In an isolated DC-DC converter, in response to a switching transistor connected to the primary winding of a transformer turning off, a turn-on control circuit turns on a synchronous rectification transistor on the secondary side. A turn-off control circuit determines the turn-off time point of the synchronous rectification transistor by charging a capacitor with a current commensurate with the magnitude of an induced voltage appearing in the secondary wiring of the transformer during the on-period of the switching transistor and, after the synchronous rectification transistor turns on, discharging the capacitor with a current commensurate with the output voltage of the DC-DC converter.
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