GaN Transistors for Efficient Power Conversion GaN Transistors for Efficient Power Conversion Third Edition Alex Lidow Efficient Power Conversion Corporation (EPC) USA Michael de Rooij Efficient Power Conversion Corporation (EPC) USA Johan Strydom Kilby Labs Texas Instruments USA David Reusch VPT, Inc. USA John Glaser Efficient Power Conversion Corporation (EPC) USA This edition first published 2020 © 2020 John Wiley & Sons Ltd Edition History 1e 2012 Power Conversion Publications, 2e 2015 Wiley All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. 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Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging‐in‐Publication Data Names: Lidow, Alex, author. | de Rooij, Michael, author. | Strydom, Johan, author. | Reusch, David, author. | Glaser, John (Electrical engineer), author. Title: GaN transistors for efficient power conversion / Alex Lidow, Ph.D., Efficient Power Conversion Corporation (EPC), USA, Michael de Rooij, Ph.D., Efficient Power Conversion Corporation (EPC), USA, Johan Strydom, Ph.D., Kilby Labs, Texas Instruments, USA, David Reusch, Ph.D., VPT, Inc., USA, John Glaser, Ph.D., Efficient Power Conversion Corporation (EPC), USA. Description: 3rd edition. | Hoboken, NJ : John Wiley & Sons, Inc., 2020. | Includes bibliographical references and index. | Identifiers: LCCN 2019015122 (print) | LCCN 2019017751 (ebook) | ISBN 9781119594376 (Adobe PDF) | ISBN 9781119594420 (ePub) | ISBN 9781119594147 (hardback) Subjects: LCSH: Field-effect transistors–Materials. | Power transistors–Materials. | Gallium nitride. Classification: LCC TK7871.95 (ebook) | LCC TK7871.95 .G355 2020 (print) | DDC 621.3815/284–dc23 LC record available at https://lccn.loc.gov/2019015122 Cover Design: Wiley Cover Image: Courtesy of Efficient Power Conversion Corporation Set in 10/12pt Warnock by SPi Global, Pondicherry, India 10 9 8 7 6 5 4 3 2 1 In memory of Eric Lidow, the original power conversion pioneer. vii Contents Foreword xv Acknowledgments xvii 1 GaN Technology Overview 1 1.1 Silicon Power Metal Oxide Silicon Field Effect Transistors 1976–2010 1 1.2 The Gallium Nitride Journey Begins 2 1.3 GaN and SiC Compared with Silicon 2 1.3.1 Bandgap (E ) 3 g 1.3.2 Critical Field (E ) 3 crit 1.3.3 On‐Resistance (R ) 4 DS(on) 1.3.4 The Two‐Dimensional Electron Gas (2DEG) 4 1.4 The Basic GaN Transistor Structure 6 1.4.1 Recessed Gate Enhancement‐Mode Structure 7 1.4.2 Implanted Gate Enhancement‐Mode Structure 8 1.4.3 pGaN Gate Enhancement‐Mode Structure 8 1.4.4 Hybrid Normally Off Structures 8 1.4.5 Reverse Conduction in HEMT Transistors 10 1.5 Building a GaN Transistor 11 1.5.1 Substrate Material Selection 11 1.5.2 Growing the Heteroepitaxy 12 1.5.3 Processing the Wafer 12 1.5.4 Making Electrical Connection to the Outside World 13 1.6 GaN Integrated Circuits 15 1.7 Summary 21 References 21 2 GaN Transistor Electrical Characteristics 25 2.1 Introduction 25 2.2 Device Ratings 25 2.2.1 Drain‐Source Voltage 25 2.3 On‐Resistance (R ) 30 DS(on) 2.4 Threshold Voltage 33 viii Contents 2.5 Capacitance and Charge 34 2.6 Reverse Conduction 37 2.7 Summary 39 References 40 3 Driving GaN Transistors 41 3.1 Introduction 41 3.2 Gate Drive Voltage 44 3.3 Gate Drive Resistance 45 3.4 Capacitive Current‐Mode Gate Drive Circuits for Gate Injection Transistors 46 3.5 dv/dt Considerations 48 3.5.1 Controlling dv/dt at Turn‐On 48 3.5.2 Complementary Device Turn‐On 49 3.6 di/dt Considerations 51 3.6.1 Device Turn‐On and Common‐Source Inductance 51 3.6.2 Off‐State Device di/dt 53 3.7 Bootstrapping and Floating Supplies 54 3.8 Transient Immunity 57 3.9 High‐Frequency Considerations 59 3.10 Gate Drivers for Enhancement‐Mode GaN Transistors 60 3.11 Cascode, Direct‐Drive, and Higher‐Voltage Configurations 60 3.11.1 Cascode Devices 60 3.11.2 Direct‐Drive Devices 63 3.11.3 Higher‐Voltage Configurations 64 3.12 Summary 64 References 65 4 Layout Considerations for GaN Transistor Circuits 69 4.1 Introduction 69 4.2 Minimizing Parasitic Inductance 69 4.3 Conventional Power‐Loop Designs 72 4.3.1 Lateral Power‐Loop Design 72 4.3.2 Vertical Power‐Loop Design 73 4.4 Optimizing the Power Loop 74 4.4.1 Impact of Integration on Parasitics 75 4.5 Paralleling GaN Transistors 76 4.5.1 Paralleling GaN Transistors for a Single Switch 76 4.5.2 Paralleling GaN Transistors for Half‐Bridge Applications 79 4.6 Summary 83 References 83 5 Modeling and Measurement of GaN Transistors 85 5.1 Introduction 85 5.2 Electrical Modeling 85 5.2.1 Basic Modeling 85 Contents ix 5.2.2 Limitations of Basic Modeling 88 5.2.3 Limitations of Circuit Simulation 90 5.3 Measuring GaN Transistor Performance 91 5.3.1 Voltage Measurement Requirements 94 5.3.2 Probing and Measurement Techniques 96 5.3.3 Measuring Non‐Ground‐Referenced Signals 99 5.3.4 Current Measurement Requirement 100 5.4 Summary 101 References 102 6 Thermal Management 105 6.1 Introduction 105 6.2 Thermal Equivalent Circuits 105 6.2.1 Thermal Resistances in a Lead Frame Package 105 6.2.2 Thermal Resistances in a Chip‐Scale Package 107 6.2.3 Junction‐to‐Ambient Thermal Resistance 108 6.2.4 Transient Thermal Impedance 109 6.3 Improving Thermal Performance with a Heatsink 110 6.3.1 Selection of Heatsink and Thermal Interface Material (TIM) 111 6.3.2 Heatsink Attachment for Bottom‐Side Cooling 112 6.3.3 Heatsink Attachment for Multisided Cooling 113 6.4 System‐Level Thermal Analysis 114 6.4.1 Thermal Model of a Power Stage with Discrete GaN Transistors 115 6.4.2 Thermal Model of a Power Stage with a Monolithic GaN Integrated Circuit 117 6.4.3 Thermal Model of a Multiphase System 118 6.4.4 Temperature Measurement 120 6.4.4.1 Optical 120 6.4.4.2 Physical Contact 121 6.4.4.3 Temperature‐Sensitive Electrical Parameter 122 6.4.5 Experimental Characterization 122 6.4.6 Application Examples 124 6.5 Summary 128 References 128 7 Hard‐Switching Topologies 131 7.1 Introduction 131 7.2 Hard‐Switching Loss Analysis 131 7.2.1 Hard‐Switching Transitions with GaN Transistors 132 7.2.2 Output Capacitance (C ) Losses 135 OSS 7.2.3 Turn‐On Overlap Loss 138 7.2.3.1 Current Rise Time 139 7.2.3.2 Voltage Fall Time 142 7.2.4 Turn‐Off Overlap Losses 145 7.2.4.1 Current Fall Time 146 7.2.4.2 Voltage Rise Time 147 x Contents 7.2.5 Gate‐Charge (Q ) Losses 147 G 7.2.6 Reverse Conduction Losses (P ) 147 SD 7.2.6.1 Impact of Dead Time Selection on Reverse Conduction Loss 147 7.2.6.2 Adding an Anti‐Parallel Schottky Diode 150 7.2.6.3 Dynamic C ‐Related Reverse Conduction Losses 153 OSS 7.2.7 Reverse Recovery (Q ) Losses 153 RR 7.2.8 Hard‐Switching Figure of Merit 154 7.3 Impact of Parasitic Inductance on Hard‐Switching Losses 154 7.3.1 Impact of Common‐Source Inductance (L ) 154 CS 7.3.2 Impact of Power‐Loop Inductance on Device Losses 157 7.4 Frequency Impact on Magnetics 160 7.4.1 Transformers 160 7.4.2 Inductors 161 7.5 Buck Converter Example 162 7.5.1 Comparison with Experimental Measurements 169 7.5.2 Consideration of Parasitic Inductance 170 7.6 Summary 174 References 174 8 Resonant and Soft‐Switching Converters 177 8.1 Introduction 177 8.2 Resonant and Soft‐Switching Techniques 177 8.2.1 Zero‐Voltage and Zero‐Current Switching 177 8.2.2 Resonant DC–DC Converters 179 8.2.3 Resonant Network Combinations 179 8.2.4 Resonant Network Operating Principles 180 8.2.5 Resonant Switching Cells 181 8.2.6 Soft‐Switching DC–DC Converters 182 8.3 Key Device Parameters for Resonant and Soft‐Switching Applications 182 8.3.1 Output Charge (Q ) 182 OSS 8.3.2 Determining Output Charge from Manufacturers’ Datasheets 183 8.3.3 Comparing Output Charge of GaN Transistors and Si MOSFETs 184 8.3.4 Gate Charge (Q ) 185 G 8.3.5 Determining Gate Charge for Resonant and Soft‐Switching Applications 186 8.3.6 Comparing Gate Charge of GaN Transistors and Si MOSFETs 187 8.3.7 Comparing Performance Metrics of GaN Transistors and Si MOSFETs 187 8.4 High‐Frequency Resonant Bus Converter Example 188 8.4.1 Resonant GaN and Si Bus Converter Designs 191 8.4.2 GaN and Si Device Comparison 191 8.4.3 Zero‐Voltage Switching Transition 193 8.4.4 Efficiency and Power Loss Comparison 195 8.4.5 Impact of Further Device Improvements on Performance 197 8.5 Summary 199 References 199 Contents xi 9 RF Performance 201 9.1 Introduction 201 9.2 Differences Between RF and Switching Transistors 202 9.3 RF Basics 204 9.4 RF Transistor Metrics 205 9.4.1 Determining the High‐Frequency Characteristics of RF Transistors 206 9.4.2 Pulse Testing for Thermal Considerations 207 9.4.3 Analyzing the s‐Parameters 209 9.4.3.1 Test for Stability 209 9.4.3.2 Transistor Input and Output Reflection 210 9.4.3.3 Transducer Gain 211 9.4.3.4 Unilateral/Bilateral Transistor Test 211 9.5 Amplifier Design Using Small‐Signal s‐Parameters 212 9.5.1 Conditionally Stable Bilateral Transistor Amplifier Design 213 9.5.1.1 Available Gain 213 9.5.1.2 Constant Available Gain Circles 213 9.6 Amplifier Design Example 214 9.6.1 Matching and Bias Tee Network Design 216 9.6.2 Experimental Verification 219 9.7 Summary 221 References 221 10 DC–DC Power Conversion 223 10.1 Introduction 223 10.2 Non‐Isolated DC–DC Converters 223 10.2.1 The 12 V –1.2 V Buck Converter with Discrete Devices 224 IN OUT 10.2.2 The 12 V –1 V Monolithic Half‐Bridge IC‐Based IN OUT Point‐of‐Load Module 228 10.2.3 Very‐High‐Frequency 12 V Monolithic Half‐Bridge IC‐Based IN Point‐of‐Load Module 230 10.2.4 The 28 V –3.3 V Point‐of‐Load Module 233 IN OUT 10.2.5 The 48 V –12 V Buck Converter with Parallel GaN Transistors IN OUT for High‐Current Applications 233 10.3 Transformer‐Based DC–DC Converters 239 10.3.1 Eighth‐Brick Converter Example 239 10.3.2 High‐Performance 48 V Step‐Down LLC DC Transformer 243 10.3.2.1 Circuit Overview 243 10.3.2.2 GaN Transistor Advantage in the LLC Converter 244 10.3.2.3 A 1 MHz, 900 W, 48 V–12 V LLC Example Using GaN Transistors 245 10.3.2.4 A 1 MHz, 900 W, 48 V–6 V LLC Example Using GaN Transistors 248 10.4 Summary 249 References 250 11 Multilevel Converters 251 11.1 Introduction 251 11.2 Benefits of Multilevel Converters 251 xii Contents 11.2.1 Applying Multilevel Converters to 48 V Applications 252 11.2.2 Multilevel Converters for High‐Voltage (400 V) Applications 254 11.3 Gate Driver Implementation 255 11.4 Bootstrap Power Supply Solutions for GaN Transistors 256 11.5 M ultilevel Converters for PFC Applications 261 11.6 E xperimental Examples 263 11.6.1 Low Voltage 263 11.6.2 High Voltage 264 11.7 S ummary 264 R eferences 265 12 Class D Audio Amplifiers 269 12.1 Introduction 269 12.1.1 Total Harmonic Distortion 271 12.1.2 Intermodulation Distortion 272 12.2 GaN Transistor Class D Audio Amplifier Example 273 12.2.1 Closed‐Loop Amplifier 274 12.2.2 Open‐Loop Amplifier 276 12.3 Summary 278 References 278 13 Lidar 281 13.1 Introduction to Light Detection and Ranging (Lidar) 281 13.2 Pulsed Laser Driver Overview 281 13.2.1 Pulse Requirements 282 13.2.2 Semiconductor Optical Sources 284 13.2.3 Basic Driver Circuits 285 13.2.4 Driver Switch Properties 286 13.3 Basic Design Process 288 13.3.1 Resonant Capacitive Discharge Laser Driver Design 288 13.3.2 Quantitative Effect of Stray Inductance 289 13.4 Hardware Driver Design 290 13.5 Experimental Results 291 13.5.1 High‐Speed Laser Driver Design Example 291 13.5.2 Fastest 292 13.5.3 Highest Current 293 13.5.4 Low Voltage 293 13.6 Other Considerations 294 13.6.1 Resonant Capacitors 294 13.6.2 Charging 295 13.6.3 Voltage Probing 295 13.6.4 Current Sensing 296 13.6.5 Dual‐Edge Control 297 13.7 Summary 299 References 299