ATX Version 3 Multi Rail Desktop Platform Power Supply
Design Guide
DC Output Current – REQUIRED
The below table summarizes the expected output transient step sizes for each output. All items in the below table are REQUIRED, unless specifically called out as RECOMMENDED.
Table 4-3: DC Output Transient Step Sizes
Output | Maximum Step Size (% of rated output amps) | Maximum Step Size (A) |
+12V1DC 5 | 40% (Required)70% (Recommended) | - |
+12V2DC 5 | 85% of CPU supported in Table 2-12 | - |
+12V3/4 5 | Steps from 100% à 300%4 30% à 100% 3 | |
+5VDC | 30% | - |
+3.3VDC | 30% | - |
-12VDC | - | 0.16 |
+5VSB | - | 0.5 |
12V2 rails are typically used for CPU power. CPU step size will have more updated values in the Power Supply Design Guide Addendum (# 621485) which will be used to determine the 85% value
12V3/V4 rails are typically used for PCIe* Add-in Card connectors. This recommendation is based on Chapter Chapter 3 where PCIe* Add-in Card needs are discussed. The step size will come from the amount of PCIe* Add-in Card power supported based on the size of the PSU in Table 3-2.For more detail reference the Desktop Platform Form Factor Power Supply Test Plan (Doc #338448)
If a power supply has multiple 12V-2x6 connectors, Dynamic testing shall be done on each 12V-2x6 connector. For more detail reference the Desktop Platform Form Factor Power Supply Test Plan (Doc #338448)
Power supplies that have one combined 12V rail shall perform Dynamic testing on the one 12V rail with multiple tests which simulate different system level workloads: 12V1 (total system), 12V2 (CPU Load), and 12V3/V4 (PCIe AIC).
“-12VDC” Voltage rail Dynamic Step Size levels are RECOMMENDED.
Output voltages should remain within the regulation limits of Table 4-2, for instantaneous changes in load as specified in Table 4-4 and for the following conditions:
- Simultaneous load steps on the +12 VDC, +5 VDC, and +3.3 VDC outputs (all steps occurring in the same direction)
- Load-changing repetition rate of 50 Hz to 10 kHz
- AC input range per Section Section 2.1 and Capacitive loading per Table 4-7
The transient load slew rate is defined in Table 4-4 based on whether the PSU supports the PCIe* 12V-2x6 Connectors. This usually can be correlated to the PSU’s Rated Wattage, which is listed as a guidance.
Table 4-4: DC Output Transient Slew Rate
Output | PSU without PCIe* 12V-2x6 Connectors (Recommended Size ≤450 watts) | PSU with PCIe* 12V-2x6 Connector(Recommended Size >450 watts) |
All +12V | 2.5 A/µs | 5.0 A/µs |
+5V | 1.0 A/µs | 1.0 A/µs |
+3.3V | 1.0 A/µs | 1.0 A/µs |
+5VSB | 0.1 A/µs | 0.1 A/µs |
-12V | 0.1 A/µs | 0.1 A/µs |