What is a GPU Riser Card
GPU riser cards serve a critical purpose in a mining rig. They allow you to locate the GPUs away from the motherboard (MB). This allows more GPUs to connect to a motherboard and allows space between GPUs to improves airflow around the GPUs. Thus ensuring the GPUs continue to run for a long time and allow for configurations like 18 GPUs with 3 PSUs. In this configuration each PSU would power 6 GPUs.
Powering a GPU
To understand how a GPU riser card works we should first discuss how a GPU is powered. All high-performance GPUs need more power than the 75 Watts that thePCIe slot was designed to deliver. So, the solution is to have an additional power connected to deliver 12-volt power directly to the GPU and this is typically provide via a connector on the top of the GPU farthest from the GPU connectors. These connectors come it two variety’s 6-pin and 8-pin. The 6-pin is designed to deliver an additional 75 Watts of power and the 8-pin is designed to deliver an additional 150 Watts of power. More details on the pinout is available here.
Now the other part of a GPU riser card is the connector that attaches to the motherboard (MB). This is a 1x PCIe connector that can go in any PCIe slot on the MB. While the PCIe slot of the motherboard provides power, the riser card will not us any power from the PCIe slot. The only connections to the MB is the PCIe data signal lines. If you know a little about electronics you might be thinking that the MB and the riser card must share a ground? In-fact they don’t need to since the data signal lines of PCIe are “differential” lines. This means that data signals are based on a difference in voltage between these pairs, not by a difference between the signal and a common ground. Each pair creates a very fast serial communication port and is electrically isolated from each side.
This electrical isolation is essential for us to use a different PSU on the MB from the one connected to the GPU(s). However it is critical that the PSU connected to the riser card is the same PSU powering the top power connector. If these happened to be two different PSU you will have stability issues and could burn out one or both of your PSUs.
GPU Riser Card Efficiency
When optimizing cost and performance in a large-scale mining operation efficiency is one of the most important aspects of your plan. Before we can talk about GPU Riser efficiency we need to understand the two main types of riser cards. Most riser cards actually have the PCB design to handle both types but will only populate them with the parts needed for the type used. The voltage needed is a key differentiator between these types of riser cards. The first type uses only 12V power. This work well with server style PSUs. The other design uses both 5V and 12V power which is easy to provide with a standard ATX style PSU.
PCIe Power Requierments
The PCIe slot needs both 12V power and 3.3V power. So, in both designs we’ll need to generate 3.3V power from somewhere. The power connectors that drive GPU riser cards come in 3 flavors: 6 Pin PCIe power plugs, 4-pin Molex Connectors and SATA power. The 6 Pin plug is 12V only, both the Molex and SATA can provide 12V and 5V power. Ironically the SATA cable can provide 3.3V power but that is never used in any riser card design. Now the key difference between these designs is how they generate 3.3V.
For the 12V only parts they use DC-DC power converter. For the 12V/5V designs they use a linear voltage regulator (LVR). The difference between these devices is cost and efficiency. A Linear voltage regulator is inexpensive, but not efficient. It drops voltage by converting the voltage difference to heat. I’ve included a table below that shows the relative performance of the LVR design with various input voltages with the DC-DC design. The LVR with a 12V input power would generate 26.1 Watts of heat. That would melt most LVRs. So if 12V power for your riser is the only option then the DC-DC type is far better. But at the cost of $1-$2 for the converter chip and $2-$3 for cost to purchase these risers. However, this negative of an LVR goes away as the input voltage approaches the output voltage. So, in the 12V/5V riser cards we can use the LVR at lower cost and only generate 5.1W of heat. But in our systems at New School Mining we can reduce that even more, by lowering the 5V down to low 4V (4.2-4.3V). The end result is the 12V/5V design will consume about 2.2W Max more than the DC-DC design. At $0.07 per kWh that’s, worst case, $1.34 per year extra cost to run.
|Input Voltage||Output Voltage||Input Amps||Watts of Heat||Annual Overhead Cost ($0.07/kWh)|
|DC-DC (95% Efficent)||12||3.3||0.87||0.52||$0.32|
Now that you understand the differences in the designs you might be asking, “how can I tell the difference?” I’ve included some pictures below that show the two designs. The key difference is the DC-DC voltage regulator is missing in the LVR design. The Blue one is the DC-DC voltage regulator and if you look at the left side just below the mid point you will see a black square that has “100” printed on it. That is the DC-DC design’s inductor. Just below the inductor is a small chip. This chip is the actual DC-DC voltage regulator. You will notice the the LVR is actually in both designs. It’s the black square near the top with 2/3 pins coming off the bottom. This provides a voltage reference for the DC-DC voltage regulator. So you can see why the DC-DC design is more costly, it basically as all the components of the LVR design and then some additional components. All this said if your starting out, I suggest getting the DC-DC design, these are more costly, but unless you are purchasing high volume, this is a small difference. Furthermore there are less pitfalls with the DC-DC design and it tends to be more robust for novice users. But if you are looking to scale, you really have to weight the initial cost savings vs the on-going expense. If you power is cheap enough, that ongoing expense may be minimal.
If you looking for DC-DC Riser feel free to contact us at firstname.lastname@example.org for recommendations.