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26.2 Volts, Amps, Watts, and Regulation

Power supplies are
rated in watts, but that cumulative figure doesn't
tell the real story. Watts (W) is calculated by multiplying Volts (V)
and Amperes (A). A power supply must provide specific amperages (or
currents) at numerous voltageshow many and which depend on the
type of power supply. All PC power supplies provide at least +5VDC,
+12VDC, and -12VDC. Some power supplies also provide +3.3VDC, -5VDC,
and/or 5V_SB.

Another important aspect of voltageone that varies greatly
between power suppliesis regulation,
which specifies how tightly voltages are controlled. For example, a
memory module that expects +3.3V may work at +3.2V or +3.4V, but will
probably not work at +3.1V or +3.5V. Regulation may be specified as a
maximum percentage variation or as a maximum variation in absolute
voltage.

No standards body produced a formal specification for all aspects of
the AT power supply or its BAT and LPX variants. However, the ATX
power supplyalong with its variants, the NLX and SFX power
suppliesis completely defined in a group of documents, many of
which are referenced in the following descriptions, and can be
downloaded from
http://www.formfactors.org/DeveloperResources.asp.

26.2.1 ATX/ATX12V Power Supply Specifications

ATX Specification
Version 2.1 and associated documents define the ATX
voltage rails and tolerances shown in Table 26-2.
An ATX 2.1-compliant power supply must provide these voltages at
these tolerances or better. High-quality power supplies provide
tighter tolerances, sometimes much tighter, such as 1% across all
positive voltages. Cheap power supplies often do not meet the
required tolerances for one or more voltages, and are therefore
technically not ATX power supplies. However, they look like ATX power
supplies, quack like ATX power supplies, and are sold as ATX power
supplies. Avoid any power supply that does not meet the following
standard. V_min and V_max
are calculated values, provided for the convenience of those testing
power supplies with a DMM.

The ATX/ATX12V Power Supply Design Guide Version
1.2 defines two distinct types of power supply, the ATX
power supply and the ATX12V power supply. An ATX12V power supply is a
superset of an ATX power supply, and is backward-compatible with an
ATX unit. ATX power supplies support motherboards that use +5VDC or
+3.3VDC voltage regulator modules (VRMs). ATX12V power supplies
support motherboards that use +5VDC, +3.3VDC, or +12VDC VRMs. The
sole advantage of ATX relative to ATX12V is that ATX power supplies
cost a bit less to produce, but that advantage is sufficient to
ensure that the standard ATX power supply definition will be
maintained in parallel with ATX12V. Standard ATX power supplies will
continue to be produced for use in high-volume, low-end applications.

For ATX power supplies, the

ATX/ATX12V Power Supply Design
Guide Version 1.2 recommends (but does not require) the
power distribution levels listed in Table 26-3.
A_min specifies the highest minimum amperage
load the power supply should require to function.
A_max specifies the lowest maximum amperages
the power supply should provide continuously.
A_peak describes startup surge current required
to spin up disk drives. The 250W and 300W units include the ATX
Auxiliary Power Supply Connector, and may or may not include the ATX
Optional Power Supply Connector, both of which are described later in
this chapter.

The ATX12V power supply definition is a superset of the ATX power
supply definition. An ATX12V power supply comprises an ATX power
supply with the following changes:

ATX systems use +12V primarily for running drive motors, which do not
require critical voltage regulation. The higher power requirements of
fast modern processors have mandated a shift from 3.3V and 5V VRMs to
12V VRMs. Because such systems use +12V to power the processor, they
require a power supply that provides higher current on the +12V
voltage rail and that also addresses such issues as cable voltage
drops, capacitive loading, cross-regulation, transient surge
tolerance, and cooling. ATX12V power supplies meet these additional
+12V requirements.

ATX12V defines a new 4-pin power connector, described later in this
chapter, that supports delivery of additional +12V current. The
presence of this connector indicates that a power supply is an ATX12V
unit. The absence of this connector indicates that a power supply is
an ATX unit.

Legacy support of ISA slots is the only reason for the presence of
the -5VDC rail. The ATX specification requires the -5VDC rail, but
the ATX12V specification omits it. Technically, that means an
ATX12V-compliant power supply may not function with some older
motherboards, but in practical terms most ATX12 power supplies
provide the -5VDC rail.

ATX12V power supplies include additional 2X2 +12V and Aux power
connectors and are intended for use with motherboards that require
more than the 6A per contact supported by the ATX main power
connector. For ATX12V power supplies, the

ATX/ATX12V Power
Supply Design Guide Version 1.2 recommends (but does not
require) the power distribution levels listed in Table 26-4. Although ATX12V does not specify -5VDC
current requirements, most real-world ATX12V power supplies provide
the minimum -5VDC current listed in Table 26-3. For
more information about ATX12V, see http://www.formfactors.org/developer%5Cspecs%5CATX_ATX12V_PS_1_1.pdf.

26.2.2 NLX Power Supply Specifications

NLX Power Supply Recommendations Version
1.1

defines the NLX voltage rails and tolerances shown in Table 26-5. An NLX 1.1-compliant power supply must
provide these voltages at these tolerances or better. Note that NLX
has tighter requirements than ATX on some rails. This document also
recommends (but does not require) the power distribution levels
listed for a typical 145W sustained (160W peak) power supply.
A_min, A_max, and
A_peak are as described earlier. NLX power
supplies may or may not include the NLX Optional Power Supply
Connector described later in this section.

For more information about NLX power supplies, see http://www.formfactors.org/developer%5Cspecs%5Cnlxps11.pdf.

26.2.3 SFX/SFX12V Power Supply Specifications

Although it is derived from the
ATX/ATX12V and NLX specifications, SFX/SFX12V makes compromises to
permit lower-cost power supplies that meet the needs of inexpensive
systems. SFX systems are designed to be low-cost, and the SFX
specification reflects this with less-stringent requirements than the
ATX and NLX specifications.

First-generation SFX 1.1 power supplies were designed to provide 90W
maximum continuous power, with peak power of 135W for 15-second
durations on a 5-minute duty cycle. Current SFX 2.1 power supplies
provide higher currents to support the Pentium 4 and other
power-hungry processors. The SFX Power Supply Design Guide
Version 2.1 defines the SFX voltage rails and tolerances
shown in Table 26-6. An SFX 2.1-compliant power
supply must provide these voltages with these tolerances or better.

The SFX Power Supply Design Guide Version 1.1
defined only one wattage for an SFX power supply. The
SFX Power Supply Design Guide Version 2.1
defines three SFX configurations, shown in Table 26-7.

The SFX Power Supply Design Guide Version 2.1
defines two SFX12V configurations, shown in Table 26-8. For both SFX12V power supplies, the combined
output of the +3.3VDC and +5VDC rails is less than 61W.

For more information about SFX and SFX12V power supplies, see
http://www.formfactors.org/developer\specs\SFX12V_2_3dg.pdf.

26.2.4 TFX12V Power Supply Specifications

TFX12V
is the newest standard power supply form factor. Like SFX/SFX12V,
TFX12V is derived from ATX/ATX12V. TFX12V power supplies have a long,
narrow physical shape that is optimized for small and low-profile
microATX and FlexATX systems. TFX12V has an improved airflow design
that allows the power supply fan to exhaust air from the area of the
motherboard occupied by the processor, chipset, and other
heat-generating components. TFX12V also uses fan speed control and
other techniques for reduced noise relative to other power supply
types. The TFX12V Power Supply Design Guide Version 1.01
defines the TFX12V voltage rails and tolerances shown in
Table 26-9. A TFX12V 1.01-compliant power supply
must provide these voltages with these tolerances or better.

The TFX Power Supply Design Guide Version 1.01
defines two TFX12V configurations, shown in Table 26-10. For both TFX12V power supplies, the combined
output of the +3.3VDC and +5VDC rails is less than 61W.

For more information about TFX12V power supplies, see http://www.formfactors.org/developer\specs\TFX12V_1_2dg.pdf.