Hold-up time represents the amount of time, usually measured in milliseconds, that a PSU can maintain output regulations as defined by the ATX specification without input power. Put simply, hold-up time is the amount of time that the system can continue to run without shutting down or rebooting during a power interruption.
According to the ATX spec, the PWR_OK is a “power good” signal. This signal should be asserted high, at 5V, by the power supply to indicate that the +12V, 5V, and 3.3V outputs are within the regulation thresholds and that sufficient mains energy is stored by the APFC converter to guarantee continuous power operation within specification for at least 17 ms. Conversely, PWR_OK should be de-asserted to a low state, 0V, when any of the +12V, 5V, or 3.3V output voltages falls below its under voltage threshold, or when mains power has been removed for a time sufficiently long such that power supply operation cannot be guaranteed. The AC loss to PWR_OK minimum hold-up time is set at 16 ms, a lower period than the hold-up time described in the first paragraph and ATX spec sets also a PWR_OK inactive to DC loss delay which should be more than 1 ms. This means that in any case, the AC loss to PWR_OK hold-up should be lower than the overall hold-up time of the PSU and this ensures that in no case the power supply will continue sending a power good signal, while any of the +12V, 5V, and 3.3V rails are out of spec.
The ATX specification sets the minimum hold-up time to 17 ms with the maximum continuous output load. In many cases, manufacturers use smaller capacitors in the APFC converter, resulting in a measurement of less than 17 ms. Manufacturers do this mostly to cut production costs, as these capacitors are expensive. The smaller bulk capacitors also improve efficiency by a little bit.
Measuring the hold-up time is a dangerous procedure since you have to connect an oscilloscope to the main grid. Unless you are taking the right precautions, you never want to do this; it's dangerous and you could harm yourself and your equipment!
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