A detection circuit for detecting the demagnetizing time of a magnetic device is provided. An input circuit is coupled to the magnetic device for detecting a magnetizing voltage and a demagnetizing voltage of the magnetic device. A control circuit is coupled to the input circuit for generating a demagnetizing-time signal in response to the magnetizing voltage, the demagnetizing voltage, and a magnetizing time. The magnetizing time is correlated to the enable period of the magnetizing voltage. The demagnetizing time of the magnetic device is represented by the demagnetizing-time signal.
