Why get a Micro ATX Power Supply?
Micro ATX power supplies are a smaller / mini form factor to fit better into SFF & compact systems. They are also generally lower power and have somewhat fewer connections as the systems they are used for are not performance systems as might be powered by ATX PSUs. There are some high performance Micro ATX PSUs that have PCI-Express connections, and that have EPS12V 4+4 pin connectors that will allow for a high performance motherboard to be powered for a compact gaming system or other enthusiast PC.
What Micro ATX Power Supply should I buy?
The right power supply for you will have the connectors and enough power you need to run the hardware in your PC build. You don't need much power of you are going to build a simple system, with just enough hardware to fill out a complete system, so maybe a 150W to 200W power supply. But if you something a bit more substantial, make sure you have connections for PCI-Express 6 or 6+8 pins, and EPS12V 4+4 pin connections on a mATX PSU with 400W or more.
What is an SFX Power Supply?
An SFX power supply oftentimes is another name for what many manufacturers refer to as uATX or MicroATX. SFX is a reduced size design closely based on the ATX form factor. The power specifications for SFX vs ATX are nearly the same meaning that SFX PSUs have mostly the same cabling and pin outs. The primary difference between the two is the size of the casing. The only electrical difference is that the SFX specifications do not require the −5 V rail, which is not really in use so much anymore so this isn't a big deal.
SFX has dimensions of 125 x 100 x 63.5 mm (width x depth x height), with a 60 mm fan, compared with the standard ATX dimensions of 150 x 86 x 140 mm. Optional 80 or 40 mm fan replacement increases or decreases the height of an SFX unit.
A Mini ATX power supply is sometimes the name referring to what actually is a Micro ATX compatible PSU. The word mini is used to imply that the supply is smaller than a standard model, but in reality the agreed upon specification is called Micro.
Dimensions & Form Factor 1)Before anything else, consider the physical requirements you will have to meet in order to mount your power supply unit to your case. People who overlook this requirement can sometimes purchase a unit that is either too big or too small for its housing. As far as the dimensions are concerned, ensure that the width, length, and height of the power supply unit you are purchasing will fit those of your case enclosure. The best bet here would be to purchase a unit which is nearly identical to the one you are looking to replace or, if setting up a new system, looking at your case manual for help with specifications. Standard ATX power supply dimensions will usually be in the range of 3.25" x 6" x 5.5" (H x W x D) while typical mATX (Micro ATX) power supply dimensions are in the range of 2.5" x 5" x 4" (H x W x D). While these dimensions may shift slightly based on the unit and manufacturer, mATX units will always be smaller than ATX units.
In regard to form factor, it is important that you purchase a unit that correctly matches your case size. A mATX size PSU will more than likely be too small for a full-sized ATX case, whereas an ATX supply would be too large for a mATX case. It is important to understand the internal layout of your case/motherboard and to select the correct form-factor power supply for the job. This is particularly important if you are looking to replace a Dell power supply, HP / Compaq power supply, Sony Power Supply, Gateway Power Supply, etc. Computer manufacturers have traditionally broken away from industry standards to release custom fitted form factors for their system lines. The reasons for this are twofold: 1) it allows them to configure the shapes of their cases to meet their own personal requirements; 2) it ensures that much of the 'replacement parts' revenue will channeled back through their suppliers. The most common mistake in power supply dimension considerations is related directly to this and it should not be taken lightly. If you are unsure whether or not your power supply requirement fits a standard ATX or mATX mold or if your manufacturer took on a new design, it is best to look it up before you start searching for your replacement unit.