Abstract: A circuit, which can detect a wrong operation of a flyback transformer having a low voltage coil and a high voltage coil, includes a voltage sensing coil which is disposed at the flyback transformer in order to generate a sensed voltage corresponding to a high voltage generated by the high voltage coil. A voltage sensing circuit generates a voltage-sensed signal corresponding to the sensed voltage and provides the voltage-sensed signal to a determination circuit. The determination circuit determines whether the flyback transformer is out of order based on the voltage-sensed signal and the horizontal frequency signal, and generates a determination signal according to the determination result. The shut-down circuit shuts down the flyback transformer in response to the determination signal, thereby preventing the abnormal flyback transformer from generating a dangerously high voltage.