In this paper is proposed a method to improve the tracking performance of repetitive controllers in Uninterruptible Power Supplies (UPS). The main idea is to simultaneously compensate the gain reduction and frequency shift caused by the introduction of a low-pass filter in series with the delay element. Also, an optimization problem based on LMI constraints is presented to determine controller gains that guarantee stability and performance of the closed loop system. Simulation results based on the output stage of 3.5kVA UPS demonstrate the effectiveness of the proposed method.