Small PC Power Supplies

Power supply form factors are surprisingly fragmented. There are several very different shapes and sizes that are called "SFX", and significant variations in the sizes of TFX and FlexATX power supplies. Some commercial systems use custom power supplies.

The more promising power supply form factors seem to be:

• "SFX12V-J" which is 63.5mm x 125mm x 100mm (WxDxH 2.5"x4.9"x3.9") with a large 80mm-92mm side fan. This fits well in a case that also places HDDs and/or a memory card reader vertically and on one side.
• TFX which is 70mm x 175mm x 85mm (2.8"x6.9"x3.4") with a 60mm-80mm side fan. This is a better fit for lower-profile cases that accommodate low-profile expansion slots but not a vertical HDD.
• FlexATX which is 40.5mm x 150mm x 81.5mm (1.6"x6"x3.2") with a 40mm fan at the inner face for very small cases.
• Mini power supplies that plug into the motherboard power connector and have a single-voltage input. However, for best usability, there should be another, 12V-only power supply in the case, and there appear to be no standard form factors for that.
  • These mini power supplies are just one step removed from integrating all electronics for generating the motherboard voltages from a single-voltage 12V supply, which would simplify system design and distribute heat sources. It would require additional motherboard real estate for more on-board voltage regulators but replace the large 24-pin power connector with a much smaller one.

A side fan seems desirable, to draw air across the PCI slots or across the CPU depending on the placement. A side fan also has a larger diameter than an end fan, reducing fan speed and noise.

In terms of power, some commercially available SFF PCs have power supplies with as little as 150W of output.

Some 220W-270W FlexATX power supplies are "80 plus" certified. Higher efficiency significantly reduces cooling requirements. For example, for 180W output:

• 60% efficient: 300W input, 120W waste heat to dissipate
• 80% efficient: 225W input, 45W waste heat to dissipate