When we first turn on the power supply, voltages are not immediately available on the power supply outputs: they increase until reaching their correct values. This increase happens is a fraction of a second (maximum of 20 ms or 0.02 s to be more exact).
In order to prevent these lower-than-normal voltages to be provided to the computer, the power supply has a signal called “power good” (also called “PWR_OK” or simply “PG”), which tells to the computer that the +12 V, +5 V and +3.3 V outputs are in their correct value and thus can be used, and the power supply is ready to work in a continuous fashion. This signal is available through pin eight (gray wire) from the main power supply connector.
There is also another reason for this signal to exist: the under voltage protection (UVP). As we will see in the next page, the under voltage protection shuts down the power supply if the outputs have a voltage below a certain level. If the UVP is active when the power supply is first turned on, the power supply would never turn on, because voltages are below the UVP trigger point. In other words, because when you first turn on the power supply voltages are below their values for a fraction of second, the UVP would prevent the power supply from being turned on. Therefore the under voltage protection circuit has to wait until the power good signal is active to be turned on.
This signal is generated by the monitoring integrated circuit or by the PWM controller (in the case of power supplies based on the half-bridge topology).
Below you can see the time diagram for the power good signal as available on the ATX12V specification. “VAC” is the input alternating voltage, i.e., the voltage from the wall. PS_ON# is the “power on” signal (i.e., you pushed the “standby” button from the computer case). “O/P’s” stand for “operating points.” And PWR_OK is the power good signal.
T1 is less than 500 ms, T2 is between 0.1 ms and 20 ms, T3 is between 100 ms and 500 ms, T4 is less or equal 10 ms, T5 is greater or equal to 16 ms and T6 is greater or equal to 1 ms. Just remembering that ms stands for millisecond and equals to 0.001 second.