Patent ID: 8315073

Claim:
An isolated switching power supply device having a primary circuit side and a secondary circuit side, comprising: a main transformer including a primary coil located on the primary circuit side and a secondary coil located on the secondary circuit side so as to transmit power from the primary circuit side to the secondary circuit side; at least one power switch connected to the primary coil of the main transformer in series and arranged to switch a current flowing to the primary coil of the main transformer from a DC input power supply; a rectifier circuit arranged to rectify the voltage generated in the secondary coil of the main transformer; a smoothing circuit arranged to smooth the voltage rectified by the rectifier circuit; a voltage clamping circuit including a clamp switch and arranged to absorb a surge voltage generated when the voltage applied to the main transformer is reversed, and thus clamping the voltage applied to the main transformer to a predetermined upper limit value; a switching control circuit including a first output terminal arranged to output a signal to drive the power switch and a second output terminal arranged to output a signal for driving the clamp switch; a first pulse transformer including a primary coil connected to the second output terminal and a secondary coil, the first pulse transformer arranged to transmit an edge signal of a pulsed signal for driving the clamp switch; and a clamp switch driving circuit connected to the secondary coil of the first pulse transformer and arranged to generate a clamp switch driving signal using the edge of the signal to drive the clamp switch outputted from the first pulse transformer as a trigger, the clamp switch driving signal arranged to control the on/off states of the clamp switch.