Hardware Reference
In-Depth Information
responsible system manufacturers tried to “key” the motherboard and power supply connectors so you
couldn't install them backward or out of sequence. However, most vendors of cheaper systems did
not feature this keying on the boards or supplies they used. The ATX form factor includes intelligently
designed and keyed power plugs to prevent users from incorrectly plugging in their power supplies.
The ATX connectors also supply +3.3V, reducing the need for voltage regulators on the motherboard
to power +3.3V-based circuits.
Besides the new +3.3V outputs, ATX power supplies furnish another set of outputs that is not
typically seen on standard power supplies. The set consists of the Power_On (PS_ON) and
5V_Standby (5VSB) outputs mentioned earlier, known collectively as
Soft Power
. This enables
features to be implemented, such as Wake on Ring or Wake on LAN, in which a signal from a modem
or network adapter can actually cause a PC to wake up and power on. Many such systems also have
the option of setting a wakeup time, at which the PC can automatically turn itself on to perform
scheduled tasks. These signals also can enable the optional use of the keyboard to power the system
on—an option you can set on some systems. These features are possible because the +5V Standby
power is always active, giving the motherboard a limited source of power even when off. Check your
BIOS Setup for control over these types of features.
SFX/SFX12V
Intel released the smaller microATX motherboard form factor in December 1997. At the same time, it
released the small form factor (SFX) power supply design to go with it. Even so, most microATX
chassis continued to use the standard ATX power supply instead. Then in March 1999, Intel released
the FlexATX addendum to the microATX specification, which was a small board designed for low-
end PCs or PC-based appliances. Since then, the SFX supply has found use in many new compact
system designs. Unlike most of the power supply form factor specifications in which a single
mechanical or physical outline is defined, the SFX standard actually defines five different physical
shapes, some of which are not directly interchangeable. In addition, there have been changes to the
connectors required as the specification has evolved. Therefore, when replacing an SFX/SFX12V-
type supply, you need to ensure you are purchasing the correct type—which is to say the type that will
physically install in your chassis—as well as have the correct connectors for your motherboard.
The number and types of connectors have changed over the life of the specification. The original SFX
power supply specification included a single 20-pin motherboard connector. The 4-pin +12V
connector to provide independent CPU power was added as an option in the 2.0 revision (May 2001)
and was made a requirement in revision 2.3 (April 2003), causing the spec to be renamed as SFX12V
in the process. SFX12V version 3.0 changed the main motherboard power connector from 20 pins to
24 pins and made Serial ATA power connectors a requirement. The current SFX12V version 3.1 was
released in March 2005 and contains a few additional minor revisions, including a change to High
Current System (HCS) terminals in the connectors. SFX12V includes several physical designs,
including one called the PS3 form factor.
On a standard SFX/SFX12V power supply, a 60mm diameter cooling fan is located inside the power
supply housing, facing the inside of the computer's case (see
Figure 18.2
)
. The fan draws the air into
the power supply housing from the system cavity and expels it through a port at the rear of the system.
Internalizing the fan in this way reduces system noise and results in a standard negative-pressure
design. The system can also use additional processor and chassis cooling fans, which are separate
from the power supply.
