Practical Switching Power Supply Design Marty Brown Mokorola Semiconductor M0rOROL.A Series in Solid State Electronics Practical Switching Power Supply Design A Division of Harcouri Brace & C San Diego New York Boston Lon Motorola reserves the right to make changes without funher notice to any products herein to improve reliability. function or design. Motorola does not mssume my liability arising out of the application or use of my product or cir- cuit described herein: neither does it convey any license under its patent rights nor the rights of others. Motorola products are not authorized for use as com- ponents in life supporr devices or systems intended for surgical implant into the body or intended to iuppon or sustain life. Buyer agrees to notify Motor- ola of any such intended end use whereupon Motorola shall determine availa- bility and suitability of its product or products for the use intended. Motorola and are registered tmdcmarkc of Motorola. Inc. Motorola. Inc. is an Equal Employment Opponunity/Affinnative Action Employer. This book is printed on acid-free paper. @ Copyright 0 1990 by Academic Press All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo- copy, recording. or any information storage and retrieval system, without permission in writing from the publisher. Academic Press A Harcourl Science and Technology Company 525 B Street. Suite IWO. San Diego. California 92101-4495. USA http:/lwww. apnet.com Academic Press Harcourt Place. 32 Jamestown Road. London NWI ?BY.U K http://www.hbuk.co.uklap/ Library of Congress Cataloging-in-Publication Data Brown, Marty. Practical switching power supply design I Marty Brown. p. cm. ISBN 0-12-137030-5 (alk. paper) 1. Switchingc ircuits--l)esign and conslruclion. 2. Pswer semiconductors-Design and construction. 3. Semiconductor switches- -Design and construction. 1. Title. TK7868S9866 1990 621.381'S--dc20 89- I75 I8 CIP Printed In The United States Of America 01 02 03 IBT 11 10 9 8 7 Preface ix CHAPTER 1 Why Use Switching Power Supplies? I CHAPTER 2 How a Switching Power Supply Works s 2. I Forward-Mode Switching Regulators 5 2.2 Flyback-Mode Switching Regulators 7 CHAPTER 3 A Walk through a Representative Switching Power Supply 9 3.1 The EM1 Filter 9 3.2 Bulk Input Filter (Storage) Capacitor 9 3.3 Transformer I I 3.4 Power Switches 12 3.5 Output Rectifiers 12 3.6 The Output Filter Section I2 3.7 Current Sense Elements 13 3.8 Voltage Feedback Elements 13 3.9 The Control Section 14 CHAPTER 4 Switching Power Supply Topologies I7 4.1 Factors Affecting the Choice of an Appropriate Topology 17 4.2 Non-Transformer-IsolatedS witching Power Supply Topologies 20 vi Contents 4.2.I The Buck Regulator Topology 20 4.2.2 The Boost Regulator Topology 24 4.2.3 The Buck-Boost Regulator Topology 26 4.3 Transformer-Isolated Switching Power Supply Topologies 29 4.3.I The Flyback Regulator Topology 29 4.3.2 The Push-Pull Regulator Topology 34 4.3.3 The Half-Bridge Regulator Topology 38 4.3.4 The Full-Bridge Regulator Configuration 40 CHAPTER 5 Semiconductors Used in a Switching Power Supply 43 5.1 Bipolar Power Transistors 43 5.2 PowerMOSFETs 50 5.3 Rectifiers 56 5.4 Switching Power Supply Control Integrated Circuits 5x 5.4. I Voltage-Mode Control 60 5.4.2 Current-Mode Control 61 5.4.3 Quasi-Resonant-ModeC ontrol 63 CHAPTER 6 The Magnetic Components within a Switching Power Supply 67 6.1 Basic Magnetism and Ferromagnetism 68 6.2 The Forward-Mode Transformer 76 6.3 The Flyback Transformer 83 6.4 The Forward-Mode Filter Choke 90 6.5 Mutually Coupled Forward-Mode Filter Inductors 04 CHAPTER 7 Cross-Regulation of the Outputs 97 7.1 Transformer Techniques 99 7.2 The Voltage-Sensing Network 99 7.3 Mutually Coupled Output Filter Chokes 100 CHAPTER 8 Protection I03 8. I Protecting the Supply and the Load from the Input Line 103 8. I. I AC Line Input Adverse Operating Conditions 104 8. I .2 DC Line Input Adverse Operating Conditions 10.5 Contents 8.2 Protecting the Load from the Supply and Itself i07 8.2. I Hardware Implementations to Address Overvoltage 109 8.2.2 Hardwarc lniplemeritations to Address Overcurrent Ill CHAPTER 9 MiscellaneousT opics I I5 9. I Power Supply and System Grounds I 15 9.2 The Use and Design of Clamps and Snubbers I I9 9.3 RFI and EM1 Design Considerations 125 9.4 Power Supply and Product Safety Considerations I 28 9.5 Testing Power Supply Units 132 9.5. I Line Regulation 132 9.5.2 Load Regulation 133 9.5.3 Dynamic Load Response Time 133 9.5.4 1)ielectric Withstanding Voltage I35 9.5.5 Holdup Time 137 9.5.6 Overcurrent Limit Test 138 CHAPTER 10 Closing the loop-Feedback and Stabillty 14I IO. I The Bode Plot as a Basic Tool 141 10.2 Closing the Loop 14s 10.3 The Stability Criteria Applied to Power Supplies 136 10.4 The Control-to-Output Transfer Functions of Common Switching Power Supply Topologies 148 10.4.1 Forward-hlode Control-to-Output Transfer Functions ( Voltage-Mode Control) I49 IO 4.2 Flyhack-hlode and Curtent-Mode Controlled Forward Converters 15 I 10.5 Common Error Amplifier Compensation Techniques I54 10.5.I Single-Pole Conipcnsation 155 10.5.2 Zero-Pole Pair Compensation 158 10.5.3 Two-Pole-Two-Zero Compensation I62 10.6 Attempting to Compensate for a Right-Half-Plane Zero 167 CHAPTER 11 Resonant Converters-An Introduction 169 1 I. I Why Resonant Switching Power Supplies'? 170 I I .2 Basic Quasi-Resonant Converter Operation 172 1 1.3 The Resonant Switch-A Method of Creating a Quasi- Resonant Family 178