Abstract: An observation is made that the peak voltage value for a rectified AC voltage signal is substantially the same from cycle to cycle. Using this observation, a method of measuring an AC voltage is used to determine a more accurate RMS voltage value under light load conditions. The method includes rectifying the AC voltage signal to form a rectified signal, sampling the rectified signal to obtain a set of sampled values for each half-cycle of the AC voltage signal, searching the sampled values for each half-cycle to determine a local minimum value for each half-cycle, searching the sampled values following the local minimum value to determine a local maximum value for each half-cycle, and calculating a root mean square value from the local maximum value for each half-cycle thereby determining the root mean square value for each half-cycle of the AC voltage signal.

Abstract: A method of optimally generating a power failure warning (PFW) signal has been disclosed here in such a manner that adjusts timing of PFW signal generation according to load conditions in case of input AC voltage loss. A PFW voltage threshold value can be set at a lower value under light load conditions and at a higher value under heavy load conditions. PFW signal generation can also be triggered by a timing mechanism that is set when a bus voltage drops to a voltage threshold value. A countdown time of the timing mechanism is set according to a determined bus voltage drop rate. In this manner, issuance of the PFW signal is delayed for lighter load conditions and the power supply unit is capable of extending normal operation under lighter load conditions before the PFW signal is issued.