A simple method for improving the input power factor of a single-phase online UPS (uninterruptible power supply) by making the DC-bus capacitance as small as possible is proposed. A feedforward modulation scheme is provided for solving the rapid DC-bus voltage fluctuation problem due to insufficient filtering. A discrete-time model of the UPS with the proposed scheme and a design technique for the compensators are also developed. Finally, a laboratory prototype 1-kVA single-phase online UPS is set up to experimentally verify the predicted and analyzed results. The proposed UPS features a high input power factor, fast transient response, high efficiency, light weight, and small size.