Patent ID: 7262977

Claim:
A switching-mode power supply for DC-to-DC conversion, comprising: (A) DC input means for inputting a DC voltage; (B) DC output means for outputting a DC voltage; (C) inductance means connected between the DC input means and the DC output means; (D) switching means including an active switch connected between the DC input means and the inductance means for switching the DC input voltage on and off; (E) an active switch control circuit connected to the active switch for on/off control thereof; (F) a smoothing capacitor connected between the inductance means and the DC output means; (G) a synchronous rectifier switch connected in parallel with a serial connection of the inductance means and the smoothing capacitor; (H) a rectifying diode connected in parallel with the synchronous rectifier switch; and (I) a synchronous rectifier control circuit connected to the synchronous rectifier switch for on/off control thereof, the synchronous rectifier control circuit comprising: (a) a synchronous rectification period determination capacitor for providing a voltage for determination of periods of time during which the synchronous rectifier switch is to conduct; (b) a first current source connected to the synchronous rectification period determination capacitor for charging the same with a current corresponding to a voltage across the inductance means during the conducting periods of the active switch; (c) a second current source connected to the synchronous rectification period determination capacitor for permitting the flow of a discharge current corresponding to the DC output voltage; (d) active switch status means for providing an active switch status signal indicative of whether the active switch is on or off; (e) conduction period detect means connected to the active switch status means for providing, in response to the active switch status signal, a signal indicative of periods of time during which the synchronous rectifier switch is to be held conductive; and (f) a synchronous rectifier switch control pulse forming circuit having inputs connected to the synchronous rectification period determination capacitor and the conduction period detect means, and an output connected to the synchronous rectifier switch, for causing conduction through the synchronous rectifier switch when the voltage across the synchronous rectifier period determination capacitor is higher than a prescribed value and at the same time when the signal from the conduction period detect means indicates that the synchronous rectifier switch be held conductive.