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3
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* spec introduces four significant updates that directly
affect this power supply specification:
1. A Power Excursion allowance was introduced to support brief, high current
demands on power, beyond the rated TDP.
2. The maximum power consumption for a single Add-in Card was doubled to 600 W.
This is the per-card limit from all sources combined.
3. A new 48V (nominal) power rail was added.
4. Two new Auxiliary Power Connectors were introduced to provide the full 600 W on
a single cable connector. The new 12VHPWR connector supports 600W on the 12V
rail while the 48VHPWR provides 600W on the 48V rail. Four new sideband signal
conductors permit simple signaling between the Add-in Card and power supply.
3.1
PCIe* Add-in Card Power Excursions
PCI Express* CEM specifications prior to Revision 5.0 did not provide any allowance to
permit an Add-in Card to exceed the TDP power for its designated power range. This
effectively limited the absolute power consumption of each Add-in Card to a hard limit
such as 10 W, 75 W, 150 W, 225 W, or 300 W, even when it would be advantageous
for it to make short-duration high-current demands on a power rail.
It is recognized that while many existing PCIe* CEM products already exceed the card
power limits, in violation of prior PCIe* CEM specs, their power supplies were never
explicitly designed to withstand these excursions. Consequently, power excursions
beyond these limits, however brief, might cause unexpected card or system
malfunctions, potentially triggering PSU overcurrent protection (OCP) or voltage
droop. This risk increases when multiple PCIe* cards are installed in a system.
The PCIe* CEM 5.0 spec addresses the need for occasional power excursions by
permitting the card to briefly exceed the existing limits on supply power while still
abiding by the power limits on a time-averaged basis. This allows the power supply
and Add-in Card to jointly withstand increased power demands with a limited duration
and magnitude.
The PCIe* CEM 5.0 specification introduces the concept of Sustained Power, the
average power delivered though a single power cable in a 1-second moving interval.
This allows the card and power supply to operate within existing power and thermal
envelopes, since the excursions' durations are very short and do not measurably
increase the average temperature of any component.
These updates are described in an Engineering Change Notice (ECN) to the PCI
Express* Card Electromechanical (CEM) Specification, Revision 5.0, and will be
20
PCI Express* Add-in
Card Considerations
336521
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