PURPOSE: To allow provision of a specific output voltage by providing a switching circuit forming the main circuit of a converter along with a transformer, and a circuit for selecting a switching element depending on the input voltage.

CONSTITUTION: Positive pole side of an input voltage V is branched through switching elements 11, 12 and diodes D1, D2 into terminals T11, T12 of a transformer T having terminal Tc connected through a switching circuit 13 with the negative polse side of the input voltage V. It is further branched to a selecting circuit 14 having output connected individually with the switching elements 11, 12. An output voltage Vo is produced from the output winding N2 of the transformer T through terminals T21, T22. When the input voltage V is applied to the input winding N1 of the transformer T, the switching circuit 13 interrupts primary current flowing through the input winding N1 by means of a control circuit and feeds a specific output voltage Vo to the output winding N2.