NTT has developed a small, lightweight, high-efficiency, and high-power-factor power supply system for fiber-optic subscriber loop transmission systems. The AC-DC converter in the power supply is a flyback converter that can correct the power factor and convert 100 V AC to -48 V DC in only one stage. The power supply has a volume of about 3700 cc, which is one-third that of the conventional power supply. The efficiency of the AC-DC converter is 90.4% at full load and the power factor is 0.995.