Disclosed a PWM cycloconverter switches power supplies without interrupting operation if a fault occurs in a power supply. In case of trouble with a 3-phase AC power supply (1), a fault detector circuit (30) produces a signal (120) indicative of trouble, and a power switch (20) selects the output voltage from an uninterruptible power supply (10). A phase detection switch (43) selects the phase information from a phase detector (41) of the uninterruptible power supply. The phase detector (41) continues to detect the phase of the uninterruptible power supply (10) before a fault occurs in the power supply so that the exact phase information on an uninterruptible power supply (10) can be available immediately after a change of phase information from the phase detection switch (43). Therefore, operation will not be interrupted in switching power supplies. |