Our First Foray Into ATX 3.0 PSU Testing: A High Hurdle to Clear

In light of the new ATX 3.0 standard, we took a shot at adding power excursion compliance testing into our articles. Given that this is the big addition to the ATX 3.0 specification– and indeed its very reason for being – it's where we would like to see if PSUs are truly living up to the very high standards set by the new specification.

Unfortunately, the testing requirements of the new standard have proven too high for our equipment - and that of the majority of small laboratories - to meet. The power excursion specifications suggest electrical current-to-time figures that are extremely short. For example, taking the MEG Ai1300P of this review into consideration, for the single case of the 200% power excursion testing, we would need to test that it can handle 2600 Watts for 0.1 ms. Assuming a starting load of about 800 Watts and 65 A on the 12 V rail, the 12 V load would have to get up to 215 A and back down to 65 A within 0.1 ms. Furthermore, according to Intel's testing guide, this would have to continue for at least a minute, which means at least five hundred cycles in this scenario.

In an ideal world, we would just enter the current and time figures into the software and our electronic loads would run the test, instantly getting the load up to 215 A for 0.1 ms and then immediately back down to 65 A for 1.9 ms, according to the guide's requirements.

In the real world, however, there is no such thing as "instantly". Electronic loads, like any other device that is bound by the laws of physics, require time to react. The speed at which an electronic load can increase its amperage is called Ramp (or Slew) Rate and our larger loads have an ideal Slew Rate of 0.5 A/μS. Assuming that they operate linearly and exactly as specified, which no electronic load does for a variety of reasons, our two primary electronic loads in parallel would require at least 0.15 ms (150 μS) just to get the load up at 215 A. They would also require time, albeit less than half of it, to get the load back down to 65 A. When the test dictates a test time of 0.1 ms and the testing equipment requires at least twice that much time just to react, it goes without saying that testing results are highly unreliable.

Nevertheless, we took a shot at testing the power excursion capabilities of the MSI MEG Ai1300P PCIE and of the few ATX 3.0 compliant units that we currently have available. We took two approaches: one by assuming that our electronic loads are "ideal" and programmed the exact duty cycle figures that Intel dictates in their guide, and one by trying to take into account the real slew rate times of our loads and calculate the RMS equivalent duty cycle.

Both of our approaches ultimately failed, as all of the PSUs we currently have available would shut down at most tests above 120% power excursion - therefore we need not worry about our loads being insufficient to test the MEG Ai1300P at 200% excursion (we are also currently limited to 2400 Watts on the 12V line). Theoretically, testing with the RMS-equivalent duty cycle times should work and the PSUs should not be shutting down, yet we cannot claim that the units are not technically capable of meeting their specifications when our equipment is not meant to be running such tests.

Intel requires the PSUs to have a slew rate of at least 5 A/μS, so an electronic load must be at least as fast as that figure to be able to perform ATX 3.0 compliance testing. From a professional's point of view, proper testing would require the testing equipment to be at least 30% faster than the absolute minimum required. This requires a highly advanced (and expensive) electronic load with multiple modules, like the Chroma Mainframe and High-Speed modules Intel themselves is using, which has a total slew rate of 8 A/μS and it would need only 0.02 ms to get the load from 65 A all the way up to 215 A - and that still is 20% of the test's required 0.1 ms time in our example, a figure that many experts would find far too great for precise measurements.

The MSI MEG Ai1300P PCIE5 1300W : Inside & Out Cold Test Results (~22°C Ambient)
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  • Dug - Thursday, December 8, 2022 - link

    I wish they would have changed the form factor. So tired of these unnecessarily large bulky designs.
  • TheinsanegamerN - Thursday, December 8, 2022 - link

    Tell me about it, why is the industry so allergic to SFX?
  • at_clucks - Thursday, December 8, 2022 - link

    Getting high power and high efficiency and ideally low noise in a small package like SFX is both hard and expensive, and depending on the combination even impossible. For now SFX isn't really great as you get closer to the the power levels in this review. And the price certainly isn't.
  • meacupla - Thursday, December 8, 2022 - link

    Asus offers 1200W in SFX-L form factor.
    At 1200W, I think it would handle transient spikes from a 3090Ti no problem.

    Now how much does this Asus 1200W SFX-L PSU cost? No clue.
  • at_clucks - Tuesday, December 20, 2022 - link

    SFX-L is better than standard SFX in the sense that it can fit a 120mm fan, which should make a difference. Regardless, the main limitation will always be there, making a smaller or larger difference: The SFX package is smaller. Everything else being identical (components, quality, etc.) there will always be the same heat concentrated in a smaller package (so hotter, less reliable, or noisier). To mitigate this SFX PSUs always have to be better. Better costs which makes the industry position SFX as niche and premium. People are allergic to spending too much so this is why SFX takes time to replace ATX.

    Add on top that companies like Nvidia are pushing power consumption for performance rather than efficiency and I'm not surprised ATX still rules.
  • Oxford Guy - Thursday, December 8, 2022 - link

    Because 50 dB isn’t “audible” enough.
  • Eliadbu - Friday, December 9, 2022 - link

    I don't know about SFX but EVGA released some exceptionally good PSUs like th G7 at small spectrum of ATX PSU. Personally I have the P6 and working with it inside a case like O11 is a treat.
  • QueBert - Thursday, December 8, 2022 - link

    Poorly designed for future proofing, it's ATX 3.0 ready, that's great. But it only has a single 12VHPWR, next gen Nvidia's will probably need 2 of those suckers I bet. And then this PSU will already be pointless.
  • Samus - Friday, December 9, 2022 - link

    I don't think we will have consumer graphics cards use two 12VHPRW connectors anytime soon because that would imply over 600w draw just for the graphics card. Since these kinds of cards would likely be used in monstrously powerful systems, paired with a 200-250w TDP CPU, add platform overhead and an SSD, that's 1000w at load. With the way climate change is a buzz word and rising energy costs, it's about time these companies focus on efficiency.

    The problem is, much like the automotive industry and so many others, the efficiency gains made by technological, engineering and manufacturing innovations is wasted by constantly delivering more power to the consumer. This is why we have 300+HP 3-cylinder engines in grocery getter Corollas that get 25MPG instead of 150HP 3-cylinder engines that get 50MPG.
  • TheinsanegamerN - Saturday, December 10, 2022 - link

    The hybrid gets 44 MMPG, nothing is stopping you from buying it.

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