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This paper proposes a DSP-controlled transformerless common-neutral single-phase on-line UPS with balanced charging/discharging control. All the control functions for an on-line UPS, which includes power-on start-up control, input stage power factor control, battery charging and boosting control, output stage AC voltage regulation, and shut-down control, were realized by using a single-chip DSP controller. A multiple rate digital controller generates all the PWM control signals for the power stage by using a set of detected feedback signals. Software current control scheme with nonlinear pulsewidth compensation has been developed to eliminate the nonlinearity caused by the dead-lock protected PWM converters. A nonlinear digital control scheme with lower switching frequency has been developed for the power factor control of the AC-DC converter. Computer simulation and experimental results have been given to verify the proposed digital control scheme. The constructed DSP-controlled UPS system can achieve fast dynamic response for nonlinear loads and high power factor under various loading conditions.