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# ATX 2.52 (June 2018 rev 002) - Simplified

Introduction

Today I'm going to break down what are in my eyes the most important parts of the Intel ATX Power Supply Specification, but to a more understandable level. I’ll link the full specification below. This document will be based on ATX Design Guide June 2018, revision 002 and will include some outside info.

2.1 Processor Configurations - Recommended

This paragraph talks about the second 12v rail, meant only for the CPU. Intel recommends how much current it should allow before shutting down for some of their TDP's. I noted an example of a CPU with the TDP to give an idea, and the amount of watts that would be on this rail.

 PSU 12V2 Capacity Recommendations Processor TDP Continous Current Peak Current 165w (9980XE) 37.5A (450w) 45.0A (540w) 140w (7900x) 28.0A (336w) 39.0A (468w) 95w (9900k) 22.0A (264w) 29.0A (348w) 65w (9400) 21.0A (252w) 28.0A (336) 35w (9100) 13.0A (156w) 16.5A (198w)

you could manually calculate this the following two ways:

12v2 Continuous Current = (SoC sustained Power / VRM efficiency) / 11.4v

12v2 Continuous Current = (SoC Peak Power / VRM efficiency) / 11.4v

3.1: AC Input - Required

This part goes into detail regarding what voltages the PSU should be able to handle as input or AC current. If it's rated for 115v, it doesn't mean it can't handle a spike to 130v for example. The two most important ones are listed below in a tableThis part goes into detail regarding what voltages the PSU should be able to handle as input or AC current. If it's rated for 115v, it doesn't mean it can't handle a spike to 130v for example. The two most important ones are listed below in a table.

 Voltage Minimum Nominal Maximum 115v AC 90v 115v 135v 230v AC 180v 230v 265v

3.1.1: Input Over Current Protection - Required

Many will think about Over Current Protection with this, but it's not quite the same, since that's on the other side of the PSU. This follows a similar idea, but on the AC side. It uses fuses to protect the PSU from too high current on the AC side in case of a PSU hardware failure.

3.1.2 Inrush Current - Required

Inrush current is the high input current that a PSU or other electrical device pulls for an instant when turning on. Usually, this is caused by charging capacitors It is required to limit it, since you could trip a breaker or even damage the PSU itself without it.

3.1.3: Input Under Voltage - Required

Many will again think of the DC side protection, but this is when voltage would drop below the minimum shown in the table above. So if it drops below 90v for 115v or 180v for 230v it will shut itself down to limit damage done to the PSU.

3.2.1: DC Voltage Regulation - Required

Now we're going to the other side, the output or DC side. These have to stay inside a margin of 5-10% depending on the rail. You can see this below in a table.

 Output Range Minimum Nominal Maximum +12V 5% +11.40V +12.00V +12.60V +5V 5% +4.75V +5.00V +5.25V +3.3V 5% +3.14V +3.30V +3.47V +5VSB 5% +4.75V +5.00V +5.25V -12V 10% -10.80V -12.00V -13.20V

3.2.5: Output Ripple Noise - Required

Ripple is the AC noise that's still left after conversion to DC voltage. This you should try to keep as low as possible, but Intel set some limits to this in their specification. In the first table you can see Intel's limits, in the second what I personally consider for a PSU within normal operation range, but it's a lot harder to meet.

Intel:

 Output Maximum Ripple +12V 120mV +5V 50mV +3.3V 50mV -12V 120mV +5VSB 50mV

Personal:

 Output Maximum Ripple +12V 50mV +5V 30mV +3.3V 30mV -12V 50mV +5VSB 30mV

3.2.8: +5v DC / +3.3v DC Power Sequencing - Required

DC Power Sequencing is the time it has to take between certain rails to start up. This is best explained by looking at the image Intel provides for this.

As you can see here, the 3.3v line should always be lower than the others, because of the way a system would boot up. If this fails by a bigger margin, the system won't power on. The impact of a smaller fail is unknown to me.

3.2.9: Voltage Hold-Up Time - Required

Voltage hold-up time is simply said that a PSU should be able to at least supply it's maximum rated continuous load for 17ms if the AC input suddenly shuts off. Intel requires this to be at a minimum load of 0A. This does not mean that this will prevent it from cutting power on a longer run. A longer hold-up time (let's say for example 23ms) won't improve this for users except if they own a UPS, as this is the time a good UPS would be able to continue operation.

3.3.1: PWR_OK - Required

PWR_OK or Power Good is a signal the PSU sends that the 12V, 5V and 3.3V rails are within limits and that there's enough energy left in the converter to supply it with the specified load. If this signal indicates differently, the PSU or motherboard will shut itself down.

3.3.4: +5VSB - Required

5 Volt StandBy (5VSB) is a rail that supplies power to components when the system isn't powered up. This would for example be to keep motherboard LEDs on, allow for Wake on LAN to work or power USB devices.

3.5.1: Over Voltage Protection (OVP) - Required

Over Voltage Protection or OVP is a protection against a too high voltage on a rail. This is technically required for everything but the 5VSB, but is highly recommended to be present there as well. It's generally integrated into the protection IC. In the table below you can see the voltages Intel recommends to set it to.

 Output Minimum Nominal Maximum +12V 13.40V 15.00V 15.60V +5V 5.74V 6.30V 7.00V +3.3V 3.76V 4.20V 4.30V +5VSB 5.74V 6.30V 7.00V

3.5.2: Short Circuit Protection (SCP) - Required

Short Circuit Protection or SCP measures the resistance on each rail, and will shut down when resistance is lower than 0.1 Ohms. Generally this goes combined with OPP, OCP, OVP and UVP.

It's generally integrated into the protection IC, and is required on ATX spec, with separate circuits per rail.

3.5.4: Over Current Protection (OCP) - Required

The term Over Current Protection or OCP has two types of protection included into the name, being OCP and OPP.

Over Power Protection or OPP is a protection that will shut down the PSU when too much power on all rails combined is drawn, generally this is between 110 and 140% of the advertised wattage. This is a protection that works as a limit, shutting down when a certain point is reached, but doesn't actively monitor the amount of current.

It's generally integrated into the PWM controller.

Over Current Protection or OCP has the same purpose, but a different concept than OPP. OCP will generally be faster than OPP, since it uses shunt resistors to check the amount of current on each individual rail, and will shut down if a certain point is reached. OCP on 12V is generally only found on PSUs with multiple rails, since OPP can handle a single rail just fine.

It's generally integrated into the protection IC combined with shunt resistors.

To explain the difference very simply is that OPP is a limit for the whole rail and OCP is a more continuous check of every single rail.

3.5.5 Over Temperature Protection (OTP) - Required

Over Temperature Protection or OTP protects the PSU against overheating,for example due to a fan failure. it's generally a thermistor combined with a protection IC that supports this, but there have also been cases where it was integrated into the fan controller. Most reviewers stop measuring after 200°C, but it depends on the place the thermistor is integrated what recommended limits are.

3.5.7: Separate Current Limit for 12V2 - Recommended

This is basically a different wording for multirail. Multi rail these days aren’t physical rails, rather they have multiple points where they measure the current (generally 2-8). It can shut down the PSU earlier to protect itself, with a lower chance of burning through connectors and/or cables with a catastrophic failure.

3.5.9: Power Supply Efficiency for Energy Regulations, Energy Star and CEC PC Computers with High Expandability Score - Recommended

This part of the documentation includes 3 examples of efficiency requirements, being the ones from Energy Star, CEC and Efficiency for Energy Regulations. I'll include the ones from Cybenetics and 80+ as a comparison.

Efficiency for Energy

Energy Star (version 6.1/7.0)

CEC

Cybenetics

 Efficiency levels (115V) Efficiency 5VSB Efficiency A++ =>94% - <97% >79% A+ =>91% - <94% >77% A =>88% - <91% >75% A- =>85% - <88% >73% Standard =>82% - <85% >71%

 Efficiency levels (230v) Efficiency 5VSB Efficiency A++ =>96% >78% A+ =>93% - <96% >76% A =>90% - <93% >74% A- =>87% - <90% >72% Standard =>84% - <87% >70%

80 plus

 Rating 10% (very low load) 20% (low load) 50% (typical load) 100% (Full load) 80+ 115v 80% 80% 80% 80+ 230v 82% 85% 82% 80+ Bronze 115v 82% 85% 82% 80+ Bronze 230v 85% 88% 85% 80+ Silver 115v 85% 88% 85% 80+ Silver 230v 87% 90% 87% 80+ Gold 115v 85% 89% 85% 80+ Gold 230v 90% 92% 89% 80+ Platinum 115v 90% 94% 89% 80+ Platinum 230v 92% 94% 90% 80+ Titanium 115v 90% 92% 94% 90% 80+ Titanium 230v 90% 94% 96% 94%

4.2.1: AC Connector - Required

In this part Intel mentions a IEC 320 or equivalent plug, which might sound complicated, but this is just the name of the well known 3-pin plug you find on most PSUs

With that it needs to have a dedicated on/off switch next to it.

4.2.2.1: Main Power Connector - Required

orange=3.3v

blue=-12v

black=ground/communication

green=power on

red=5v

gray=power good

purple=5vsb

yellow=12v

the main power connector, generally called 24 pin connector because of it's 20+4 pins supplies its power via the motherboard to many components, including part of the GPU, memory, the motherboard itself and so on.

4.2.2.2: Peripheral Connectors - Required

yellow=12v

black=ground/communicaton

red=5v

Peripheral connectors, or better known as the Molex standard is a connector that's getting more and more rare in favor of other connectors these days. It's used on old GPUs, expansion cards and so on. Fun thing is that Molex was the base for many connectors, including PCI-E, EPS and ATX Main Power. They’re part of the so-called Micro Fit series.

4.2.2.4: PCI-Express (PCI-E) Graphics Card Connector - Required

yellow=12v

black=communication/ground

The PCI-E Graphics Card Connector, or generally called PCI-E connector supplies power to the GPU. A 6 pin is rated for up to 75W, while the 8 pin goes up to 150W.

4.2.2.5: +12V Power Connector - Required

yellow=12v

black=communication/ground

The +12V power connector or 8 (4+4) pin connector is a cable that provides current to the VRM, which then supplies it to the CPU. It follows the EPS standard.

4.2.2.6: Serial ATA (SATA) Connectors - Required

orange=3.3v

black=communication/ground

red=5v

yellow=12v

Serial ATA or SATA power is mostly used for drives using the SATA standard, but can be used for other things as well.

6.0: Environmental - Recommended

A PSU has to be able to:

• Operate at +10 to +50 degrees Celsius at full load.
• Survive -40 to +70 degrees Celsius while not operating
• Operate at a humidity up to 85%
• Survive a humidity up to 95% while not operating
• Operate at up to 3.048 meters high
• Survive non-operational at up to 15.240 meters high
• Survive a mechanical shock of 50 grams while not operating

8.4: Safety - Required

Should a component failure occur, the power supply should not exhibit any of the following:

• Flame
• Excessive smoke
• Burnt PCB
• Fused PCB conductor
• Unusual noise
• Emission of molten material
• Fail to ground

9.1: Reliablity - Recommended

This is relatively simple. Make a unit that's expected to work at least for it's rated lifetime (generally by warranty) by selecting the right components. This would mainly include capacitor and fan lifetime and reliability.

10.0-15.0: CFX12V/LFX12V/ATX12/SFX12V/TFX12V/FLEX ATX Specific Guidelines - Required

The last paragraph goes into the many shapes you find power supplies in. here a quick breakdown

CFX12V: CFX is a formfactor only really used in SFF sized prebuilts. It's simply said an ATX sized PSU with a cut in it.

ATX12V: Standardized formfactor PSU, which is the most common to find.

SFX12V: A formfactor for small sized PSUs for use with SFF builds.

SFX-L12V: A slightly bigger version of SFX, which allows a 120mm fan to fit up top

TFX12V: A formfactor closer to server PSUs in its shape. These are rare to find, mostly in office/HTPCs, but even there SFX is getting more common.

FLEX ATX: A formfactor mostly used in small cases, but just like TFX hard to find as SFX becomes more common

From Left to right: SFX -> SFX-L -> ATX

Sources:

Credit:

Moritz Plattner - Tech-review.de

Spoiler

These works by Luke Savenije et al. are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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1 hour ago, LukeSavenije said:

3.2.9: Voltage Hold-Up Time - Required

Voltage hold-up time is simply said that a PSU should be able to at least supply it's maximum rated continuous load for 17ms if the AC input suddenly shuts off. Intel requires this to be at a minimum load of 0A. This does not mean that this will prevent it from cutting power on a longer run. A longer hold-up time (let's say for example 23ms) won't improve this for users except if they own a USP, as this is the time a good UPS would be able to continue operation.

I didn't check the document, but I think it's actually 1 AC Cycle, not 17ms ... that would translate to 1000 ms / 60 = 16.66 ms in 60Hz countries, or 1000ms / 50 Hz = 20ms in 50Hz countries.

Also, maybe make correction and change USP to UPS in the text.

Other than that there was one more sentence I had an issue with, but wasn't a big enough deal to remember it as I skimmed through the post.

Ah .. in the colors for connector, maybe point out that the connector may or may not have a white wire, which was used for -5v and which was deprecated (obsolete and made optional) years ago. So it's perfectly normal for the connector to have one missing wire.

Another observation... while the standard doesn't require it (again, if my memory is correct), some manufacturers will connect thinner wires to some of the wires going to the 24 pin connector ... these are called sense wires, because they're used like the probes of a digital multimeter, to measure the voltage right by the connector. This way, the power supply can account for losses in the wires between the power supply and the connector, and produce voltages closer to the ideal ones.

So, it's not uncommon to see 25-26 wires going to the 24 pin connector (23 wires because the 24th -5v no longer exists, plus sense wires for 3.3v , 5v and 12v)

Good job on the post, thanks for writing down in such a well formatted layout.

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4 minutes ago, mariushm said:

I didn't check the document, but I think it's actually 1 AC Cycle, not 17ms ... that would translate to 1000 ms / 60 = 16.66 ms in 60Hz countries, or 1000ms / 50 Hz = 20ms in 50Hz countries.

to quote the specification itself

Quote

The power supply should maintain output regulations per Table 3-2 despite a loss of input power at the low-end nominal range-115 VAC / 47 Hz or 230 VAC / 47 Hz – at maximum continuous output load as applicable for a minimum of 17ms (T5+T6).

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• 3 months later...
On 3/7/2020 at 3:38 PM, LukeSavenije said:

4.2.2.6: Serial ATA (SATA) Connectors - Required

orange=12v

black=communication/ground

red=5v

yellow=12v

Serial ATA or SATA power is mostly used for drives using the SATA standard, but can be used for other things as well.

Orange should be 3.3V not 12V, just a nitpick

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• 7 months later...

This is missing the two things that actually make the 2.52/June 2018 rev 002 different from previous versions.

Section 3.5.8:  Required minimum efficiency at 10W/2% > 60%.

Section 3.3:  Recommended power supply timing to support alternative sleep mode needs to be < 150ms for T1 and 100-150ms for T3.