DC POWER SUPPLY UNIT

PURPOSE:To form a device small in size and light in weight with high density by setting a reference signal to a pulse width modulation circuit forcedly to fixed voltage higher than a peak value of a chopping wave signal in the point of riseup time of output voltage of the pulse width modulation circuit. CONSTITUTION:A pulse waveform generating circuit output signal VS rises, thereafter a pulse width modulation circuit output voltage VD rises up simultaneously transferring a switching element 2 to an ON-condition, and a select switch output voltage VTS becomes a fixed voltage generating circuit output signal VC. At this time, even in the case that a value of the select switch output voltage VTS is fluctuated in a ringing state and converged at time t3 by an electromagnetically inductive or conductive noise generated from the switching element 2, high frequency transformer 3, rectifier circuit 4, etc., a phase compensation circuit output voltage Ve obtains a sufficiently low value relating to the select switch output voltage VTS, so that a condition after the riseup is stably maintained in the pulse width modulation circuit output voltage VD.