| 摘要 |
PROBLEM TO BE SOLVED: To obtain stable operation even if a voltage of supplied electric power drops. SOLUTION: The switching of an excited current that flows in the primary winding side of a transformer is controlled by FETs 12, 13 connected to a resonance circuit consisting of a resonance inductor 14 and a resonance capacitor 16. By reducing an operating frequency when an input voltage Vin drops and by increasing the frequency when the input voltage Vin increases, the control is effected so that a desired output voltage Va can be obtained from the secondary winding side 15sa. When it is detected that the input voltage Vin has dropped lower than a prescribed voltage at a frequency varying portion 60, the frequency of an oscillating signal Sf that is outputted from an oscillating part 42 is raised according to the drop of the input voltage Vin. The stable operation can be effected by preventing the operating frequency from dropping lower than the resonance frequency of the resonance circuit. The abrupt strain of electric power supplied to a load is preventable, because the operating frequency is raised according to the drop of the input voltage Vin.
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