AMD and NVIDIA in the mass loop: Why AMD should rethink its board design and NVIDIA is hardly in a better position

1 - Introduction and problem definition 2 - Mainboard and graphics card board layout 3 - Measurements on current NVIDIA cards 4 - Measurements on current AMD cards 5 - Summary and conclusion

After all my measurements on the new Blackwell and AMD graphics cards, I decided to write another in-depth article about ground loops in computers, especially in the context of the latest graphics cards and their power feedback. The article will examine the role of the PCIe motherboard slot and its ground pins in detail and explain why they are often underestimated. I will also look at the power flows through the mainboard, EPS and 24-pin connector and how this affects the PCB layout of graphics cards, particularly in relation to ground planes.

If you want to read the old article again, you can find it as a link at the end of today’s review. The article at that time dealt exclusively with the 12VHPWR connection. Today I measured a total of four graphics cards of different performance classes, including two models from NVIDIA and two from AMD. I will also compare the new 12V-2×6 connector with the conventional 6 2-pin connectors, so it will certainly be exciting!

What we all know, of course: In a computer, electrical current flows in closed circuits. Whatever energy flows into the system via the supply lines (e.g. 12-volt rails) must also flow out again via ground connections. This return flow of current to ground is often overlooked, but is just as important for the stability of a PC as the power supply itself. Especially with modern graphics cards with high power requirements – such as current high-end GPUs with 8-pin or 12VHPWR power connectors – the distribution of return currents plays a major role. Unintentional ground loops (i.e. loops formed by the current via different ground paths) can occur time and again.

They occur because electric current, “like water, always seeks the easiest and shortest path” back to the source – Mr. Kirchhoff sends his regards. And this is precisely why we sensitize ourselves a little and clarify why, in addition to the 12V connections, the return paths of the current must also be considered, what role the PCIe mainboard slot with its many ground pins plays, how the mainboard (via 24-pin and EPS connectors) is involved in the current return and what effects all this has on the board layout of graphics cards and system stability.

Power supply vs. power return – all currents must return!

A central principle of electrical engineering is Kirchhoff’s first theorem, which states: In a node (such as a component or a connector), the sum of the incoming currents is equal to the sum of the outgoing currents. Applied to the PC power supply, this means that Every ampere that flows into the graphics card via the 12V lines must flow back out via the ground. In practice, PC enthusiasts often concentrate on how much current flows to a graphics card via the 12V rails – e.g. via an 8-pin PCIe connector or the new 12VHPWR or 12V2X6 connector – and deliberately overlook the fact that exactly the same current must also be dissipated again.

However, it is precisely this return flow that is crucial. Let’s formulate a rather provocative question: With connectors such as the 12VHPWR or 12V2x6 or 6 2 pin (conventional PCIe power connectors), the 8-pin EPS and the 24-pin mainboard connector, does exactly the same current really flow back to ground via the corresponding pins of the respective connectors as it flows in? The answer is: Not at all! i.e. not necessarily via the same paths, but in total, of course. Ok, let’s take a look at what we have here, because in a PC there are several parallel ground paths via which the current can flow back to the power supply unit. It is important to understand which paths the return current prefers to take:

• Via the ground lines of the graphics card power cables: each 6-/8-pin PCIe connector of a graphics card has several ground pins (e.g. five ground contacts for 8-pin). Part of the graphics card current then flows directly back to the power supply unit via these thick ground wires.
• Via the PCIe slot (mainboard slot): The graphics card is permanently connected to the mainboard via the PCIe slot. In addition to data lines, the slot also provides 12V and numerous ground pins. This allows a considerable amount of the current to flow into the mainboard ground and from there back to the power supply unit (but I’ll come to that in a moment).
• Via the mainboard 24-pin connector: The mainboard is connected to the power supply unit via the 24-pin ATX connector. It contains several ground wires. Current that has been fed into the mainboard ground plane (e.g. from the PCIe slot) can flow back to the power supply unit via these ground lines.
• Via the EPS/CPU connector: Modern mainboards have a 4- or 8-pin EPS12V connector (sometimes 2× 8-pin) for the CPU supply. This also provides ground connections to the power supply unit. Under certain circumstances, graphics card return current flows via the common ground plane of the mainboard to the ground pins of the EPS connector.
• Via the PC housing (chassis ground): The metal housing of a PC is usually connected to the power supply housing and thus coupled to ground (earth). The graphics card is also usually electrically connected to the housing via its slot bracket and the screw connection. In some cases, this allows part of the return current to flow through the housing, which can lead to ground loops and cause interference.

This large number of return paths means that the entire current does not necessarily flow back via the same connector through which it came. The distribution is based on the lowest resistance. Ideally, all these paths are low impedance enough to absorb the current without large voltage drops. However, if individual return lines offer a significantly easier path, the return current is concentrated there – sometimes to the detriment of other components.

A practical example of this is the phenomenon of melting power connector pins. In the case of overloaded 12V GPU connectors (such as the 12VHPWR), it was mainly the 12V pins that melted, while the ground pins usually remained undamaged. This seems paradoxical at first, as the same current flows through every circuit. However, the explanation is plausible, as the graphics card distributes the return current over several paths, so that less current flows back through the ground contacts of the same connector than through the 12V contacts. This relief of the ground pins in the connector means that primarily only the 12V pins that have become hot are damaged and the ground pins are only damaged indirectly.

This is exactly what my measurements at that time had already confirmed and I had shown that the return flow of the GPU current is distributed in the system “like a watering can”, i.e. distributed over various ground connections.

The PCIe slot and its ground pins – the underestimated backbone

Modern graphics cards draw up to 75 W via the PCIe slot (also known as the PEG slot). The x16 slot provides three 12V pins (in total typically ~5.5 A at 12V) as well as several 3.3V pins for lower power levels. However, considerably more of the slot’s pins are designed as ground – according to the ATX specification, dozens of the 82 contacts in an x16 slot are connected to ground. These many ground pins fulfill two purposes: they serve as a reference potential for the high-speed data lines (signal feedback to avoid interference) and they can transport current. In fact, the specified power consumption via the slot may of course also flow back via its ground and the slot ground must also be able to carry this ~6 A safely. Thanks to the large number of parallel ground pins, this is not a problem, but then comes the ampere hammer.

It is always underestimated that significantly higher currents can flow via the PCIe slot’s ground contacts than the nominal 12V power drawn from the slot. The reason is again the parallelism of the return paths: If, for example, the graphics card draws a further 200 W via external 8-pin connectors, most of this current must also return. If the ground lines of the PCIe power cables and the mainboard are all connected together in the computer, it can happen that a significant proportion of the externally supplied current flows back via the slot ground because this path is perhaps “shorter” or lower impedance for some of the current. I would like to remind you of my experiment at the time with a graphics card drawing approx. 380 W, where I was able to record over 10 A return current at the PEG slot, although the slot itself only fed in ~3.4 A via 12V. Other tests with an RTX 4090 (~516 W) resulted in around 13 A ground return current at the slot, with extreme BIOS up to 15 A!

These values are well above the actual “75W limit” of the slot (which corresponds to around 6.25 A on 12 V), but relate to the ground connection, not the 12 V supply line. Thanks to over 30 parallel ground contacts, the PCIe slot can still withstand this in purely electrical terms without being damaged immediately. However, the important role that the slot plays for current feedback becomes clear here. A robust ground connection of the graphics card in the slot is therefore essential. In practice, this is ensured by the many ground pins and the wide ground plane in the slot connector of the mainboard to which these pins are connected. Nevertheless, you should be aware that a high load on the slot via the ground can have consequential effects on the mainboard (more on this in the next sections). Engineers from graphics card manufacturers are already considering how the mass flows can be controlled more consciously in future in order to relieve the PCIe slot somewhat. It would be conceivable, for example, to design the voltage converter distribution on the card in such a way that a larger proportion of the current is fed back locally via the external connectors instead of via the slot. This would make it possible to influence the current distribution in a targeted manner so as not to overload the slot.