Patent ID: 7729136

Claim:
An isolated DC-DC converter, comprising: a main transformer including at least a primary winding and a secondary winding; a first switching device arranged to switch an input voltage applied from an input power supply to the primary winding of the main transformer; a PWM control circuit arranged to output a pulse for switching having an on-duty ratio in accordance with an input voltage of a feedback signal input terminal; and a rectifying and smoothing circuit arranged to rectify and smooth a voltage induced in the secondary winding of the main transformer; wherein the input voltage of the feedback signal input terminal of the PWM control circuit is set such that the on-duty ratio of the pulse signal for switching becomes a maximum or approximately a maximum, and the PWM control circuit includes: an on-period control circuit arranged to detect an output voltage of the rectifying and smoothing circuit and to generate, when the output voltage exceeds a reference voltage, an off-timing signal which is a timing signal for causing the first switching device to be turned off; a pulse signal transmission unit arranged to transmit the off-timing signal to a primary side in an isolated state; a signal reception/power switch driving circuit arranged to cause the first switching device to be turned on based on the pulse signal for switching that is output from the PWM control circuit and to cause the first switching device to be turned off based on the off-timing signal transmitted by the pulse signal transmission unit; and a booster circuit arranged to boost a power supply voltage for the signal reception/power switch driving circuit with the pulse signal for switching that is output from the PWM control circuit.