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 voltages?how 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 voltage?one that varies greatly between power supplies?is 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 supply?along with its variants, the NLX and SFX power supplies?is 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:

Enhanced +12VDC output

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.

Additional power connector

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.

Removal of -5VDC requirement

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.