For years, power supplies have barely been discussed in the ongoing dialog on computing technologies. People tend to think first about motherboards, processors, hard disks and RAM. The PSU has traditionally been simply necessary but unappreciated component in a system build, usually included as an afterthought if not thrown in as part of a case purchase. These days, however, the PSU is an extremely important PC component, subject to the same kinds of strict specifications and requirements as a motherboard.
You can read about PSU specifications in a document entitled "ATX12V Power Supply Design Guide" at the formfactors.org Web site. Over time, this design guide has continued to track the latest developments in power conservation and delivery technology. This guide was most recently updated in March, 2005, as version 2.2. These latest specifications are comprehensive, in that they encompass not only the form factor and the dimensions for a typical 12-Volt power supply, but also state operating voltages and tolerances, and where cooling fans must be positioned to provide proper ventilation for such devices. These same specifications also require that the voltage sources that a PSU delivers be managed independently of one another. Such individual voltage sources are called rails in PSU-speak.
In certain technical dimensions, the ATX12V specifications are so thorough that the only decisions a power supply vendor has to make deal with which requirements a power supply actually meets. That said, the specifications shouldn't be interpreted as being an instrument for quality control. As always, the quality of a PSU depends on the engineering that the vendor puts into its design, and into the components that go into the device they build to implement that design.
Be Cautious About Fanless PSUs
The power consumption of a computer is as idiosyncratic as the user it serves. To meet all the various needs for energy consumption that a multitude of users presents, vendors typically offer PSUs rated at multiple different wattages. This is an area where vendors have a pretty free hand to anticipate and meet user demands. Most PSU models start at ratings of 300 Watts. For a long time, higher wattages came at increments of 50 watts to deliver higher levels of power. When ratings climb above 500 watts, though, the increments between models also tend to increase as well.
Other technical areas of freedom in PSU design permit the creation of actively and passively cooled models, whose selection and use depend on how a PC will be used. An actively cooled PSU includes at least one fan, and contributes to the ventilation of the entire computer case, as well as handling its own internal cooling needs. This situation plays a role in the evolution of the ATX standards, which requires the PSU to contribute to the overall ventilation of the PC's case. Older power supplies typically include fans that are no more than 80 mm in diameter; they're designed to suck warm air out of the case and blow it out of the back. Today, more and more PSUs include heftier 120 mm fans. Because of the increased diameter, more air gets moved at the same RPMs. Larger PSU fans can also run at lower rotational speeds to produce less noise. Most 120 mm fans are mounted at the bottom center of a PSU, whereas 80 mm models are mounted at the rear, which puts them right at the back of the PC case as well - a longer path from fan to outlet also means less audible noise from that outlet.
The Gigabyte Odin series uses quiet and efficient 120 mm fans.
Ironically, those who decide to use passively-cooled PSUs in an ATX case must usually install a system fan somewhere else in their PCs. Otherwise, they risk overheating key components to the point where they might be damaged or destroyed.
In some situations, noise output from a PC is a critical factor. For example, Media Center PCs or Home Theater PCs usually reside in a living or family room, where their primary function is entertainment. These kinds of PCs are often equipped with passively-cooled power supplies to keep noise levels to an absolute minimum. If these PCs also omit active cooling entirely, the remaining system components must be carefully chosen to produce less heat than typical desktop system components, to avoid potential overheating problems.