SWITCHING POWER SUPPLY DEVICE

PROBLEM TO BE SOLVED: To provide a switching power supply device capable of operating respective switching elements independently of a level of an input DC voltage by using a low-voltage transformer.SOLUTION: A switching power supply device has a pulse generation circuit outputting a first (a second) pulse signal depending on a switching state of a switching element Q11 (Q12). A current having a phase delayed by 90° from the first pulse signal flows in a first series resonance circuit L1, C1 to which the first pulse signal is applied. A switching element Q21 is on/off controlled by the current of the first series resonance circuit. A current having a phase delayed by 90° from the second pulse signal flows in a second series resonance circuit L2, C2 to which the second pulse signal is applied. A switching element Q22 is on/off controlled by the current of the second series resonance circuit. Therefore, a phase of an operation of a second converter 4 is shifted by 90° from that of the first converter 3. The pulse generation circuit has a third transformer T3 having a secondary coil Na3 outputting the first pulse signal and a secondary coil Na4 outputting the second pulse signal, and to which a voltage synchronized with a driving signal for the switching elements Q11 and Q12 is applied.