Intel ATX 3.0, Design Manual

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PCI Express* Add-in Card Considerations
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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 3.2 and Table 3-5 .
Table 3-1: PCI Express* CEM Add-in Card Power Excursion Limits Table 45
Average Power Calculation
Interval Length in
microseconds(µs)
“T”
Ratio of Average Power1 in
Interval “T” Divided by
Maximum Sustain Power
Notes
≤ 100 3 2
>100 and
<1,000,000 µs (1 sec)
4 – 0.2171 x ln(T) 3