An AC-DC converter 1 has, e.g.: a primary winding 11 to which an alternating-current input voltage Vi is applied; a secondary winding 12 magnetically coupled with the primary winding 11; a bidirectional switch 20 connected in series with the primary winding 11; a resonance capacitor 30 connected in parallel with at least one of the bidirectional switch 20 and the primary winding 11; a resonance inductance component (e.g., a coil 120); a full-wave rectification circuit 40 performing full-wave rectification on an induced voltage appearing in the secondary winding 12; and a smoothing capacitor 50 smoothing the output of the full-wave rectification circuit 40. The alternating-current input voltage Vi is converted directly into a direct-current output voltage Vo, with both a forward voltage and a flyback voltage extracted from the secondary winding 12. The converter 1 further has a switch 200 switching the number of turns of the primary winding.