Undervoltage protection (UVP) is very similar to Overvoltage Protection in concept, just flipped. UVP shuts the PSU off if the voltage the PSU is providing to the PC drops below accepted values, and was a source for concern when ultra high-efficiency CPUs began shipping (like Haswell chips, which idled below 1W in some cases). It should be noted that like OVP, accepted values for undervoltage protection do vary from PSU to PSU.
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