ATX Version 3 Multi Rail Desktop Platform Power Supply
Design Guide
PCIe* Add-in Card Power Excursions
Prior to Revision 5.0, the PCI Express* CEM specification did not provide any allowance that would permit an Add-in Card to exceed the TDP power for its designated power range. This effectively confined 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 the Add-in Card to make short-duration high-current demands on a power rail.
It is recognized that while many existing PCIe* CEM products frequently exceeded 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, could cause unexpected (and untraceable) card or system malfunctions, potentially triggering PSU overcurrent protection (OCP) or voltage droop. This risk increased when multiple PCIe* Add-in Cards were installed in a system.
The newest PCIe* CEM spec addresses the need for occasional power excursions by permitting the Add-in Card to briefly exceed the nominal limits on supply power while still abiding by the limits on a time-averaged basis. This allows the power supply and Add-in Card to jointly endure increased card power demands having a limited duration and magnitude.
The PCIe* CEM 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 infrequent, and will not measurably increase the average temperature of any component.
These updates are described in the PCI Express* Card Electromechanical (CEM) Specification, Revision 5.1. This section of the ATX Power Supply Design Guide will draw on this PCIe* CEM Rev 5.1 content to provide design guidance for power supplies that meet the permitted power excursions of PCIe* Add-in Cards.
Power excursions are allowed in a PCIe* Add-in Card when it draws power through either a connected 12V-2x6 Auxiliary Power cable connector or a 48VHPWR Auxiliary Power cable connector. 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.
Power excursions are also permitted on the 12 V rail drawn from the baseboard through PCIe Add-in Card connector’s card edge. No excursions are permitted on the 3.3V and 3.3Vaux domains; these two power rails must continue to abide by the legacy Add-in Card power limits.
The power supply must be able to provide voltages that remain within the requirements defined in Table 4-2 (Section 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.
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 tolerances 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 Table
Average Power Calculation Interval Length in micro-seconds(µs) “T” | Ratio of Average Power in Interval “T” Divided by Maximum Sustained Power “R” Max | Notes | ||
On Any Power Rail Before “Set_Slot_Power_Limit” Message | +12V Power Rail in CEM Connector for all Add-in Cards | Total Card Power for > 75 W Cards | ||
≤ 100 µs | 1 | 2.5 | 3 | 1,2,4 |
>100 and <1,000,000 µs (1 sec) | 1 | 3.25–0.1628 x ln(T) | 4 – 0.2171 x ln(T) | 1,3,4 |
≥1,000,000 µs (1 sec) | 1 | 1,4 | ||
This is also the max ratio of instantaneous power relative to maximum sustained power.
ln() is the natural logarithm function.
The Add-in Card must always 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.