| 摘要 |
PROBLEM TO BE SOLVED: To suppress noises due to the high-frequency components of a main control signal of a main switching element of a quasi-resonant switching power supply unit, by varying an operation frequency of the main switching element. SOLUTION: The switching power supply unit is provided with a primary winding (2a) of a transformer (2), connected in series with a DC power supply unit (1), and a main MOS-FET (3); a voltage resonant capacitor (12), connected in parallel to the main MOS-FET (3); a rectifying smoothing circuit (4), having a rectifying MOS-FET (51) and a smoothing capacitor (6) and connected to a secondary winding (2b) of the transformer (2); a synchronous rectifying control circuit (52) switching on the rectifying MOS-FET (51), after turning off the main MOS-FET (3) and switching off the rectifying MOS-FET (51), after turning on the main MOS-FET (3); a pulse width extending circuit (55) for extending the on-pulse width of the rectifying MOS-FET (51); and a main control circuit (8) for switching on and off the main MOS-FET (3) at an operation frequency, corresponding to the on-pulse width of the rectifying MOS-FET (51) extended by the pulse width extending circuit (55). COPYRIGHT: (C)2009,JPO&INPIT
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