motherboard How does a GPU pull power out of a motherboard?

[Carlos1010]

Hello guys I know with the RX 480 and it's power issue a lot of people have been talking about the maximum draw from a motherboard, but how does this work? As far as I know the GPU only takes power from the PSU and that;s it? What happens after that, why does it have to take additional power from the motherboard. Thanks in advanced!

[Electronics Wizardy]

The power can come from the psu from either the pcie power cable or the pcie slot.

[Carlos1010]

3 minutes ago, Electronics Wizardy said:

The power can come from the psu from either the pcie power cable or the pcie slot.

It can also come from the cables from the PSU right?

[SeanAngelo]

1 minute ago, Carlos1010 said:

It can also come from the cables from the PSU right?

the psu powers the card

the pci-e lane is for communicating with the processor

[Electronics Wizardy]

1 minute ago, Carlos1010 said:

It can also come from the cables from the PSU right?

Yes, all the power is comming from the psu.

[i_build_nanosuits]

From the 24 pin power connector on the motherboard it's distributed through the motherboard PCB to all the components.

[Carlos1010]

And why is it that it pulls less power in the pcie lane?

[SageOfSpice]

I don't really understand it myself. I assumed there would've been some sort of fail-safe preventing the motherboard from burning itself to death. Apparently not.

 

If I had to hazard a guess, it'd be that motherboards are more analog than they seem.

All of the power comes from the PSU, and it doesn't seem like there's all that much to really regulate the power on the board, so the motherboard, CPU, and GPU communicate with the PSU and essentially tell it to send more voltage down the line. The traces responsible for carrying the power to the PCI-E rails is probably like, "Yo bro, I'm not equipped to handle this" but it can't do anything about it, and it isn't fused or anything, so the excessive current super-heats the trace due to it's thermal losses due to resistance and burns itself out.

 

If I had to guess why less power is transferred through the PCI-E, that would probably be due to EMI. The PCI allows the GPU to communicate with the northbridge. While it's important to power the card, you still want to be able to get your information to the component relatively intact. Furthermore, traces are really only equipped to transfer so much current due to their relatively small size, so it makes more sense to just power the card directly from the PSU, which is capable of transferring significantly larger amounts of power due to it's larger conductors.

 

The same thing goes for the PSU connectors though, they too can only supply so much power, but you can't just keep adding on more and more power connectors or people aren't going to be able to use your cards.

 

I'm not really sure on the process of approving the use of a standardized connector, and how that would be orchestrated though.

[Oshino Shinobu]

6 minutes ago, SeanAngelo said:

the psu powers the card

the pci-e lane is for communicating with the processor

 

6 minutes ago, Electronics Wizardy said:

Yes, all the power is comming from the psu.

The PCIe slot can also provide power to the GPU. Not a huge amount, but it does provide power (some cards use it more than others) 

 

Though the power is still technically coming from the PSU.

[Carlos1010]

Why does the gpu have to pull power from the pcie slot when it can from the psu?

[MightyMic]

The PCI-E slot spec is 75 watts of power. You typically have 6 pin connectors or 6+2 pin connectors which provide most of the power. The problem with the RX 480 is that it only has a 6 pin power connector which is also only supposed to provide 75 watts of power. 75 from PCI-E and 75 from the 6 pin gives you 150 watts of power. The RX 480 would pull more than 75 watts of power from the PCI-E slot. On new motherboards this usually isn't a problem. I've read that they usually (not in spec) have an extra 4 pins that can provide more power than the standard 75 watts. But since this is outside of PCI-E specs, not all motherboards have these extra 4 pins (especially older motherboards). Radeon's new driver is supposed to limit the draw through the PCI-E to 75 watts and, if the card does need more power, will draw it from the 6 pin connection directly from the psu.

 

That's my understanding. If someone spotted an error, please point it out, because I'm trying to learn this too.

[agent_x007]

Quote

4. A x16 graphics card is limited to 75 W. The 75 W maximum can be drawn via the
combination of +12V and +3.3V rails, but each rail draw is limited as defined in
Table 4-1, and the sum of the draw on the two rails cannot exceed 75 W. 

^Direct quote from PCI-e electrical specification.
Source : LINK (.pdf file, power specs are on pages 28-30, and yes they are the same for PCI-e 3.0).

GPU pulls power from 12V and 3,3V rails embeded in PCI-e slot, exact pinout : LINK (wikipedia)
Number of "ground" pins tells how much power a PCI-e device can use.

[Glenwing]

1 hour ago, Carlos1010 said:

As far as I know the GPU only takes power from the PSU and that;s it?

 

59 minutes ago, Electronics Wizardy said:

Yes, all the power is comming from the psu.

How do you think lower end graphics cards with no supplemental power connectors work?

 

That big 24-pin cable from the PSU to the motherboard carries power (what else would it be for, right?) and some of that power can be distributed through the PCIe slots. The front section of the PCIe slot, that small part separated off with the notch, is for power delivery. PCIe is for other things besides graphics cards, like network cards, audio cards, storage cards, and so forth. It would be a bit ridiculous if all of these things needed a power cable from the PSU, so the PCIe slot can provide a certain amount of power. But high-power devices like graphics cards need supplemental power cables.

 

As for why power can't be taken only from the PSU on graphics cards with additonal power connectors, I don't know enough to answer, but I don't see what the point would be. If the motherboard can provide some power, you might as well use it.

[Carlos1010]

So the motherboard has a 2 pin connector than can give the graphics card an additional 150W and the PSU gives 150W with its 6 pin connector. So its like a puzzle like you have to add each one. Also do all motherboards give power to the GPU and if a GPU pulls really low power can it only get power from the PCI-e slot? So pretty much the PCI-e slot is for power and to connect it with the mobo? Please correct me if i'm wrong because this has been a subject I wanted to learn for years!

[BrightCandle]

PCIe slots are always capable of providing power and if you get a sound card it doesn't have a PSU direct connection, it powers entirely from the motherboard which in turn is powered from the 24pin ATX connector.

 

Graphics cards however pull more power than the motherboard can reasonably provide, because the traces in copper across the board and especially the pins of the slot are limited in the amps they can carry to around 1.1A per pin. The PCIe specification defines what slots must be capable of and what cards will maximally attempt to draw and there is a certification process in place to stop either side from releasing products that break the spec. For a full length PCIe slot its 75W total, 66W of which is 12V and the rest is 3.3V. But its only 25W for a 1x slot.

 

What the PCIe specification also provides is the details of what additional power can be drawn from the PSU for cards. The 6 pin can be used to draw 75W and the 8 pin can be used to draw 150W. A card can also use 2x 6 pin or a 6 pin and an 8 pin for a total of 300W (75W slot + 75W 6 pin + 150W 8 pin). It doesn't define in its standard the 375W option that is clearly possible if you use 2x 8pin connectors.

 

How does this relate to the RX 480? Well despite its specified TDP of 150W the card will consistently pull more than that in some scenarios, up to 170W. Since it has a 6 pin connector (75W maximum) and the PCIe slot (75W maximum) its clearly out of spec, its drawing more power than its allowed. Before the patch it pulled that power pretty equally from the slot and the 6 pin connector. This pushed both out of spec but the motherboard side of things is more concerning because they really aren't designed to be a big source of power and no card has ever overdrawn from the slot before and when pcper asked the motherboard manufacturers they were concerned it could cause damage.

 

On the other hand we have had multiple cards pull more than 75W out of a 6 pin connector, its not common but its certainly happened before especially with AIB cards. The PSU 6 pin connectors make better contact and are made of better materials and hence often can provide a lot more power, although it really depends on your PSU and whether it supports the updated standards for the higher quality connectors, thicker than the minimum cabling and the appropriate support internally to actually deliver the power. Its very common to find PSUs supporting more so its less concerning for the power to be pulled from there. Its important to note however that the card remains outside the PCIe specification and it ought to be rated as a 170W TDP and use an 8 pin connector or 2x 6 pins.

 

So the fix for the rx 480 is two options. With the new driver the excess power will be pulled from the 6 pin and not the PCIe slot (although AMD has failed still to meet the specification for the PCIe slot but its reduced it at least- http://www.pcper.com/reviews/Graphics-Cards/AMD-Radeon-RX-480-Power-Consumption-Concerns-Fixed-1671-Driver/Power-Testing-). It also provides a compatibility mode that constrains the power of the GPU to 155W total (but its still out of spec on the PCIe slot again overdrawing on 12V current and also still on the 6 pin as well). So while the problem has partly been fixed the testing of it shows its still not actually in specification.

 

TLDR is the RX 480 pulls too much power from all its power sources and even after the fix it still does. This could cause damage to the motherboard and the PSU, their protection mechanism is that the specification defends them but AMD has thrown the spec out and is just doing its own thing and its causing motherboard manufacturers to be concerned about damage.