A three-level DC-DC converter using zero voltage and zero current switching is provided to reduce a loss of electric conduction of a main circuit and a loss of a transformer by lowering a circulation current. A flying capacitor(Css) and auxiliary diodes(Dh1,Dh2) are connected in parallel between a contact point of a source of a switch(S1) and a drain of a switch(S2) and a contact point of a source of a switch(S3) and a drain of a switch(S4). Coupling inductors(Lc1,Lc2) are connected to a contact point of the auxiliary diodes and one terminal of a primary coil. One of the coupling inductors is connected to a cathode of the first auxiliary diode and an anode of the second auxiliary diode. An anode of the first diode is connected between the first and second switches, and a cathode of the second auxiliary diode is connected between the third and fourth switches.
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