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High-precision control of a single-phase PWM inverter for constant voltage and constant frequency applications, such as uninterruptible power supplies, is presented. The proposed control employs a capacitor current feedback with a phase-locked loop (PLL) compensator that minimises the steady-state error of the output voltage. The PLL compensator utilises the capacitor voltage and current with a phase difference of 90°. This concept is similar to the dq transform of the three phase variables, and thus the error of the output voltage can be easily minimised by using a proportional-integral type controller. A computer simulation and experiment have been carried out for the actual single-phase PWM inverter controlled by a digital signal processor, and the results clearly demonstrate the effectiveness of the proposed control.