摘要 |
PROBLEM TO BE SOLVED: To provide a switching power supply unit which is of a simple two-stone forward type having a voltage of a switching element limited to not more than a constant value and capable of being properly operated even within the range of an on-time ratio of not less than 50%.SOLUTION: An input winding 44 of a transformer 42 is constituted by connecting an upper-stage winding 44(1), a middle-stage winding 44(2) and a lower-stage winding 44(3) in series. One end of the upper-stage winding 44(1) side is connected with an upper-side switching element 18a, and one end of the lower-stage winding 44(3) is connected with a lower-side switching element 18b. A first and a second diodes 26a, 26b are provided. In the first diode 26a, an anode terminal is connected with a connection point between the middle-stage winding 44(2) and the lower-stage winding 44(3), and a cathode terminal is connected with a plus-side line 22(+). In the second diode 26b, an anode terminal is connected with a minus-side line 22(-) and a cathode terminal is connected with a connection point between the upper-stage winding 44(1) and the middle-stage winding 44(2). |