| 摘要 |
PROBLEM TO BE SOLVED: To improve the efficiency of a DC power supply, and to improve the reliability by preventing the failure of a switching element. SOLUTION: This DC power supply selects a power supply voltage appropriately with a control circuit 40 of unlimited on-duty current mode system by utilizing the characteristic that the threshold voltage Vt of an AND circuit 57 becomes approximately 1/2 the power supply voltage of the AND circuit 47, and then obtains at the input terminal of the AND circuit 57 a square wave output voltage of a constant width of an range which exceeds a triangular-wave voltage S40e obtainable from the control circuit 40. This triangular-wave voltage is differentiated with a capacitor 58 and a resistor 59 to extract a positive pulse voltage 860 with a diode 60. Then, the positive pulse voltage S60 is superposed on a current signal Vc proportional to the current flowing in the primary windings 23a of a transformer 23 and given to the control circuit 40. This structure prevents the on-duty of the control signal S40c for driving a field-effect transistor(FET) 24 from expanding to more than a fixed value.
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