Proposta, análise e implementação de técnicas de comutação sob tensão nula aplicadas a fontes cc-cc isoladas para telecomunicações

Abstract

Dc-dc isolated converters with Zero-Voltage-Switching are proposed and studied in this thesis. Special attention is focused on converters with series switches connections, which allow the use of switches designed for half of dc bus voltage. A new modulation entitled as interleaved-Phase-Shift-Modulation is proposed, which is applied to two half-bridges converter connected in series. The main characteristics of the proposed modulation in relation to the pulse-width-Modulation are the input capacitors voltage equalization and the switches currents symmetrical sharing. This way, low frequency voltage oscillations are eliminated, making possible the implementation of faster control law, and better switches utilization can be obtained, producing higher efficiency. It is proposed an optimization process to design auxiliary circuits of low cost which perform Zero-Voltage-Switching in entire load range. The utilized circuit is composed of inductors connected in series and in parallel with the transformer(s). The optimization method is applied to half-bridge converters connected in series with soft commutation in entire load range with different modulations, and to the fullbridge isolated converter. By the use of the optimization process, it is obtained the set of series and parallel inductances values, which maximize the converters efficiency. Theoretical and experimental results obtained from the converters with output of 60V/25A demonstrate that converters with optimized designs can present efficiency even 2% higher than converters with non-optimized designs. Moreover, it is demonstrated that by utilizing an optimization process, converters with soft commutation in entire load range can present efficiency higher than converter with soft commutation in only a load range, despite the inclusion of auxiliary shunt inductors. An alternative for the use of phase-shift-modulation in half-bridge converter connected in series is presented. The proposed converter utilizes a three-level boost rectifier, which performs the input current power factor correction and maintains the dc bus voltage equally shared. The series converter, with phase-shift modulation is connected on the rectifier output. The main advantages of the phase-shift modulation utilization in relation to the pulsewidth- modulation are the symmetrical current share through the switches and the smaller current peaks on auxiliary circuits, resulting in smaller reactive energy in the converter and higher efficiency. A comparative study among the full-bridge converter and converters with series arrangement of half-bridges with different modulations is performed, considering the efficiency and the cost of the converters as parameters. As a result it can be observed the best performance and smaller cost of the full-bridge converter for 400V applications, while for 800V applications, the series converters present higher efficiency and smaller cost, considering the actual technology.