An uninterruptible power supply (“UPS”) has a phase-controlled rectifier coupled to a source of AC power and having an output providing a DC bus, the output of the phase-controlled rectifier coupled to an inverter. A first controller generates a firing angle for the rectifier and a fuzzy logic controller generates a firing angle for the rectifier. In an aspect, the rectifier is controlled by the firing angle generated by the first controller during normal operating conditions of the UPS and the rectifier is controlled by the firing angle generated by the fuzzy logic controller during abnormal operating conditions of the UPS. The abnormal operating conditions can include loss of a direct DC bus voltage measurement and or a period of time after the UPS experiences a large load change. In an aspect, the firing angle generated by the first controller is compared to the firing angle generated by the fuzzy logic controller and a rectifier fault condition determined to exist when the two firing angles differ by at least a threshold amount. In an aspect, the first controller is a PI controller.