PCI Express* Add-in Card Considerations

The PCI Express* (PCIe*) Card Electromechanical Specification (CEM Spec) provides thermal, power, mechanical, and signal integrity design guidance for the PCI Express* Add-in Card (AIC) form factor. This includes the card’s electrical and mechanical interface with a host system board, chassis, and power supply.

The 5.0 Revision of the PCIe* CEM specification introduced multiple updates that directly affect this power supply specification. The newer, 5.1 Revision of the PCIe CEM spec provides important corrections, revisions, and clarifications of the features first introduced in CEM 5.0. This ATX power supply specification has been updated to reflect the content in the PCIe 5.1 CEM specification.

1. Two new Auxiliary Power Connectors were introduced to deliver up to 600 W through a single cable connector. The 12V-2x6* connector supports 600 W on the 12V rail while the 48VHPWR connector provides 600 W on a 48V rail. Four sideband signal conductors on each connector permit simple, direct signaling between the Add-in Card and power supply to aid in power supply and card configuration.
2. A Power Excursion allowance was established to allow Add-in Cards to make brief, high-current demands on power while keeping average power within specified limits.
3. The maximum power consumption for a single PCIe Add-in Card was increased from 300 W to 675 W.
4. A 48V (nominal) power rail was added. The 48V rail is expected to be deployed chiefly in large data centers. No support for this 48V rail is proposed in this ATX power supply specification. PSU vendors are free to include 48V support, at their discretion.

   *The 12V-2x6 connector described in PCIe CEM 5.1 replaces the 12VHPWR connector introduced in PCIe CEM 5.0, to address reliability concerns that were observed in the 12VHPWR connector. New power supply designs should mount only the 12V-2x6 connector and the 12VHPWR connector should be deprecated.

   While many of the relevant PCIe specification updates are duplicated here for convenience, designers should confirm that they have the most up-to-date information by consulting the reference documentation on https://www.pcisig.com. The information below is drawn from the PCIe* documentation at the time of publication.

   Historically, prior to the PCIe* CEM 5.0 specification, Add-in Card power consumption beyond 300 W was not allowed. Earlier generations of PCIe* Add-in Cards were limited to 300 watts or below, drawing up to 75 W from the card slot, and additional increments of 75 W and 150 W delivered through one or more 2x3 and 2x4 12V Auxiliary Power Cables, to reach the aggregated card maximum power of 300 W.

   The 5.1 CEM specification allows cards to consume 675 W of continuous power. This power is the maximum that may be drawn from all power rails, combined. For example, an Add-in Card might draw 75 W through its card edge connector and 600 W through a 12V-2x6 cable plug, for a total of 675 W.