Abstract: A PFC converter includes a bridge diode, a booster section controlled for improving a power factor of the bridge diode, a transformer section supplied with an output of the booster section via a primary coil side to excite it to a secondary coil side, a switch section for switching the output of the booster section, an output section for rectifying and smoothing the output of secondary coil side of the transformer section, a control section for sensing to feedback an output voltage of the output section and thus control the switch section, and a delay section operated for applying a stable voltage to the transformer section by controlling the booster section according to the output of the control section, so that the booster section is controlled for decreasing the harmonic current by the delayed output of the delay section to constantly maintain the voltage of a bulk condenser and thereby improve the power factor while controlling double stage PFC converter and DC--DC converter by the single phase.