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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 integrated into the specification itself in upcoming releases. This ECN is titled “Power Excursion Limits for 300W-600W PCIe* AICs” (Doc# 16495). This ECN defines power excursions, which are a “temporary condition in which the power exceeds the maximum sustained power”. The ECN states the purpose of the document is to “allow system designers to properly design power subsystems to enable these excursions”. Based on this PCIe* ECN, this section of the ATX Power Supply Design Guide will provide guidance for power supplies to be designed to meet the permitted power excursions of PCIe* Add-in Cards.
Note that Add-in Cards with power from 300-600 watts did not exist prior to the PCIe* CEM 5.0 specification. Earlier generations of PCIe* Add-in Cards were limited to 300 Watts or below. While many of the relevant 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 of this document.
The power consumption excursions allowed in a PCIe* Add-in Card rated at 300 watts to 600 watts, only when the mount one of new 12VHPWR or 48VHPWR power connectors, also introduced in PCIe* CEM 5.0. The 48VHPWR connector is not relevant to this document and therefore will not be discussed further. Similar power excursions are not permitted for the legacy 2x3 and 2x4 PCIe* Auxiliary Power connectors since that would introduce backward compatibility risks with legacy power supplies.
A second Engineering Change Request, “Power Excursion Limits up to 300 Watts” is under consideration. This ECR would define similar power excursions for the 12VHPWR Auxiliary Power connector for cards less than 300 Watts. It is likely that an identical Power Excursion allowance will be adopted for Add-in Cards rated at 300 W or less, so power supply vendors are advised to provision any 12V rails supporting the 12VHPWR connector with support for Power Excursions at all power levels.
The power supply must be able to provide voltages that remain within the requirements defined in Table 4-2 (Section 4.2) during the defined power excursions. Add-in Card power consumption excursion limits are defined by the maximum ratio (R) of average power consumption in any continuous time interval (T) relative to the maximum sustained (average) power of that Add-in Card.
At all times, the Add-in Card must concurrently adhere to the power excursion limits for all time interval lengths as defined in Table 3-1 and Figure 3-1 as well as the rolling time average of the sustained power of the card. Table 3-1 shows the power excursion limits for all time intervals in which “R” is calculated by dividing the average power consumption in a continuous time interval of length “T” but the maximum sustained power of that Add-in Card. The Add-in Card must also stay within all voltage tolerance and current as defined in Section Section 3.2 and Table 3-5.
Table 3-1: PCI Express* CEM Add-in Card Power Excursion Limits Table45
Average Power Calculation Interval Length in microseconds(µs)
Ratio of Average Power1 in
Interval “T” Divided by Maximum Sustain Power
<1,000,000 µs (1 sec)
4 – 0.2171 x ln(T)
≥ 1,000,000 µs (1 sec)
- R= average power during internal T / Max Sustained Power (ie. TDP).
- This is also the max ratio of instantaneous power relative to maximum sustained power.
- ln() is the natural logarithm function.
- Add-in Card must at all times and concurrently adhere to power excursion limits for all time interval lengths as well as the limits defined for Power Supply Rail Requirements in Table 3-5.
- Engineering Change Notice for up to 300 Watts (Work in Progress) will likely make this table apply to all cards mounting the 12VHPWR connector.
Figure 3-1: PCI Express* CEM Add-in Card Power Excursion Limits Chart