HIGH DENSITY POWER CONVERSION ELECTRONICS ENABLED BY GAN-BASED MODULAR TOPOLOGIES by Ansel Barchowsky B.S. in Electrical Engineering, University of Pittsburgh, 2012 M.S. in Electrical Engineering, University of Pittsburgh, 2014 Submitted to the Graduate Faculty of Swanson School of Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2017 UNIVERSITY OF PITTSBURGH SWANSON SCHOOL OF ENGINEERING This dissertation was presented by Ansel Barchowsky It was defended on March 31st, 2017 and approved by Brandon Grainger, PhD, Professor Department of Electrical and Computer Engineering Zhi-Hong Mao, PhD, Professor Department of Electrical and Computer Engineering Paul Ohodnicki, PhD Department of Energy National Energy Technology Laboratory William Stanchina, PhD, Professor Department of Electrical and Computer Engineering Dissertation Director: Dr. Gregory Reed, PhD, Professor Department of Electrical and Computer Engineering ii Copyright © by Ansel Barchowsky 2017 iii HIGH DENSITY POWER CONVERSION ELECTRONICS ENABLED BY GAN- BASED MODULAR TOPOLOGIES Ansel Barchowsky, Ph.D. University of Pittsburgh, 2017 This dissertation explores the use of modular multilevel converter (MMC) architectures, coupled with wide-bandgap semiconductors, to achieve high power-density in power electronics converters. At the converter level, the capabilities of the modular multilevel converter are investigated for their use in low voltage, low power, DC-DC and DC-AC applications. This investigation shows that the use of modular multilevel architectures enables low voltage Gallium Nitride high electron mobility transistors (GaN HEMTs) to be used in applications for which their voltage thresholds are not typically suited. This results in lightweight, compact, conversion systems. GaN HEMTs have been shown to provide a low loss, low volume alternative to Silicon transistors for power conversion, but require several enabling technologies to make them ideally suited to high-density converters. This work therefore presents two enabling technologies for GaN- based conversion circuits. First, a technique is developed that optimizes the gate resistance for driving GaN HEMTs in order to ensure safe, rapid device turn on. Next, the development of planar magnetic transformers is discussed, with a focus on high-frequency converter operation. For each of these technologies mathematical analysis, circuit simulation, and hardware development are performed and compared to ensure proper functionality. iv Taking advantage of those two enabling technologies, two converter architectures based on the MMC structure are developed. First, a DC-AC MMC is presented, taking advantage of GaN HEMTs and minimal filtering requirements to achieve high power density in low voltage systems. Next, that topology is extended and a novel DC-DC converter based on two coupled DC-AC MMCs is presented. Both systems are described mathematically, simulated, and developed as hardware prototypes to prove functionality. While both converter systems are relevant for applications in DC microgrids, the DC-AC converter will be specifically investigated for its application as a variable speed drive in naval power systems. Likewise, the DC-DC MMC will be shown to provide new solutions for high voltage spacecraft power systems. Based on the work presented in this dissertation, engineers will be presented with alternatives to traditional methods of achieving high density in power conversion systems. By coupling the low filtering requirements and low losses of the modular multilevel converter with low voltage, highly efficient GaN HEMTs, the presented converter systems achieve high power density and efficiency with minimal filtering requirements. The result of this work is two novel converter systems that will enable further research into lightweight, low volume, power conversion. v TABLE OF CONTENTS ACKNOWLEDGEMENTS .................................................................................................... XIII 1.0 INTRODUCTION ........................................................................................................ 1 1.1 OBJECTIVE ........................................................................................................ 2 1.2 ORGANIZATION ............................................................................................... 8 2.0 BACKGROUND .......................................................................................................... 9 2.1 HIGH DENSITY MULTILEVEL POWER ELECTRONICS ....................... 9 2.2 WIDE BANDGAP POWER SEMICONDUCTORS ...................................... 16 2.3 FERRITE MAGNETIC MATERIALS ........................................................... 20 2.4 SUMMARY ........................................................................................................ 24 3.0 GATE DRIVE OPTIMIZATION OF GAN HEMTS ............................................. 25 3.1 PROBLEM CONTEXT: OVERSHOOT IN GAN HEMTS ......................... 26 3.1.1 Risk of Gate Overshoot in GaN HEMTs ..................................................... 26 3.1.2 Effects of Gate Resistance on Turn On Transients .................................... 27 3.2 ANALYTICAL TURN ON MODEL FOR GAN HEMTS ............................ 30 3.3 EXPERIMENTAL VALIDATION WITH DOUBLE PULSE TEST CIRCUIT ............................................................................................................ 35 3.4 CHAPTER SUMMARY ................................................................................... 42 4.0 PCB INTEGRATED PLANAR TRANSFORMERS .............................................. 44 4.1 CORE SIZE DETERMINATION AND MATERIAL PARAMETER EXTRACTION .................................................................................................. 47 vi 4.2 FINITE ELEMENT MODELING OF TRANSFORMER DESIGNS ......... 54 4.3 PRINTED CIRCUIT BOARD IMPLEMENTATION .................................. 66 4.4 CHAPTER SUMMARY ................................................................................... 69 5.0 GAN-BASED DC-AC MODULAR MULTILEVEL CONVERTERS ................. 70 5.1 THEORY OF OPERATION ............................................................................ 70 5.2 SYSTEM SIMULATION ANALYSIS ............................................................ 75 5.3 APPLICATION OF DC-AC MMCS AS NAVAL VARIABLE SPEED DRIVES .............................................................................................................. 81 5.4 HARDWARE IMPLEMENTATION .............................................................. 84 5.4.1 Submodule PCB Design and Evaluation ..................................................... 85 5.4.2 MMC Arm PCB Design and Evaluation ..................................................... 90 5.5 CHAPTER SUMMARY ................................................................................... 97 6.0 GAN-BASED DC-DC MODULAR MULTILEVEL CONVERTERS ................. 98 6.1 THEORY OF OPERATION .......................................................................... 100 6.2 SYSTEM SIMULATION ANALYSIS .......................................................... 107 6.3 APPLICATION OF DC-DC MMCS FOR ANODE DISCHARGE POWER MODULES IN SPACECRAFT SOLAR ELECTRIC PROPULSION...... 113 6.4 HARDWARE IMPLEMENTATION ............................................................ 120 6.5 CHAPTER SUMMARY ................................................................................. 124 7.0 CONCLUSION ......................................................................................................... 126 8.0 BIBLIOGRAPHY .................................................................................................... 131 vii LIST OF TABLES Table 1. Characteristic parameters of Ferroxcube 3F4 ................................................................. 23 Table 2. Semiconductor and IC Components ............................................................................... 36 Table 3. System and Passive Parameters ...................................................................................... 36 Table 4. Transformer parameters for power converter development ........................................... 46 Table 5. Extracted core parameters for Ferroxcube 3F4 MnZn ferrite ......................................... 50 Table 6. Extracted equivalent circuit values for series planar transformer design ....................... 65 Table 7. Extracted equivalent circuit values for parallel planar transformer design .................... 65 Table 8. Extracted parameters from E43/10/28 prototype transformer ........................................ 68 Table 9. Ratings for the DC-AC MMC system ............................................................................ 78 Table 10. VSD system input at various frequencies ..................................................................... 82 Table 11. Prototype test conditions ............................................................................................... 95 Table 12. DC-DC MMC circuit parameters ............................................................................... 108 Table 13. NASA roadmap converter specifications ................................................................... 116 Table 14. Submodule integrated circuits for the developed test system ..................................... 122 Table 15. Parameters of MHz-range test converter .................................................................... 123 viii LIST OF FIGURES Figure 1. Half-bridge submodule for modular multilevel converter design ................................. 11 Figure 2. Full bridge submodule for modular multilevel converter design .................................. 11 Figure 3. Switching states of a half-bridge modular multilevel converter submodule ................. 12 Figure 4. Three-phase modular multilevel converter topology .................................................... 13 Figure 5. Device structure for a vertical SiC MOSFET................................................................ 17 Figure 6. Device structure for a lateral GaN HEMT .................................................................... 18 Figure 7. Specific Power Loss of Ferroxcube 3F4 MnZn Ferrite ................................................. 22 Figure 8. Complex Permiability of Ferroxcube 3F4 MnZn Ferrite .............................................. 22 Figure 9. Circuit layout of a resistor-protected GaN HEMT illustrating device parasitics .......... 28 Figure 10. Commonly observed turn on characteristics for a generic device, where RG1 < RG2 < RG3 ............................................................................................................................. 29 Figure 11. Equivalent circuit for sub-stage 1 of device turn on.................................................... 31 Figure 12. Equivalent circuit for sub-stage 2 of device turn on.................................................... 31 Figure 13. Equivalent circuit for sub-stage 3 of device turn on.................................................... 31 Figure 14. Double pulse test circuit for turn on measurements .................................................... 36 Figure 15. Double pulse test PCB for verification of RG selection.............................................. 37 Figure 16. Un-damped measurement of vGS vs t for calculation of Leq ..................................... 38 Figure 17. Turn on of EPC8010 with RG = 10 Ω ......................................................................... 39 Figure 18. Turn-on of EPC8010 with RG = 5 Ω .......................................................................... 40 Figure 19. Turn-on of EPC8010 with RG = 1 Ω .......................................................................... 40 Figure 20. Predicted and measured values for for vGS vs RG ..................................................... 41 ix Figure 21. PCB Integrated Planar Transformer [25] .................................................................... 45 Figure 22. Matched core loss data for Ferroxcube 3F4 MnZn ferrite .......................................... 50 Figure 23. Current densities for various winding cases and interleaving patterns in A/cm3 ....... 56 Figure 24. Solidworks drawing of transformer primary winding ................................................. 57 Figure 25. Solidworks drawing of trasnformer secondary winding ............................................. 58 Figure 26. Solidworks drawing of paired E43/10/28 cores .......................................................... 58 Figure 27. 3D model of planar transformer core with PCB integrated parallel windings ............ 59 Figure 28. Primary (low-voltage) voltage and current waveforms with 200 V sinusoidal excitation and 2 kW resistive load ............................................................................................... 60 Figure 29. Secondary (high-voltage) voltage and current waveforms with 200 V sinusoidal excitation and 2 kW resistive load .............................................................................. 60 Figure 30. Primary (low-voltage) voltage and current waveforms with 200 V square excitation and 2 kW resistive load...................................................................................................... 61 Figure 31. Secondary (high-voltage) voltage and current waveforms with 200 V square excitation and 2 kW resistive load ............................................................................................... 62 Figure 32. Highest flux point in the core when excited by 200 V square wave ........................... 63 Figure 33. Lowest flux point in the core when excited by 200 V square wave ............................ 63 Figure 34. 3D model of planar transformer core with PCB integrated series windings ............... 64 Figure 35. Equivalent circuit model for planar transformer ......................................................... 66 Figure 36. Prototype transformer PCB with layer-integrated windings ....................................... 67 Figure 37. Single-phase MMC Topology ..................................................................................... 71 Figure 38. Phase shifted pulse width modulation signal generation and output ........................... 73 Figure 39. Overview of modeled single phase MMC system ....................................................... 75 Figure 40. AC output THD as a function of switching frequency ................................................ 77 Figure 41. SM voltage ripple as a function of switching frequency ............................................. 77 Figure 42. Simulated AC ouptut voltage of 2 kW DC-AC MMC ................................................ 79 Figure 43. Simulated AC output current for 2 kW DC-AC MMC ............................................... 80 x