Zero-voltage switching power supply device

PURPOSE:To reduce the size of a power supply by using the output capacitances of MOSFETs and the leakage inductance of a transformer for causing resonance without requiring any resonance capacitor or reactor thereby effecting zero voltage switching. CONSTITUTION:In a half bridge or full bridge converter employing a MOSFET as a switch element, resonance is caused by the output capacitances 6, 7 of MOSFETa 4, 5 and the leakage inductance 9 of a transformer 8. A control circuit 14 turns the MOSFETs 4, 5 OFF during partial resonance to switch the MOSFETs at zero voltage thus controlling the ON/OFF time ratio of the to a constant value.