| 摘要 |
A flyback converter uses primary side sensing to sense the output voltage for regulation feedback. Such sensing requires a predetermined minimum duty cycle even with very light load currents. Therefore, such a minimum duty cycle may create an over-voltage condition. In the flyback phase, after a minimum duty cycle of the power switch at light load currents, a synchronous rectifier turns off approximately when the current through the secondary winding falls to zero to create a discontinuous mode. If it is detected that there is an over-voltage, the synchronous rectifier is turned on for a brief interval to draw a reverse current through the secondary winding. When the synchronous rectifier shuts off, a current flows through the primary winding via a drain-body diode while the power switch is off. Therefore, excess power is transferred from the secondary side to the power source to reduce the over-voltage so is not wasted.
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