| 摘要 |
PROBLEM TO BE SOLVED: To eliminate the trouble that occurs when, after oscillation is stopped by applying a high voltage to an output, the oscillation is restarted in a synchronous rectifying circuit-type switching power supply wherein signals from a commutation FET is excited by a secondary winding of an output choke. SOLUTION: The synchronous rectifying-type forward converter converts a direct-current input voltage into a rectangular-wave pulse voltage through a switching element, and applies the voltage to a primary winding of an output transformer, rectifies and smoothes a secondary winding-side output of the output transformer through an output-side synchronous rectifying circuit comprising a synchronous rectifying FET, the commutation FET, a choke coil, a capacitor and the like, and outputs a direct-current voltage. With respect to a power supply received from a secondary winding of the output choke as a gate signal of the commutation FET, when a high voltage is applied to the output and oscillation is stopped, the stop of the oscillation is detected and the signals from the commutation FET are interrupted and turned off. When the high voltage is applied to the output to cause a control circuit to stop the oscillation and a signal from the secondary winding of the output choke is longer in time than ordinary time, the length of the time is detected and the signals from the commutation FET are interrupted to eliminate energy stored in the output choke. Thus an abnormal voltage is prevented from being produced at the restart.
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