ATX12VO (12V Only) Desktop Power Supply

Design Guide

ID 613768
Date 01/03/2022
Public

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+12VSB (Required)

+12VSB is a standby supply output that is active whenever the AC power is present. This output provides a power source for circuits that must remain operational when the main 12V DC output rails are in a disabled state. Example uses include soft power control, Wake on LAN, wake-on-modem, intrusion detection, Alternative Low Power Modes (ALPM) or suspend state activities.

The power supply must be able to provide the required power during a “wake up” event. If an external USB device generates the event, there may be peak currents around 2.0 A or higher, lasting no more than 500 ms.

Over current protection is required on the +12VSB output regardless of the output current rating. This ensures the power supply will not be damaged if external circuits draw more current than the supply can provide.

With new modes of operation for computers like Alternative Low Power Modes (ALPM) the continuous current rating of the 12VSB rail is recommended to be at least 1.5 A (18 Watts). Some scenarios like USB Power Charging in Sleep or ALPM could require more current on the 12VSB rail like 2.0 Amps or more depending on the design.

It is recommended that power supplies with a size of 1200 watts or larger to have the 12VSB Rail of at least 3.0 Amps to support larger memory sizes with larger power systems.

Figure 4-4: +12VSB Power on Timing versus VAC

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