Switching power supply unit

A switching power supply unit comprising:

• a switching circuit including first, second, third and fourth switching elements each having a pair of electrodes, wherein respective ones of the main electrodes of said first and second switching elements are connected to each other to form a first connection point, respective ones of the main electrodes of said third and fourth switching elements being connected to each other to form a second connection point, the other main electrode of said first switching element and the other main electrode of said third switching element being connected to each other to form a third connection point, and the other main electrode of said second switching element and the other main electrode of said fourth switching element being connected to each other to form a fourth connection point;
• a pair of DC voltage input ends connected to said third and fourth connection points so as to allow a DC voltage to be applied with a unidirectional polarity between said pair of main electrodes in each of said switching elements;
• a diode disposed between said main electrodes in parallel with each of said first, second, third and fourth switching elements to have a polarity opposite to said polarity;
• a capacitor disposed between said main electrodes in parallel with each of the first, second, third and fourth switching elements;
• a transformer including first and second windings, wherein said first winding is connected between said first and second connection points to generate a switching output at said second winding in response to a switching operation of said switching element in said switching circuit;
• an output circuit for rectifying and smoothing the switching output yielded at said second winding of said transformer to generate an output voltage;
• a control circuit for operating said switching elements to generate the switching output at said second winding of said transformer, and controlling a switching phase-difference between said first and fourth switching elements or a switching phase-difference between said second and third switching elements to stabilize said output voltage of said output circuit;
• and
• an inductor inserted in a path ranging from said first connection point through said first winding to said second connection point to form a resonance circuit in conjunction with said capacitor, wherein
• the inductance value of said inductor is arranged to provide the output voltage of said output circuit larger than or equal to a rated value when the input voltage has a minimum value, the output current having a maximum value and said switching phase-difference being zero.