Switching power supply unit with wide input voltage range

摘要

A switching power supply unit is provided, in which widening of the input voltage (Vin) range can be achieved while suppressing production of a surge current. A transformer (3) having two primary windings (31A,31B) having the number of turns equal to each other, and two inductors (Lr1,Lr2) are provided correspondingly to two switching circuits (1,2) of the full bridge type. By using an input voltage detection circuit (61), a control section (7), and connection changeover switches, (S5,S6) perform connection changeover such that a relative period of parallel connection state and a relative period of series connection state are changed depending on a level of the input DC voltage, respectively.