitors. Each cycle they accumulate some energy from the input and transfer it to the output. When a semiconductor device acts as a switch, it can control large amounts of energy with a relatively low dissipation. Indeed, an ideal switch does not dissipate any energy since it has either zero voltage when it's closed or zero current when it's open. For the design principals and schematics see this guide to SMPS. A computer PSU is a typical example of an SMPS power supply.

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