A detection circuit for an AC-to-AC power converter includes a voltage divider circuit that has three AC input terminal coupled respectively to three input ends of a power converter which are to receive respectively three AC input power signals with different phases, and that receives a predetermined first reference voltage. Upon receiving one AC input power signal, the voltage divider circuit outputs a divided voltage, which during an active period of the one AC input power signal, is greater than a predetermined second reference voltage, to a comparator, which also receives the predetermined second reference voltage, such that the comparator outputs a detection signal that is in an active state when the one AC input power signal is.