A current detecting circuit detects a resonant current in a primary side of a resonant converting circuit to generate a current detecting signal. An output detecting circuit generates a feedback signal according to the output voltage. A resonant controller generates a clock signal and adjusts an operating frequency of the clock signal in response to the feedback signal for modulating the output voltage of the resonant circuit. The resonant controller includes a resonance deviation protection unit which detects the current detecting signal according to a phase of the clock signal to determine whether the resonant circuit enters a region of zero current switching or not. When the resonant circuit enters the region of zero current switching, the resonant controller executes a corresponding protection process in response to that the resonant controller operates in a starting mode or a normal operating mode.