The duration of time, typically measured in milliseconds (ms), that a power supply unit (PSU) can maintain its output voltage within its specified regulation limits after the loss or interruption of its input AC or DC power source.
Mechanism: Hold-up time relies on energy stored in the power supply's internal components—most notably the large, high-voltage bulk storage capacitors located in the input/rectifier stage of an AC/DC converter or the DC-link stage of an AC/AC converter. When input power drops out, these capacitors discharge their stored energy (E =0,5*CV^2) into the regulation circuit to temporarily sustain the load.