A High Frequency Transformer Isolation 110V/220V Input Voltage UPS System René P. Torrico-Bascopé 1 , Demercil S. Oliveira Jr. 2 , Carlos G. C. Branco 3 , Fernando L. M. Antunes 4 , Cícero M. T. Cruz 5 Energy Processing and Control Group, Electrical Engineering Department Federal University of Ceará P.O. Box: 6001 – Campus of Pici - 60.455-760 - Fortaleza – CE - BRAZIL 1 rene@dee.ufc.br, 2 demercil@dee.ufc.br, 3 cgustavo@ieee.org, 4 fantunes@dee.ufc.br, 5 cicero@dee.ufc.br Abstract – This paper proposes a double conversion UPS system with power factor correction, high frequency transformer isolation and 110V/220V input voltage characteristics. It’s suitable for rack type structure because it has small size and reduced weight. For both input voltages, the proposed converter has almost the same efficiency processing the same output power. Other relevant features of this structure are: soft commutation of the controlled switches of the chopper and of the boost stage, simple control strategy that can be implemented with well-known integrated circuits and the use of few batteries in series due to step-up stage. Qualitative analysis and experimental results obtained from 2kVA laboratory prototype are presented. I. INTRODUCTION Nowadays uninterruptible power supply systems are being used to protect sensitive loads against a wide range of utility voltage disturbances and power outages. A large amount of these systems consists in the double conversion UPS configuration that operates normally with a low frequency isolation using a silicon-steel core transformer. Such transformer is placed at the input or at output depending on the topology arrangement. The addition of this magnetic component increases both weight and volume, and also add costs and difficulties in the transportation to the installation place. During 1990s, the evolution of semiconductors (diodes and transistors) and other components have allowed the development of devices with near-ideal characteristics, making research on UPS systems with high frequency isolation possible [1-6]. Several high frequency isolation UPS topologies were proposed in the literature and some of them are here analyzed. The UPS scheme shown in Fig.1 was studied in [1-2], and it consists of a power factor correction (PFC) current-fed full- bridge pre-regulator and a voltage source full-bridge inverter. In this circuit, hard commutation of the controlled switches worsens efficiency and many batteries placed in series connection are necessary to achieve the high DC bus voltage. Also, the current drawn by the battery bank is pulsed degrading the reability of the battery bank. The UPS shown in Fig. 2 was studied in [3]. The circuit is composes by, a modified PFC current-fed full-bridge pre- regulator and a voltage source full-bridge inverter, similar to the previous one. This topology has the advantages of reduced amount of semiconductors in series during power transfer, contributing for reduction of conduction losses and improving efficiency. The disadvantages are hard commutation of the controlled switches, and many batteries in series to achieve high DC bus voltage that feed the inverter. Fig. 3 shows the series-parallel resonant system with galvanic isolation between the input, the output and the battery proposed in [4]. This system has the advantages of power factor correction, single pre-regulator stage, soft commutation of the controlled switches and few batteries in series. On the other hand, the disadvantages are: the complex control strategy and resonant parameters adjustment. In [5], as shows Fig. 4, a two power conversion stages UPS was studied. The first stage consists of a PFC-DCM flyback converter with integrated battery charger, and the second stage consists of a boost inverter. Due to the discontinuous conduction mode operation of the flyback converter, the system is suitable for low power applications (<500W). T4 T6 C2 HF L1 L2 T1 C1 T7 T8 T3 T5 T2 Vi Vo Transf. Vbat Fig. 1. UPS system proposed in [1, 2]. T4 T6 C2 HF L1 L2 T1 C1 T7 T8 T3 T5 T2 Vi Vo Transf. Fig. 2. UPS system proposed in [3]. This work was supported by Brazilian Research and Projects Financing – FINEP and CNPQ. 362 0-7803-9547-6/06/$20.00 ©2006 IEEE.