ATX12VO - Addressing some misconceptions

Apr 2, 2023

The ATX12VO caused some discussions after its release. Having read multiple articles on the topic, I felt that the benefits and downsides haven't been addressed properly by the tech media.

About me

I worked in an engineering position on server power supplies for multiple years.

In this article, I try to explain why ATX12VO makes sense from an engineering point of view.

I personally can't wait for ATX12VO to become a reality.

What is ATX12VO (12 Volt Only)

As the name suggests, ATX12VO is a new iteration of the ATX power standard that reduces the number of voltages delivered by the PSU to only one - 12 Volts. This reduces the complexity of the power supply and allows for cleaner power delivery concepts than the old multi-voltage standard.

It is nothing new

For servers, it is standard for many years to run only from one 12V or 48V voltage rail.

This brings some key benefits:

Even though those features are not as interesting in the consumer market as they are for a data center, there might still be benefits for everybody in it.

The current state

The differences between a current mainboard and a 12V-only mainboard might be smaller than you think.

Your computer is already almost 12V Only

The main power consumers in your computer, CPU and GPU, are already fed from 12V.

The power converter for your CPU (commonly referred to as VRM) is already on the mainboard next to the CPU. It has a separate power connector and converts 12V from your PSU to whatever voltage your CPU needs.

The power converter for your GPU is already in your graphics card. It has a separate power connector and converts 12V from your PSU to whatever voltage your GPU needs.

The myth of increased mainboard complexity

The voltage converters for your CPU and GPU are very complex and costly. The added complexity for the low-power 12V to 5V or 3.3V converters is neglectable by comparison.

The myth of increased cost

Most people are probably not aware of how cheap it is to convert 12V to 5V or 3.3V for low-power applications. In large quantities, the cost of the parts for such a converter is somewhere in the range of 1$.

The reason why the VRM for the CPU is expensive is that it needs to convert about 100W of power. The 5V and 3.3V rail in your computer does not need that much. Therefore it's cheap.

The problem with multiple output voltages on power supplies

If a PSU has multiple output voltage rails, the PSU design engineer does not know how much power is required on each rail. This leads to an over-specification of the 5V and 3.3 voltage rails.

Think about it like this:

If you buy an 850W PSU, how much of its power is available on the 12V rail? How much on 5V? How much on 3.3V?

Do you know how much power your computer needs on the 5V rail?

PC power supplies have already changed over the years - you just haven't noticed

Maybe you still have an old PC PSU from 15 or 20 years ago. If you would compare the power allocated to the different output rails you may notice that modern computer PSUs can output proportionally less power on the 5V and 3.3V rail but more on the 12V rail.

Over time, many components already moved from using the 3.3V and 5V directly and instead generate their own voltage rails locally similar to what the CPU and GPU are doing.

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Reliability

A well-designed (not to be confused with expensive) buck-converter will live longer than the time you want to use the same computer. (Sidenote: I used my last computer for >10 years)

Operational Reliability

Additional parts mean additional points of failure. But can additional parts also increase reliability? Sometimes, Yes.

For a reliable design, voltage rails should be generated as close as possible to the chip that is using it. This way, they are less susceptible to the environment around them and can not pick up noise or get disturbed. It is common practice and probably already the case for most bigger components on your PC mainboard.

For example, a 3.3V rail will be generated locally maybe from the 5V rail but might as well from the 12V rail. This way, in case of some instability in the 5V rail where it dips to 4V for a short time, there is still some margin left. Your computer continues running instead of crashing.

I_PSU - A truly game-changing feature

Have you ever wondered if your PSU is sized accordingly to your system's needs?

The I_PSU signal will inform the mainboard how much percent of the PSU is utilized. This means no guessing anymore if your PSU is over- or undersized. Now you will have an easy way of knowing.

Paralalling PSUs with active load sharing might be possible

Another possible use case of the I_PSU signal is that it could in combination with the 12V Voltage Sensing Pin allow for active load sharing of multiple PSUs connected in parallel.

This could be extremely useful for systems that need more power than one PSU can traditionally deliver or for systems with redundant PSUs.

The downsides

Having to convert the 5V for the USB A and SATA ports is probably the biggest change between an old mainboard and a new 12VO mainboard.

The power requirements, especially for SATA are extremely system dependent and can range from "Who uses SATA anymore" to "I need to plug in my 10 hard drives"

Maybe it would be smarter to not try to do this on the mainboard and just make some sort of external converter box.

Conclusion

Overall, the ATX12VO makes a lot of sense in today's world. It removes legacy burdens from PC PSUs. The improvements in power conversion technology of the last 10 years make this standard easily adoptable.

25 years ago it would have been stupid to do it but today it's stupid not to do it.