ENHANCE ATX12V Guide

ATX / ATX12V
Power Supply Design Guide
Version 1.1

ATX/ATX12V Power Supply Design Guide
Version 1.1
Page 2
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Copyright 2000 Intel Corporation. All rights reserved.
Version 1.1
† Third-party brands and names are the property of their respective owners.
Revision History
Version Summary of Changes Date
1.0 Initial Release Feb. 2000
1.1 Increase 3.3 V current; add more explanation for power sharing; do minor edits and
format fixes Aug. 2000

ATX/ATX12V Power Supply Design Guide
Version 1.1
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Contents
1. Introduction...................................................................................................... 6
1.1. Scope .................................................................................................................................... 6
1.2. New for ATX12V as Compared with ATX Power Supply....................................................... 6
1.2.1. ATX12V .................................................................................................................... 6
1.2.2. Increased +5 VSB Current........................................................................................ 7
2. Applicable Documents..................................................................................... 8
3. Electrical........................................................................................................... 9
3.1. AC Input................................................................................................................................. 9
3.1.1. Input Overcurrent Protection..................................................................................... 9
3.1.2. Inrush Current Limiting ............................................................................................. 9
3.1.3. Input Undervoltage....................................................................................................9
3.1.4. Immunity................................................................................................................... 10
3.1.5. Catastrophic Failure Protection ................................................................................ 10
3.2. DC Output.............................................................................................................................. 10
3.2.1. DC Voltage Regulation ............................................................................................. 10
3.2.2. Remote Sensing....................................................................................................... 11
3.2.3. Typical Power Distribution ........................................................................................ 11
3.2.4. Power Limit............................................................................................................... 14
3.2.5. Efficiency................................................................................................................... 14
3.2.6. Output Ripple/Noise.................................................................................................. 15
3.2.7. Output Transient Response...................................................................................... 16
3.2.8. Capacitive Load........................................................................................................ 16
3.2.9. Closed-loop Stability.................................................................................................16
3.2.10. +5 VDC / +3.3 VDC Power Sequencing ................................................................. 17
3.2.11. Voltage Hold-up Time............................................................................................. 17
3.3. Timing / Housekeeping / Control........................................................................................... 17
3.3.1. PWR_OK.................................................................................................................. 17
3.3.2. PS_ON#.................................................................................................................... 18
3.3.3. +5 VSB...................................................................................................................... 19
3.3.4. Power-on Time .........................................................................................................19
3.3.5. Risetime.................................................................................................................... 19
3.3.6. Overshoot at Turn-on / Turn-off................................................................................ 20
3.3.7. Reset after Shutdown ............................................................................................... 20
3.3.8. +5 VSB at AC Power-down....................................................................................... 20
3.4. Output Protection................................................................................................................... 20
3.4.1. Overvoltage Protection ............................................................................................. 20
3.4.2. Short-circuit Protection ............................................................................................. 21
3.4.3. No-load Operation .................................................................................................... 21
3.4.4. Overcurrent Protection.............................................................................................. 21
3.4.5. Overtemperature Protection ..................................................................................... 21
3.4.6. Output Bypass .......................................................................................................... 21

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4. Mechanical........................................................................................................ 22
4.1. Labeling / Marking ................................................................................................................. 22
4.2. Physical Dimensions ............................................................................................................. 22
4.3. Airflow / Fan........................................................................................................................... 25
4.4. AC Connector........................................................................................................................ 26
4.5. DC Connectors...................................................................................................................... 26
4.5.1. ATX Main Power Connector ..................................................................................... 27
4.5.2. +12 V Power Connector (for ATX12V Configurations Only)..................................... 28
4.5.3. Auxiliary Power Connector for Configurations with +3.3 VDC Output > 18 A
or +5 V Output > 24 A............................................................................................... 28
4.5.4. Peripheral Connector(s)............................................................................................ 28
4.5.5. Floppy Drive Connector............................................................................................ 28
5. Environmental .................................................................................................. 29
5.1. Temperature.......................................................................................................................... 29
5.2. Thermal Shock (Shipping)..................................................................................................... 29
5.3. Humidity................................................................................................................................. 29
5.4. Altitude................................................................................................................................... 29
5.5. Mechanical Shock ................................................................................................................. 29
5.6. Random Vibration.................................................................................................................. 30
5.7. Acoustics ............................................................................................................................... 30
6. Electromagnetic Compatibility........................................................................ 31
6.1. EMI ........................................................................................................................................ 31
6.2. Input Line Current Harmonic Content and Line Flicker Required for Sales
in Europe and Japan.............................................................................................................. 31
6.3. Magnetic Leakage Fields....................................................................................................... 31
7. Reliability.......................................................................................................... 32
7.1. Component Derating ............................................................................................................. 32
7.2. Mean Time Between Failures (MTBF) .................................................................................. 32
8. Safety ................................................................................................................ 33
8.1. North America........................................................................................................................ 33
8.2. International........................................................................................................................... 34
8.3. Proscribed Materials.............................................................................................................. 34

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Figures
Figure 1. Differential Noise Test Setup................................................................................................. 15
Figure 2. Power Supply Timing............................................................................................................. 17
Figure 3. PS_ON# Signal Characteristics............................................................................................. 19
Figure 4. Power Supply Dimensions for Chassis in Which the P/S Does Not Cool Processor ............ 23
Figure 5. Power Supply Dimensions for Chassis in Which the P/S Cools the Processor..................... 24
Figure 6. ATX and ATX12V Power Supply Connectors........................................................................ 27
Tables
Table 1. Power Supply Feature Summary, ATX versus ATX12V......................................................... 7
Table 2. AC Input Line Requirements................................................................................................... 9
Table 3. DC Output Voltage Regulation................................................................................................ 10
Table 4. Typical Power Distribution for a 160 W ATX Configuration .................................................... 11
Table 5. Typical Power Distribution for a 200 W ATX Configuration .................................................... 12
Table 6. Typical Power Distribution for a 250 W ATX Configuration .................................................... 12
Table 7. Typical Power Distribution for a 300 W ATX Configuration .................................................... 12
Table 8. Typical Power Distribution for a 200 W ATX12V Configuration.............................................. 13
Table 9. Typical Power Distribution for a 250 W ATX12V Configuration.............................................. 13
Table 10. Typical Power Distribution for a 300 W ATX12V Configuration............................................ 14
Table 11. DC Output Noise/Ripple........................................................................................................ 15
Table 12. DC Output Transient Step Sizes........................................................................................... 16
Table 13. Output Capacitive Loads....................................................................................................... 16
Table 14. PWR_OK Signal Characteristics .......................................................................................... 18
Table 15. PS_ON# Signal Characteristics............................................................................................ 18
Table 16. Overvoltage Protection.......................................................................................................... 20

ATX/ATX12V Power Supply Design Guide
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1. Introduction
1.1. Scope
This document provides design suggestions and reference specifications for a family of
power supplies that comply with the ATX Specification, Version 2.03*for motherboards
and chassis. It includes supplementary information not expressly detailed in the ATX
Specification, such as information about the physical form factor of the power supply,
cooling requirements, connector configuration, and pertinent electrical and signal timing
specifications.
This document is provided as a convenience only and is not intended to replace the user’s
independent design and validation activity. It should not be inferred that all ATX or
ATX12V power supplies must conform exactly to the content of this document. Neither
are the design specifics described herein intended to support all possible system
configurations, because system power supply needs vary widely depending on factors such
as the application (that is, for desktop, workstation, or server), intended ambient
environment (temperature, line voltage), or motherboard power requirements.
1.2. New for ATX12V as Compared with ATX Power Supply
This section briefly summarizes the major changes made to this document that now
includes the ATX12V power supply. An earlier design guide covered only the ATX power
supply. There are also a few changes from Version 1.0 to 1.1 (increased 3.3 V current,
more explanation for power sharing).
1.2.1. ATX12V
A new superset of the original ATX power supply has been defined. Named “ATX12V,”
this new power supply is comprised of a standard ATX unit plus the following
enhancements:
•Increased +12 VDC output capability. Motherboard components with unique voltage
requirements are increasingly expected to be powered via DC/DC converters off the
+12 VDC power supply output. This trend is due primarily to the higher power
conversion and transmission efficiencies of +12 VDC relative to +5 VDC or +3.3 VDC.
ATX12V power supplies should be designed to accommodate these increased +12 VDC
current requirements and to address associated issues such as cross-regulation,
capacitive loading, transient surge tolerance, cable voltage drop, and cooling.
*2.03 is the current version of the ATX Specification as of this writing. Future references to the ATX
Specification in this document imply version 2.03 or later, as applicable.

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•Power Connectors. To enable the delivery of more +12 VDC current to the
motherboard, a new 4-pin receptacle/header combination—the +12 V power
connector—has been defined. The presence of the +12 V power connector indicates
that a power supply is ATX12V; the absence of the +12 V power connector
indicates that a supply is ATX. To allow for greater than 3.3 V current, the Aux
Power Connector is recommended for ATX and ATX12V power supplies with 3.3 V
current > 18 A. See Sections 3.2.3.2 and 4.5 for details on the +12 V and AUX power
connector.
ATX12V power supplies are intended to be downward compatible with ATX power
supplies. Consequently, it is required that an ATX12V power supply be able to provide the
same typical +5 VDC and +3.3 VDC maximum continuous output currents as an ATX
supply of the same total output power. To minimize cost, this may be accomplished via a
load sharing arrangement, whereby the individual +12 VDC, +5 VDC, and +3.3 VDC
output currents may trade off against each other but the combined total output power is
constrained to not exceed a limit specified by the power supply designer. Such an approach
can effectively support both +5 VDC- or +12 VDC-centric motherboards.
The standard ATX power supply definition will be maintained in parallel with ATX12V—
despite the downward compatibility of ATX12V—because both offer distinct advantages to
the industry:
•An ATX power supply supports motherboards that rely on DC/DC conversion from
+5 VDC or +3.3 VDC only. This limitation and an initial volume advantage make it
more attractive for focussed, cost-sensitive system platforms.
•An ATX12V power supply supports motherboards that rely on DC/DC conversion from
either +12 VDC, +5 VDC, or +3.3 VDC, yielding more application flexibility for future
platforms.
1.2.2. Increased +5 VSB Current
Trends in PC system power management solutions (for example, “Instantly Available PC,”
“Suspend-to-RAM”) are driving a need for increased +5 VSB current capability for all
ATX-family power supplies. The previous +5 VSB output requirement is being raised to
1.0 amps minimum, with 2.0 amps preferred. Recommendations for momentary peak
current have also been added to enable USB "wake on" devices. See Section 3.3.3 for
details.
Table 1. Power Supply Feature Summary,
ATX versus ATX12V
ATX ATX12V
+12 V power connector No Yes
+5 VSB current 1.0 amps 1.5 Peak (required)
2.0 amps 2.5 Peak (recommended) 1.0 amps 1.5 Peak (required) 2.0 amps 2.5 Peak (recommended)

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2. Applicable Documents
The latest revision in effect of the following documents forms a part of this document to the
extent specified.
Document title Description
AB13-94-146 European Association of Consumer Electronics Manufacturers (EACEM)
Hazardous Substance List / Certification
ANSI C62.41-1991 IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Circuits
ANSI C62.45-1992 IEEE Guide on Surge Testing for Equipment Connected to Low-Voltage AC Power
Circuits
MIL-STD-105K Quality Control
MIL-STD-217F Reliability Predictions for Electronic Equipment
MIL-C-5541 Chemical Conversion Coatings on Aluminum and Aluminum Alloys
CSA C22.2 No.234, Level 3 Safety of Component Power Supplies (Intended for use with Electronic Data
Processing Equipment and Office Machines)
CAN/CSA C22.2 No.950-95,
3rd edition Safety of Information Technology Equipment Including Electrical Business
Equipment
UL 1950, 3rd edition, without D3
Deviation Safety of Information Technology Equipment Including Electrical Business Equipment
IEC 60950 plus A1, A2, A3, A4 Safety of Information Technology Equipment Including Business Equipment
EN 60950 plus A1, A2, A3, A4 Safety of Information Technology Equipment Including Business Equipment
EMKO-TSE (74-SEC) 207/94 Nordic national requirement in addition to EN 60950
CISPR 22:1997 3rd edition
EN 55022:1998 Limits and Methods of Measurements of Radio Interference Characteristics of Information Technology Equipment, Class B
ANSI C63.4 – 1992 American National Standard for Methods of Measurement of Radio-Noise
Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9
kHz to 40 GHz for EMI testing
EN 55024:1998 Information technology equipment—Immunity characteristics—Limits and methods
of measurement
EN 61000-3-2 Electromagnetic compatibility (EMC)—Part 3: Limits—Section 2: Limits for
harmonic current emissions, Class D
IEC 61000-4- Electromagnetic compatibility (EMC) for industrial-process measurement and
control equipment—Part 4: Testing and measurement techniques
Section -2: Electrostatic discharge
Section -3: Radiated, radio-frequency, electromagnetic field
Section -4: Electrical fast transient / burst
Section -5: Surge
Section -6: Conducted disturbances, induced by radio-frequency fields
Section -8: Power frequency magnetic fields
Section -11: Voltage dips, short interruptions, and voltage variations
Japan Electric Association Guidelines for the Suppression of Harmonics in Appliances and General Use
Equipment
IEC Publication 417 International Graphic Symbol Standard
ISO Standard 7000 Graphic Symbols for Use on Equipment
CFR 47, Part 15, Subpart B FCC Rules

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3. Electrical
The electrical requirements that follow are to be met over the environmental ranges
specified in Section 5 unless otherwise noted.
3.1. AC Input
Table 2 lists AC input voltage and frequency requirements for continuous operation. The
power supply shall be capable of supplying full-rated output power over two input voltage
ranges rated 100-127 VAC and 200-240 VAC rms nominal. The correct input range for
use in a given environment may be either switch-selectable or autoranging. The power
supply shall automatically recover from AC power loss. The power supply must be able to
start up under peak loading at 90 VAC.
Table 2. AC Input Line Requirements
Parameter Min. Nom. (1) Max. Unit
Vin (115 VAC) 90 115 135 VACrms
Vin (230 VAC) 180 230 265 VACrms
Vin Frequency 47 -- 63 Hz
(1) Nominal voltages for test purposes are considered to be within ±1.0 V of nominal.
3.1.1. Input Overcurrent Protection
The power supply shall incorporate primary fusing for input overcurrent protection to
prevent damage to the power supply and meet product safety requirements. Fuses should
be slow-blow–type or equivalent to prevent nuisance trips.
3.1.2. Inrush Current Limiting
Maximum inrush current from power-on (with power on at any point on the AC sine) and
including, but not limited to, three line cycles, shall be limited to a level below the surge
rating of the input line cord, AC switch if present, bridge rectifier, fuse, and EMI filter
components. Repetitive ON/OFF cycling of the AC input voltage should not damage the
power supply or cause the input fuse to blow.
3.1.3. Input Undervoltage
The power supply shall contain protection circuitry such that the application of an input
voltage below the minimum specified in Section 3.1, Table 2, shall not cause damage to the
power supply.

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3.1.4. Immunity
At a minimum, a system and power supply typically must pass testing per the limits and
methods described in EN 55024 prior to sale in many parts of the world. Additional
requirements may depend on the design, product end use, target geography, customer, and
other variables. Consult your company’s Product Safety and Regulations department for
more details.
3.1.5. Catastrophic Failure Protection
Should a component failure occur, the power supply should not exhibit any of the
following:
•Flame
•Excessive smoke
•Charred PCB
•Fused PCB conductor
•Startling noise
•Emission of molten material
3.2. DC Output
3.2.1. DC Voltage Regulation
The DC output voltages shall remain within the regulation ranges shown in Table 3 when
measured at the load end of the output connectors under all line, load, and environmental
conditions. The voltage regulation limits shall be maintained under continuous operation
for a period of time equal to or greater than the MTBF specified in Section 7.2 at any steady
state temperature and operating conditions specified in Section 5.
Table 3. DC Output Voltage Regulation
Output Range Min. Nom. Max. Unit
+12VDC (1) ±5% +11.40 +12.00 +12.60 Volts
+5VDC ±5% +4.75 +5.00 +5.25 Volts
+3.3VDC ±5% +3.14 +3.30 +3.47 Volts
-5VDC ±10% -4.50 -5.00 -5.50 Volts
-12VDC ±10% -10.80 -12.00 -13.20 Volts
+5VSB ±5% +4.75 +5.00 +5.25 Volts
(1) At +12 VDC peak loading, regulation at the +12 VDC output can go to ± 10%.

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3.2.2. Remote Sensing
The +3.3 VDC output should have provisions for remote sensing to compensate for
excessive cable drops. The default sense should be connected to pin 11 of the main power
connector. The power supply should draw no more than 10 mA through the remote sense
line to keep DC offset voltages to a minimum.
3.2.3. Typical Power Distribution
DC output power requirements and distributions will vary widely based on specific system
options and implementation. Significant dependencies include the quantity and types of
processors, memory, add-in card slots, and peripheral bays, as well as support for advanced
graphics or other features. It is ultimately the responsibility of the designer to derive a
power budget for a given target product and market.
Tables 4 through 10 provide sample power distributions and cross-loading diagrams as a
reference only. The tabular data summarizes maximum and minimum loadings on each
output, regardless of cross-regulation. The diagrams show typical assumptions for cross-
loading: The area within each plotted perimeter represents the power usage of a
motherboard and system platform. At a minimum, the power supply must have the
capability to provide power for the area inside the plotted area. The power supply may
provide additional margin for the area outside the perimeter. In each graph, the vertical line
on the right side shows the recomended combined power from 3.3 V and 5 V; the upper
horizontal line is the recommended 12 V power; and the total power is the power expected
from all rails for any system configuration. The power supply will share power between the
5 V, 3.3 V, and 12 V to provide the maximum rated power under the possible loading
conditions shown in each graph. It should not be inferred that all power supplies must
conform to these tables, nor that a power supply designed to meet the information in the
tables will work in all system configurations.
3.2.3.1. ATX Configurations
Table 4. Typical Power Distribution for a 160 W ATX Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 6.0 8.0
+5 VDC 1.0 18.0
+3.3 VDC 0.3 14.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 1.5 2.5
See graph at right for power sharing.
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100 120
+5V & +3.3V combined load (W)
+12V load (W)

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Table 5. Typical Power Distribution for a 200 W ATX Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 8.0 10.0
+5 VDC 1.0 21.0
+3.3 VDC 0.3 14.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 1.5 2.5
See graph at right for power sharing.
Table 6. Typical Power Distribution for a 250 W ATX Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 10.0 12.0
+5 VDC 1.0 25.0
+3.3 VDC 0.3 16.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 1.5 2.5
See graph at right for power sharing.
Table 7. Typical Power Distribution for a 300 W ATX Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 12.0 14.0
+5 VDC 1.0 30.0
+3.3 VDC 0.3 20.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 1.5 2.5
See graph at right for power sharing.
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100 120 140 160 180 200
+5V & +3.3V combined load (W)
+12V load (W)
0
20
40
60
80
100
120
0 20406080100120140
+5V & +3.3V combined load (W)
+12V load (W)
0
20
40
60
80
100
120
140
0 20406080100120140160
+5V & +3.3V combined load (W)
+12V load (W)

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3.2.3.2. ATX12V Configurations
ATX12V power supplies—with their additional 2x2 +12V & Aux power connectors—are
intended for applications where the motherboard demand for current exceeds the ATX
main power connector capability of ~6 A per contact. In general, the +12 V power
connector should not be implemented on any power supply with a total +12 VDC
continuous output capability of less than 10 A.
Table 8. Typical Power Distribution for a 200 W ATX12V Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 10.0 12.0
+5 VDC 0.1 21.0
+3.3 VDC 0.3 14.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 1.5 2.5
See graph at right for power sharing.
Table 9. Typical Power Distribution for a 250 W ATX12V Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 13.0 16.0
+5 VDC 0.1 25.0
+3.3 VDC 0.3 20.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 1.5 2.5
See graph at right for power sharing.
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100 120 140 160
+5V & +3.3V combined load (W)
+12V load (W)
0
20
40
60
80
100
120
140
0 20406080100120140
+5V & +3.3V combined load (W)
+12V load (W)

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Table 10. Typical Power Distribution for a 300 W ATX12V Configuration
Output
Min.
Current
(amps)
Max.
Current
(amps)
Peak
Current
(amps)
+12 VDC 0.0 15.0 18.0
+5 VDC 0.1 30.0
+3.3 VDC 0.3 28.0
-5 VDC 0.0 0.3
-12 VDC 0.0 0.8
+5 VSB 0.0 2.0 2.5
See graph at right for power sharing.
3.2.4. Power Limit
Under short-circuit or overload conditions, no output shall exceed 240 VA under any
conditions including single component fault conditions, per IEC 60950 requirements.
3.2.5. Efficiency
3.2.5.1. General
The power supply should be a minimum of 68% efficient under maximum rated load. The
efficiency of the power supply should be met over the AC input range defined in Table 2,
under the load conditions defined in Section 3.2.3, and under the temperature and operating
conditions defined in Section 5.
3.2.5.2. Energy Star
The “Energy Star” efficiency requirements of the power supply depend on the intended
system configuration. In the low-power Energy Star state, the AC input power is limited to
30 W or 15% of the rated maximum DC output power for the configuration, whichever is
greater. For example, in a 160 W system configuration, the Energy Star input power limit
is 160 W × 0.15 = 24 W ⇒30 W; for a 300 W configuration, 300 W × 0.15 = 45 W.
While a minimum power supply efficiency of 56% is often recommended at Energy Star
operating levels, compliance to the guideline will also depend on the system’s DC power
consumption. In cases where the system Energy Star power consumption for each DC
output is known, the system designer should provide this information to assist the power
supply designer.
3.2.5.3. Blue Angel, RAL-UZ 78
The +5VSB standby supply efficiency should be a minimum of 50% at 500 mA output.
Standby efficiency is measured with the main outputs off and with PS_ON# high. To meet
Blue Angel requirements, the AC input power shall not exceed 5 W when the main outputs
are in the “DC disabled” state with 500 mA load on +5VSB and a 230 VAC/50 Hz input.
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80 100 120 140 160 180 200
+5V & +3.3V combined load (W)
+12V load (W)

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3.2.6. Output Ripple/Noise
The output ripple/noise requirements listed in Table 11 should be met throughout the load
ranges specified in Section 3.2.3 and under all input voltage conditions as specified in
Section 3.1.
Ripple and noise are defined as periodic or random signals over a frequency band of 10 Hz
to 20 MHz. Measurements shall be made with an oscilloscope with 20 MHz bandwidth.
Outputs should be bypassed at the connector with a 0.1 µF ceramic disk capacitor and a
10 µF electrolytic capacitor to simulate system loading. See Figure 1.
Table 11. DC Output Noise/Ripple
Output Max. Ripple & Noise (mVpp)
+12 VDC 120
+5 VDC 50
+3.3 VDC 50
-5 VDC 100
-12 VDC 120
+5 VSB 50
Scope
Load
V out
V return
Power Supply
AC Hot
AC Neutral
AC Ground
Scope Note:
Use Tektronix TDS460 Oscilloscope or
equivalent and a P6046 probe or equivalent.
Filter Note:
0.1uf - Kemet, C1206C104K5RAC or equivalent
10uf - United Chemi-con, 293D106X0025D2T or
equivalent
General Notes:
1. Load the output with its minimum load
current.
2. Connect the probes as shown.
3. Repeat the measurement with maximum
load on the output.
Load must be
isolated from the
ground of the
power supply.
0.1uf
10uf
Figure 1. Differential Noise Test Setup

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3.2.7. Output Transient Response
Table 12 summarizes the expected output transient step sizes for each output. The transient
load slew rate is = 1.0 A/µs.
Table 12. DC Output Transient Step Sizes
Output Max. step size (% of rated output amps per Sec 3.2.3) (1) Max. step size
(amps)
+12 VDC 50%
+5 VDC 30%
+3.3 VDC 30%
-5 VDC 0.1 A
-12 VDC 0.1 A
+5 VSB 0.1 A
(1) For example, for a rated +5 VDC output of 18 A, the transient step would be 30% × 18 A = 5.4 A
Output voltages should remain within the regulation limits of Section 3.2.1, and the power
supply should be stable when subjected to load transients per Table 12 from any steady
state load, including any or all of the following conditions:
•Simultaneous load steps on the +12 VDC, +5 VDC, and +3.3 VDC outputs
(all steps occurring in the same direction)
•Load-changing repetition rate of 50 Hz to 10 kHz
•AC input range per Section 3.1
•Capacitive loading per Table 13
3.2.8. Capacitive Load
The power supply should be able to power up and operate normally with the following
capacitances simultaneously present on the DC outputs.
Table 13. Output Capacitive Loads
Output ATX
Capacitive load (µF) ATX12V Capacitive load (µF)
+12 VDC 1,000 20,000
+5 VDC 10,000 10,000
+3.3 VDC 6,000 6,000
-5 VDC 350 350
-12 VDC 350 350
+5 VSB 350 350
3.2.9. Closed-loop Stability
The power supply shall be unconditionally stable under all line/load/transient load
conditions including capacitive loads specified in Section 3.2.8. A minimum of 45 degrees
phase margin and 10 dB gain margin is recommended at both the maximum and minimum
loads.

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3.2.10. +5 VDC / +3.3 VDC Power Sequencing
The +12 VDC and +5 VDC output levels must be equal to or greater than the +3.3 VDC
output at all times during power-up and normal operation. The time between the +12 VDC
or +5 VDC output reaching its minimum in-regulation level and +3.3 VDC reaching its
minimum in-regulation level must be ≤20 ms.
3.2.11. Voltage Hold-up Time
The power supply should maintain output regulation per Section 3.2.1 despite a loss of
input power at the low-end nominal range—115 VAC / 57 Hz or 230 VAC / 47 Hz—at
maximum continuous output load as applicable for a minimum of 17 ms.
3.3. Timing / Housekeeping / Control
PS_ON#
PWR_OK
PWR_OK Sense Level = 95% of nominal
95%
10%
~
~
~
T2 T3
T4 T6
timing_3_5_12b
+12VDC
+5VDC
+3.3VDC O/P's
}
VAC
~
T5T1
Figure 2. Power Supply Timing
Notes: T1 is defined in Section 3.3.4. T2 is defined in Section 3.3.5. T3, T4, T5, and T6 are defined in Table 14
3.3.1. PWR_OK
PWR_OK is a “power good” signal. It should be asserted high by the power supply to
indicate that the +12 VDC, +5VDC, and +3.3VDC outputs are above the undervoltage
thresholds listed in Section 3.2.1 and that sufficient mains energy is stored by the converter
to guarantee continuous power operation within specification for at least the duration
specified in Section 3.2.11, “Voltage Hold-up Time.” Conversely, PWR_OK should be
deasserted to a low state when any of the +12 VDC, +5 VDC, or +3.3 VDC output voltages
falls below its undervoltage threshold, or when mains power has been removed for a time
sufficiently long such that power supply operation cannot be guaranteed beyond the power-
down warning time. The electrical and timing characteristics of the PWR_OK signal are
given in Table 14 and in Figure 2.

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Table 14. PWR_OK Signal Characteristics
Signal Type +5 V TTL compatible
Logic level low < 0.4 V while sinking 4 mA
Logic level high Between 2.4 V and 5 V output while sourcing 200 µA
High-state output impedance 1 kΩfrom output to common
PWR_OK delay 100 ms < T3< 500 ms
PWR_OK risetime T4≤10 ms
AC loss to PWR_OK hold-up time T5≥16 ms
Power-down warning T6≥1 ms
3.3.2. PS_ON#
PS_ON# is an active-low, TTL-compatible signal that allows a motherboard to remotely
control the power supply in conjunction with features such as soft on/off, Wake on LAN†,
or wake-on-modem. When PS_ON# is pulled to TTL low, the power supply should turn on
the five main DC output rails: +12VDC, +5VDC, +3.3VDC, -5VDC, and -12VDC. When
PS_ON# is pulled to TTL high or open-circuited, the DC output rails should not deliver
current and should be held at zero potential with respect to ground. PS_ON# has no effect
on the +5VSB output, which is always enabled whenever the AC power is present. Table
15 lists PS_ON# signal characteristics.
The power supply shall provide an internal pull-up to TTL high. The power supply shall
also provide debounce circuitry on PS_ON# to prevent it from oscillating on/off at startup
when activated by a mechanical switch. The DC output enable circuitry must be SELV-
compliant.
Table 15. PS_ON# Signal Characteristics
Min. Max.
VIL, Input Low Voltage 0.0 V 0.8 V
IIL, Input Low Current (Vin = 0.4 V) -1.6 mA
VIH, Input High Voltage (Iin = -200 µA) 2.0 V
VIH open circuit, Iin = 0 5.25 V

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Enable
Disable
PS_ON# Voltage
0.8 2.0
5.25 = Maximum Open-
Circuit Voltage
≥2.0 V
PS is
disabled
≤0.8 V
PS is
enabled
Hysteresis ≥0.3 V
Figure 3. PS_ON# Signal Characteristics
3.3.3. +5 VSB
+5 VSB is a standby supply output that is active whenever the AC power is present. It
provides a power source for circuits that must remain operational when the five main DC
output rails are in a disabled state. Example uses include soft power control, Wake on
LAN, wake-on-modem, intrusion detection, or suspend state activities.
The +5 VSB output should be capable of delivering a minimum of 1.0 A at +5 V ± 5% to
external circuits. Because trends indicate a growing demand for standby power, it is
recommended that designs be scalable to 2.0 A to meet future needs. The power supply
must be able to provide the required power during a "wake up" event. If an external USB
device generates the event, there may be peak currents as high as 2.5A lasting no more than
500mS.
Overcurrent protection is required on the +5 VSB output regardless of the output current
rating. This ensures the power supply will not be damaged if external circuits draw more
current than the supply can provide.
3.3.4. Power-on Time
The power-on time is defined as the time from when PS_ON# is pulled low to when the
+12 VDC, +5 VDC, and +3.3 VDC outputs are within the regulation ranges specified in
Section 3.2.1. The power-on time shall be less than 500 ms (T1< 500 ms).
+5 VSB shall have a power-on time of two seconds maximum after application of valid AC
voltages.
3.3.5. Risetime
The output voltages shall rise from ≤10% of nominal to within the regulation ranges
specified in Section 3.2.1 within 0.1 ms to 20 ms (0.1 ms ≤T2≤20 ms).
There must be a smooth and continuous ramp of each DC output voltage from 10% to 90%
of its final set-point within the regulation band, while loaded as specified in Section 3.2.3.

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The smooth turn-on requires that, during the 10% to 90% portion of the rise time, the slope
of the turn-on waveform must be positive and have a value of between 0 V/ms and
[Vout,nominal / 0.1] V/ms. Also, for any 5 ms segment of the 10% to 90% risetime
waveform, a straight line drawn between the end points of the waveform segment must
have a slope ≥[Vout,nominal / 20] V/ms.
3.3.6. Overshoot at Turn-on / Turn-off
The output voltage overshoot upon the application or removal of the input voltage, or the
assertion/deassertion of PS_ON#, under the conditions specified in Section 3.1, shall be
less than 10% above the nominal voltage. No voltage of opposite polarity shall be present
on any output during turn-on or turn-off.
3.3.7. Reset after Shutdown
If the power supply latches into a shutdown state because of a fault condition on its outputs,
the power supply shall return to normal operation only after the fault has been removed and
the PS_ON# (or AC input) has been cycled OFF/ON with a minimum OFF time of
1 second.
3.3.8. +5 VSB at AC Power-down
After AC power is removed, the +5 VSB standby voltage output should remain at its steady
state value for the minimum hold-up time specified in Section 3.2.11 until the output
begins to decrease in voltage. The decrease shall be monotonic in nature, dropping to
0.0 V. There shall be no other perturbations of this voltage at or following removal of AC
power.
3.4. Output Protection
3.4.1. Overvoltage Protection
The overvoltage sense circuitry and reference shall reside in packages that are separate and
distinct from the regulator control circuitry and reference. No single point fault shall be
able to cause a sustained overvoltage condition on any or all outputs. The supply shall
provide latch-mode overvoltage protection as defined in Table 16.
Table 16. Overvoltage Protection
Output Min. Nom. Max. Unit
+12 VDC 13.4 15.0 15.6 Volts
+5 VDC 5.74 6.3 7.0 Volts
+3.3 VDC 3.76 4.2 4.3 Volts