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A switched-mode power supply (SMPS) is a type of converter that converts the voltage level of a device. An SMPS produces a usable output and also allows efficient use of energy. The general purpose of this power supply is to receive electricity from an external source, and convert the power to a voltage level required by a load. These power supplies are commonly used in personal computers.
The first switched-mode power supply was patented in 1976. In the following year, Apple® Computer became the first computer manufacturer to use a SMPS in a production unit. The use of this type of power supply allowed the Apple II™ to be lighter and more compact than other competing computers.
Prior to the development of the SMPS, most personal computers used linear regulators. These earlier power units produced a set, unvaried amount of voltage. In order to keep this output constant, linear supplies typically used simple resistors. This generated a significant amount of heat, and wasted voltage during the regulation process.
Switched-mode power supplies solve this problem and do not waste energy in the form of heat. Instead of regulating voltage through resistors, a SMPS is equipped with several different transistors and capacitors. The power supply can quickly select a combination of components that is the most efficient for a specific situation. In essence, an SMPS can provide the exact amount of voltage that is demanded by a load.
In addition to the advantage of efficiency, a switched-mode power supply has several additional advantages. A SMPS is usually smaller and more lightweight than a comparable linear model, due to the lack of bulky transformers. Switched-mode supplies are less prone to overheating, and often have a longer usable life than their linear counterparts.
An SMPS is not without drawbacks, however. This type of power supply is more complex than older units, and is generally more expensive to manufacture. The rapid switching of voltage within these power supplies will also produce electromagnetic interference, which can adversely affect nearby electronic gear. Modern power supplies often use internal filters to limit this interference.
Computers are the most common devices that use switched-mode power supplies, but other devices also make use of them. Many cell phone chargers are in fact small SMPS units. This allows them to efficiently convert wall power without becoming overheated. A switched-mode supply can be used in practically any application where high efficiency is required.
Switch-mode power supplies operate by switching the power from full-on, to full-off at a very high rate, to produce any needed output. The basic concept is the same as producing one bucket of water per minute from a firehose by switching the hose full-on for just a fraction of a second, then turning it back off, and repeating until you're filling buckets at the right rate. This is as opposed to linear regulators that work more like opening the nozzle just a bit, so that water trickles out at the right rate.
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