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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 m y liability arising out of the application or use of m y product or circuit 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 components in life supporr devices or systems intended for surgical implant into r the body o intended to iuppon or sustain life. Buyer agrees to notify Motorola of any such intended end use whereupon Motorola shall determine availability and suitability of its product or products for the use intended. Motorola and are registered tmdcmarkc of Motorola. Inc. Motorola. Inc. i s 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, includingphotocopy, recording. or any informationstorage and retrieval system, without permission in writing from the publisher. Academic Press A Harcourl Science a d Technology Company n 525 B Street. Suite IWO. San Diego. California92101-4495.USA http:/lwww.apnet.com Academic Press Harcourt Place. 32 Jamestown Road. London NWI ?BY.UK http://www.hbuk.co.uklap/ Library of Congress Cataloging-in-PublicationData Brown, Marty. Practical switching power supply design I Marty Brown. p. cm. ISBN 0-12-137030-5 (alk. paper) 1. Switchingcircuits--l)esign and conslruclion. 2. Pswer semiconductors-Design and construction. 3. Semiconductor switches-Design and construction. 1. Title. TK7868S9866 1990 89- I75 I8 621.381'S--dc20 CIP Printed In The United States Of America 01 02 03 IBT 11 10 9 8 7 Preface CHAPTER 1 ix Why Use Switching Power Supplies? CHAPTER 2 I How a Switching Power Supply Works 2. I Forward-Mode Switching Regulators 2.2 Flyback-Mode Switching Regulators CHAPTER 3 s 5 7 A Walk through a Representative Switching Power Supply 9 3.1 The EM1 Filter 9 3 . 2 Bulk Input Filter (Storage) Capacitor 3 . 3 Transformer II 3.4 Power Switches 12 3.5 Output Rectifiers 12 I2 3.6 The Output Filter Section 3.7 Current Sense Elements 13 3 . 8 Voltage Feedback Elements 13 3.9 The Control Section 14 CHAPTER 4 9 Switching Power Supply Topologies I7 17 4.1 Factors Affecting the Choice of an Appropriate Topology 4 . 2 Non-Transformer-Isolated Switching 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 Topo