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Uninterruptible power supplies (UPSs) are widely used to overcome instantaneous voltage drops and complete power failures. This paper proposes a novel single-phase standby UPS with a small size and high reliability. The proposed UPS consists of a buck-boost converter and a full-bridge inverter; this device eliminates the need for bulky electrolytic capacitors or output LC-filtering. The buck-boost converter regulates the output voltage, which is derived from a full-wave rectified waveform. A sinusoidal output voltage is obtained from the full-bridge inverter by polarity switching that is synchronized with the output voltage frequency. Experiments were carried out using a pilot UPS system with a 40-μF film capacitor and an output power of 500 W. The proposed UPS provided stable power to the load under all experimental conditions. The instantaneous interruption time of the proposed system was 1 ms when using a semiconductor-type alternating current switch (ACSW) and our proposed control method. Experimental results confirmed an interruption time of 1 ms when a power failure occurred.