A method for detecting a synchronization point of an alternating power in comparing the alternating waveform with a threshold voltage is provided. The method includes the steps of detecting a rising point at which the voltage of the alternating waveform changes from a level lower than the threshold voltage to a level equal to or higher than the threshold voltage; detecting a falling point at which the voltage of the alternating waveform changes from a level equal to or higher than the threshold voltage to a level lower than the threshold voltage; measuring a first interval from the rising point to the falling point subsequent to the rising point; measuring a second interval from the falling point to the rising point subsequent to the falling point; and determining the synchronization point based on a difference between the length of the first interval and the length of the second interval.