A switched-mode power supply (switching-mode power supply/SMPS, or simply switcher) is an electronic power supply unit (PSU) that incorporates a switching regulator in order to provide the required output voltage. An SMPS is actually a power converter that transmits power from a source (e.g., a battery or the electrical power grid) to a load (e.g., a personal computer) with ideally no loss. The function of the converter is to provide a reliable output voltage often at a different level than the input voltage.
When mechanical shafts are rotating, a simple gear train can deliver power to a shaft at one speed from a shaft at a different speed. However, fluid power can be converted from a source with one pressure-flow ratio to another pressure-flow level without rotation by using the switching action of a hydraulic ram. Similarly, when AC power is being delivered from an AC source, a simple transformer can be used to convert power at one voltage level to power at another voltage level with low losses. Likewise, the switched action of an SMPS can convert DC power with low losses. http://en.wikipedia.org/wiki/Switched-mode_power_supply