[SOLVED] Noobquestion: Why a 12V rail and why do VRMs exist?

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kythQ

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I am not sure if this forum is the right place to ask this question but i saw some similar posts so here goes:

In my long-ongoing quest to understand how a computer works, I recently watched a lot of videos on pcb design and VRMs. As far as I understand it, the exclusive task of VRMs is to "convert" the 12V from the power supply to a lower voltage (usually 1.2V), that the CPU/GPU can handle. Modern PSUs have 3 power rails: one 12V, one 5V and one 3.3V, and the CPU/GPU is usually the only thing that uses the 12V rail.

So then I ask, why is the rail 12V anyways? Why is the standard not changed so that the power comes in 1.2V from the power supply? The obvious answer would be that it is not possible to carry the required amount of power on such a low-voltage rail, but then again, the current is somehow transfered from the VRMs to their respective PU, right? Why can't the same method of transfer not be used to transfer the power directly from the PSU to the PU with 1.2V.

In other words: Why do we need seperate VRMs on a Motherboard and a GPU pcb, why can't the PSU do the job of the VRMs and include a 1.2V rail?

Thank you for any answers.
 
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The first part is that conductors still have resistance to them. If the power supply supplied 1.2V, by the time it reached the CPU, it's not 1.2V anymore. It's likely not that bad of a voltage drop, but compared to 12V, it's a more significant voltage drop.

The second part, and probably biggest reason, is current. Wires of a certain thickness can only carry so much current before it heats up to the point of melting the insulator. Rather than spend a lot of material on the cable itself, just have it carry higher voltages/lower current and have the "last mile" of the circuit carry the brunt of the current.

EDIT: To explain that last part about current, power is voltage times current. For say 120W, it's less material to transfer...
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hotaru.hino

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The first part is that conductors still have resistance to them. If the power supply supplied 1.2V, by the time it reached the CPU, it's not 1.2V anymore. It's likely not that bad of a voltage drop, but compared to 12V, it's a more significant voltage drop.

The second part, and probably biggest reason, is current. Wires of a certain thickness can only carry so much current before it heats up to the point of melting the insulator. Rather than spend a lot of material on the cable itself, just have it carry higher voltages/lower current and have the "last mile" of the circuit carry the brunt of the current.

EDIT: To explain that last part about current, power is voltage times current. For say 120W, it's less material to transfer 12V at 10A most of the way, then as close to the part as you're willing, convert this down to 1.2V and beef up this section to handle the 100A of total current feeding the device. If you used 1.2V the entire way, you'd have to have the cable support 100A.

(edited for math, yay needing coffee)
 
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refillable

refillable

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It's the amount of current that is stopping the PSU to not have its dedicated VRM. Sending 100W on a 12V voltage only needs 8.3 amps, while 1.2V needs 83A! The reason why this is fine on VRMs is because the distance from the VRM to the chips are really short, and the VRM/power bus are specifically designed for such currents. They are expensive if you want something like this straight from the PSU.
 
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jonnyguru

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kythQ said:
I am not sure if this forum is the right place to ask this question but i saw some similar posts so here goes:

In my long-ongoing quest to understand how a computer works, I recently watched a lot of videos on pcb design and VRMs. As far as I understand it, the exclusive task of VRMs is to "convert" the 12V from the power supply to a lower voltage (usually 1.2V), that the CPU/GPU can handle. Modern PSUs have 3 power rails: one 12V, one 5V and one 3.3V, and the CPU/GPU is usually the only thing that uses the 12V rail.

So then I ask, why is the rail 12V anyways? Why is the standard not changed so that the power comes in 1.2V from the power supply? The obvious answer would be that it is not possible to carry the required amount of power on such a low-voltage rail, but then again, the current is somehow transfered from the VRMs to their respective PU, right? Why can't the same method of transfer not be used to transfer the power directly from the PSU to the PU with 1.2V.

In other words: Why do we need seperate VRMs on a Motherboard and a GPU pcb, why can't the PSU do the job of the VRMs and include a 1.2V rail?

Thank you for any answers.
Click to expand...

Because Watts / Voltage = Current.

I'm disappointed that they're not using 24V instead of 12V.

Your question should actually be: Why are they still using +3.3V and +5V. ;)
 
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kanewolf

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jonnyguru said:
I'm disappointed that they're not using 24V instead of 12V.
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DC standard for network equipment is 48VDC. I read that the big hosting firms, like google and AWS have custom motherboards that are DC input. This allows them to have RACK level power supplies for dozens of motherboards. Much more efficient to have one BIG power supply than many small ones.
 
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hotaru.hino

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refillable said:
As long as it is not backwards compatible people probably won't even bother...
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The 12VO spec could be pushed with a new line of motherboards and sockets. AMD and Intel are due for new sockets that aren't compatible with older hardware anyway, and you can bring over existing ATX power supplies with ease with a converter. And for quality power supplies that come from reputable brands, the 12V rail provides practically all of the power the unit is rated for. The 3.3V and 5V rails seem like extras.

EDIT: Looking it over, it seems like the only devices that use the 5V rail are SATA drives. HDDs need it and SSDs seem to exclusively use it So there would be a problem there. But again, I don't see how this can't be solved with a converter of sorts (inline with the cable? A "SATA only" port?) until a more elegant solution is found.

You could say "but what about USB?" I haven't looked at USB controllers but it's not like the motherboard can't provide 5V where needed.
 
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refillable

refillable

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hotaru.hino said:
The 12VO spec could be pushed with a new line of motherboards and sockets. AMD and Intel are due for new sockets that aren't compatible with older hardware anyway, and you can bring over existing ATX power supplies with ease with a converter. And for quality power supplies that come from reputable brands, the 12V rail provides practically all of the power the unit is rated for. The 3.3V and 5V rails seem like extras.

EDIT: Looking it over, it seems like the only devices that use the 5V rail are SATA drives. HDDs need it and SSDs seem to exclusively use it So there would be a problem there. But again, I don't see how this can't be solved with a converter of sorts (inline with the cable? A "SATA only" port?) until a more elegant solution is found.

You could say "but what about USB?" I haven't looked at USB controllers but it's not like the motherboard can't provide 5V where needed.
Click to expand...
So you have to have to buy DC-DC converters that you will put inside the case? I think that's a bit too bothersome for most people.
 
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jonnyguru

jonnyguru

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refillable said:
So you're trying to say 5V and 3.3V on the PSU is useless beside peripherals and SATA?
Click to expand...

No. I'm saying they moved the DC to DC from the PSU to the motherboard.

If you have an ATX12VO motherboard and PSU, and that motherboard has at least two SATA power ports, each supporting three SATA connectors, what do you need an additional +5V for anymore? Especially if you're using M.2.

I've already used a couple of the ASROCK boards (the same one that is in Linus's video) and I have to say I really like the idea.
 
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refillable

refillable

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jonnyguru said:
No. I'm saying they moved the DC to DC from the PSU to the motherboard.

If you have an ATX12VO motherboard and PSU, and that motherboard has at least two SATA power ports, each supporting three SATA connectors, what do you need an additional +5V for anymore? Especially if you're using M.2.

I've already used a couple of the ASROCK boards (the same one that is in Linus's video) and I have to say I really like the idea.
Click to expand...
So the goal is to remove SATA completely (or at least reduce the number), move the DC to DC converter into the motherboard, and you can start having 12VO PSUs on new systems?
 
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jonnyguru

jonnyguru

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refillable said:
Are HP prebuilds with 12VO PSUs like this?
Click to expand...

Yes. Most are. If they only have +12V on the PSU, you'll see that there are power connectors for the SATA devices on the motherboard itself. These are installed after a +12V to +5V buck converter located on the motherboard.

Note this example: https://www.amazon.com/Generic-Brand-Power-611895-001-ShipQuantity/dp/B01N7RC2N6

The six pin plugs into the HP motherboard where the +12V to +5V is. Note there is also no +3.3V. The extra unused two-pins are for an additional +5V and ground in case you wanted a cable that supported five drives instead of just three.

This is the board I have: https://www.anandtech.com/show/1576...otherboard-the-asrock-z490-phantom-gaming-4sr

There's info here as well: https://www.pcworld.com/article/357...th-atx12vo-motherboards-and-power-supply.html
 
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