PURPOSE:To perform stable DC voltage conversion without any reaction of a switch by driving a switching element by the output of a blocking oscillating circuit whose pulse width and pulse halt period are settable independently. CONSTITUTION:The conduction period of a switching transistor TR4 is determined nearly by the inductance of the primary winding 13a of a blocking oscillating circuit 3, the current amplification factor of the TR14, and the value of a resistance 17 for pulse control. The off period of the TR4 is determined nearly by a time constant determined by a resistance 15 and a capacitor 16, and the value of a resistance for break-period control. Therefore, only the pulse width of the circuit is varied by the set value of the resistance 17 without changing the basic operation conditions of the circuit 3, and any evil influence is exerted upon the process of determining a break period. Further, the break period is settable independently by the set value of the resistance 20 without exerting any influence upon the setting conditions of the pulse width. Then, output control is exerted easily by making the impedance of the resistances 17 and 20 variable and by controlling it.
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