A DC-DC converter includes: a clamp capacitor having one terminal connected with a ground terminal of a voltage source; a switching circuit including first and second switches connected with each other in series between a positive terminal and the ground terminal of the voltage source, and third and fourth switches connected with each other in series between both terminals of the clamp capacitor; and a forward-flyback transformer including a plurality of primary coils connected between a first connection node between the first and second switches and a second connection node between the third and fourth switches.