Patent ID: 8717781

Claim:
A direct current (DC)/DC converter, comprising: a transformer, comprising a primary coil having one terminal applied with an input voltage, a secondary coil, and an auxiliary coil disposed on the side of the primary coil; a first output capacitor, having one terminal with a fixed electric potential, and the other terminal connected to an output terminal; a first diode, having a cathode facing the first output capacitor and disposed between the other terminal of the first output capacitor and one terminal of the secondary coil; a switch transistor, disposed in a path of the primary coil; a second output capacitor, having one terminal with a fixed electric potential; a second rectifier element, having a cathode facing the second output capacitor and disposed between the other terminal of the second output capacitor and one terminal of the auxiliary coil; and a control circuit, for controlling the on/off of the switch transistor; wherein the control circuit comprises: a power supply terminal, connected to the other terminal of the second output capacitor; a high voltage terminal, for inputting the input voltage; a charging transistor, being a N-channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET) that is disposed between the high voltage terminal and the power supply terminal, and applied with a bias so that the charging transistor is normally on; and a current limiting circuit, for limiting a charging current flowing from the high voltage terminal through the charging transistor to the power supply terminal in a first state in which the voltage of the power supply terminal is lower than a specified first threshold voltage, and lowering the charging current substantially to zero in a second state in which the voltage of the power supply terminal is higher than a second threshold voltage that is specified to be higher than the first threshold voltage.