Switching power supply unit and electric power supply system

A switching power supply unit comprising:

• a pair of input terminals configured to receive an input voltage;
• a pair of output terminals configured to output an output voltage;
• a transformer including a primary winding and a secondary winding;
• an inverter circuit disposed between the pair of input terminals and the primary winding, and including:
• first to fourth switching devices coupled in series to each other in this order between the pair of input terminals;
• a first connection point between the first switching device and the second switching device;
• a second connection point between the third switching device and the fourth switching device;
• a third connection point between the second switching device and the third switching device;
• a first capacitor disposed between the first connection point and the second connection point;
• a resonant inductor;
• and
• a resonant capacitor, the resonant inductor, the resonant capacitor, and the primary winding being coupled in series to each other between the third connection point and one of the pair of input terminals;
• a rectifying and smoothing circuit disposed between the pair of output terminals and the secondary winding, and including a rectifying circuit and a smoothing circuit, the rectifying circuit including two or more rectifying devices, the smoothing circuit including a second capacitor;
• a switching cycle in the switching driving including:
• a first ON period during which both the first switching device and the second switching device are to be on;
• a second ON period during which both the third switching device and the fourth switching device are to be on;
• a first dead time during which both the first switching device and the fourth switching device are to be off;
• and
• a second dead time during which both the second switching device and the third switching device are to be off;
• and
• a driver configured to:
• perform switching driving to control respective operations of the first to fourth switching devices in the inverter circuit;
• and
• control a value of the output voltage by preventing the first ON period and the second ON period from overlapping each other in the switching cycle and by adjusting each of a duty ratio of the first ON period and a duty ratio of the second ON period to the switching cycle.