Power Supply Unit¶

Introduction¶

Compute nodes in Dalek are powered via a 220 V AC wall outlet. Consequently, the incoming high-voltage AC must first be converted into DC, which is the primary function of PSU. Each node is equipped with a dedicated PSU capable of delivering different current levels according to its specifications.

It should be noted that the PSU itself incurs power losses during operation. Since the Dalek nodes are equipped with 1000 W 80 PLUS Platinum-rated PSUs, an efficiency loss of approximately 6–8% can be expected when operating within a typical load range of 20–60%.

The following section details the various power connectors (PSU outputs) used within the Dalek cluster.

Power Connectors¶

The compute nodes in Dalek can be powered from many different connectors. For instance, the az4-n4090 and az4-a7900 nodes, have a connector for the motherboard (the ATX 24-pin connector), a connector for the CPU (EPS12V connector) and connectors for the GPU (typically PCIe 8-pin connectors or the recent 12VHPWR 16-pin connector). On the other side, iml-ia770 nodes are built on the top of a Minisforum AtomMan X7 Ti mini-PC and an external GPU. The mini-PC is powered via a 19 V coaxial connector while the eGPU dock uses an ATX 24-pin connector (same as those used for motherboards!) and the GPU itself (Intel Arc A770 ) is powered via PCIe 8-pin connectors. Finally, the az5-a890m nodes (based on Minisforum EliteMini AI370 mini-PCs) can be powered either via a 19 V coaxial connector or via an USB-C connector.

In fact these connectors can sometime even be decomposed in different rails. Understanding them is important because it is what cab be measured with external probing systems like NCM. The following sub-sections depict the different connectors and their use cases.

ATX 24-pin - Motherboard Main Power Connector¶

The ATX 24-pin connector provides multiple rails described below. This connector can typically deliver up to 150–200 W.

Warning

The allocation of power to each rail is provided for informational purposes only. This distribution may not be entirely accurate and can vary depending on the motherboard design.

+3.3 V rail - 3 pins
- for RAM (via onboard regulators)
- for NVMe M.2 SSDs
- for chipset / PCH
- for PCIe logic
- for some I/O controllers
+5 V rail - 5 pins
- for USB ports
- for SATA logic power (control electronics) - there is no SATA component in Dalek's compute nodes
- for legacy components - there is no legacy component in Dalek's compute nodes
- note: High-current 5 V loads are mostly gone, but USB still depends on it
+12 V rail - 2 pins
- to feed onboard DC2DC converters
- for PCIe slot powe (partially, up to 75 W)
- for CPU fan and NVMe M.2 SSDs fan
- note: The CPU and GPU do NOT rely on this connector alone - they also use EPS12V and PCIe 8-pin connectors for high power
+5 V standby (5VSB) - 1 pin
- for power button logic
- for WoL
- USB charging when PC is off
- note: Always ON when PSU is connected to AC

EPS12V - Motherboard Power Connector for the CPU¶

This connector is also called "CPU power connector" or "ATX12V CPU connector". Its only purpose is to supply dedicated 12 V power to the CPU via the motherboard's VRMs. Usually it is a 4-pin, 8-pin, or 4+4-pin connector. Modern motherboards, like the ones in Dalek az4-n4090 and az4-a7900 nodes, use the 8-pin connector that can deliver up to 200 W.

Modern CPUs primarily draw power from the EPS12V connector via the VRMs, not directly from the 24-pin connector. In other word, the CPU power consumption should be very well isolated by observing this connector.

az4-n4090 and az4-a7900 node are equipped with Minisforum BD790i motherboards that are using this connector. The TDP of the CPU sealed to these motherboards (AMD Ryzen 9 7945HX) is 75 W.

PCIe 8-pin - Classic GPU Power Connector¶

The PCIe 8-pin connector is the classic way GPUs get supplemental power from the PSU beyond what the motherboard slot provides. It is also called "6+2 pin PCIe connector" (for PSU compatibility) and it provides additional 12 V power to GPUs that need more than the 75 W from the PCIe slot. One 8-pin connector can deliver up to 150 W and this is why, depending on the GPU, multiple connectors can be used to power it. Here are the nodes that are using this connector in Dalek:

az4-a7900: AMD Radeon RX 7900 RTX GPU where 3 PCIe 8-pin connectors are used (max. 3x150 W + 75 W = 525 W).
iml-ia770: Intel Arc A770 GPU where 2 PCIe 8-pin connectors are used (max. 2x150 W + 75 W = 375 W).

Note that the TDP of the AMD Radeon RX 7900 RTX GPU is 355 W and the TDP of the Intel Arc A770 GPU is about 225 W. Clearly these GPUs will not use the maximum power they could drain from the PCIe 8-pin connectors.

12VHPWR - New GPU Power Connector¶

The 12VHPWR connector is the new standard for high-power GPUs, and it's a bit more complex than a traditional PCIe 8-pin. It is made to deliver up to 600 W to modern GPUs from a single connector. It is a 12 V connector like the PCIe 8-pin connector but it has 16 pins (12 for power and 4 for sense). Typically, from the PSU it is presented as new modular cable or an adapter from 4 traditional PCIe 8-pin connectors (4x150 W = 600 W). What is new is the 4-pin sense that communicates with the GPU to inform it with the maximum safe current available, up to 600 W.

This connector is used by the Nvidia RTX 4090 GPUs that can be found in the az4-n4090 nodes.

Coaxial 19V - Mini-PC and SBC Connector¶

Many mini-PCs are powered via a 19 V coaxial connector. This is typically true for the iml-ia770 and az5-a890m nodes, that rely on Minisforum AtomMan X7 Ti and EliteMini AI370 mini-PCs, respectively.

USB-C PD - Mini-PC and SBC Connector¶

Many current mini-PCs and SBCs can be powered via USB PD. In fact, USB PD 3.1 - through an USB type C connector - can deliver up to 240 W. For instance, the az5-a890m nodes support this protocol. Then, the USB PD protocol is in charge of negotiating the voltage and the current.

For the az5-a890m nodes we preferred to plug the legacy 19 V coaxial connector because it is simpler and more stable for measurements.