Silicon Carbide Devices in High Efficiency DC-DC Power Converters for Telecommunications Rory Brendan Shillington A thesis submitted for the degree of Doctor of Philosophy In Electrical and Electronic Engineering at the University of Canterbury, Christchurch, New Zealand. 2012 - ii - Abstract The electrical efficiency of telecommunication power supplies is increasing to meet customer demands for lower total cost of ownership. Increased capital cost can now be justified if it enables sufficiently large energy savings, allowing the use of topologies and devices previously considered unnecessarily complex or expensive. Silicon carbide Schottky diodes have already been incorporated into commercial power supplies as expensive, but energy saving components. This thesis pursues the next step of considering silicon carbide transistors for use in telecommunications power converters. A range of silicon carbide transistors was considered with a primary focus on recently developed, normally-off, junction field effect transistors. Tests were devised and performed to uncover a number of previously unpublished characteristics of normally-off silicon carbide JFETs. Specifically, unique reverse conduction and associated gate current draw relationships were measured as well as the ability to block small reverse voltages when a negative gate-source voltage is applied. Reverse recovery-like characteristics were also measured and found to be superior to those of silicon MOSFETs. These characteristics significantly impact the steps that are required to maximize efficiency with normally-off SiC JFETs in circuits where synchronous rectification or bidirectional blocking is performed. A gate drive circuit was proposed that combines a number of recommendations to achieve rapid and efficient switching of normally-off SiC JFETs. Specifically, a low transient output impedance was provided to achieve rapid turn-on and turn-off transitions as well as a high dc output impedance to limit the steady state drive current while sustaining the turned-on state. A prototype circuit was constructed using building blocks that are typically found in single chip MOSFET drivers. The circuit was shown to operate well from a single supply, alleviating the need for a split supply such as that required by many published JFET drive circuits. This demonstrated a proof of concept for a single chip JFET driver solution. An active power factor correction circuit topology was extensively modelled and a prototype designed and tested to verify the model. The circuit was able to operate at - iii - switching frequencies in excess of 100kHz when using SiC JFETs, whereas silicon MOSFETs could only achieve switching frequencies of several kHz before switching losses became excessive. The circuit was designed as the dc equivalent for a 2kW, 230V AC input power converter with a split +/-400V dc output. A commercial single phase telecommunications power converter was modified to utilise normally-off SiC JFETs in its power factor correction circuit. The converter was tested and found to achieve similar electrical efficiency with 1200V SiC JFETs to that achieved with 600V silicon MOSFETs. The performance of the 1200V SiC JFETs in this application was also compared to that of 900V silicon MOSFETs and found to be superior. Finally, a prototype three-phase cyclo-converter was modified to use 1200V normally- off SiC JFETs in place of 600V silicon MOSFETs and found to achieve similar electrical efficiency to the silicon MOSFETs in a 208V three phase system. These results strongly indicate that the 1200V SiC JFETs would provide better performance than 900V silicon MOSFETs in a 400V three phase system (that had been considered for commercial development). - iv - Acknowledgements I would like to thank a number of people without whom this work would not have been possible. Thank you to Dr Paul Gaynosr, my head supervisor, for always keeping an eye on the big picture, guiding me along the path towards gaining my PhD and ensuring that the project fulfilled my academic best interests. Thank you to Dr Bill Heffernan, my co- supervisor for sharing his technical expertise, academic experience and Oxford grammar skills with me. Thank you to Mr Michael Harrison, my co-supervisor and industry mentor for overseeing my day-to-day research, technical guidance and taking the time to share his wealth of industry experience with me. Thanks to Dr Phillip Hunter for taking over day-to-day supervision and mentoring duties in Michael's absence. I would like to thank all the engineers at Eaton who assisted me in many ways with my research, with a special mention going to Charles, Craig, Graeme, James, John, Keith, Nick, Sav, Singa and Tomasz. I would also like to thank the various academic, technical and administrative staff at the University of Canterbury who assisted me on many occasions, especially Alan, Helen, Ken, Mike, Nick, Pieter and Scott. I'd like to thank my family for their continuous encouragement, love, support, belief in my abilities and their occasional nudge in the right direction as well as assistance with proof reading. I'd also like to thank Hayden for all the love, support, humour and day- to-day encouragement along the way. Finally I would like to acknowledge the financial support that I received from the Foundation for Research Science and Technology under TIF IESL0801, without which this research would not have been possible. - v - - vi - Publications associated with this thesis Shillington, R., et al. “Applications of SiC JFETs in power converters”. 20th Australasian Universities Power Engineering Conference (AUPEC), 2010. Shillington, R., et al. “SiC JFET reverse conduction characteristics and use in power converters”, IET Power Electronics, vol: 5, no: 8, pp 1282-1290, September 2012. - vii - - viii - Contents ABSTRACT ...........................................................................................................III ACKNOWLEDGEMENTS...................................................................................... V PUBLICATIONS ASSOCIATED WITH THIS THESIS......................................... VII CHAPTER 1 INTRODUCTION............................................................................1 1.1 General ................................................................................................................................................ 1 1.2 Scope and outline of thesis.................................................................................................................. 2 CHAPTER 2 BACKGROUND.............................................................................5 2.1 Telecommunications power converters............................................................................................... 5 2.1.1 Market trends.................................................................................................................................. 5 2.1.2 Application requirements ............................................................................................................... 6 2.1.3 Potential for SiC transistors............................................................................................................ 7 2.2 Single Phase Active Power Factor Correction .................................................................................... 8 2.2.1 Boost converter based PFC topologies ........................................................................................... 9 2.2.2 Buck-boost based PFC topologies................................................................................................ 12 2.2.3 Conclusions .................................................................................................................................. 14 2.3 Specialized three-phase terminology................................................................................................. 14 2.4 Three-phase AC to DC power converters.......................................................................................... 15 2.4.1 Six primary-side bidirectional switch topology............................................................................ 15 2.4.2 Three primary-side bidirectional switch topology........................................................................ 18 2.4.3 Three-phase SEPIC rectifier with reduced transistors.................................................................. 20 2.5 Conclusion......................................................................................................................................... 21 - ix - CHAPTER 3 SIC TRANSISTORS .................................................................... 23 3.1 SiC MESFETS...................................................................................................................................23 3.2 SiC MOSFETS ..................................................................................................................................23 3.3 SiC BJTs............................................................................................................................................24 3.4 Normally-on SiC JFETs ....................................................................................................................25 3.5 Normally-off JFETs...........................................................................................................................27 3.6 Basic comparison of switching performance.....................................................................................28