EMI Filter Design, Second Edition - Richard Lee Ozenbaugh.pdf

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EMI Filter

Design

Second Edition

Revised and Expanded

Richard Lee Ozenbaugh

Consultant

Hesperia, California

ISBN: 0-8247-8924-5

This book is printed on acid-free paper.

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The publisher offers discounts on this book when ordered in bulk quantities. For more

information, write to Special Sales/Professional Marketing at the headquarters address above.

Neither this book nor any part may be reproduced or transmitted in any form or by any

means, electronic or mechanical, including photocopying, microfilming, and recording, or

by any information storage and retrieval system, without permission in writing from the

publisher.

Current printing (last digit):

10 9 8 7 6 5 4 3 2 1

PRINTED IN THE UNITED STATES OF AMERICA

This is dedicated to my wife, Pansy, for her moral support. Also, I wish to thank

her for the computer time and for the seminar time she lost (and for not

complaining when she had the right to), especially during this rewrite.

Preface

Over the last twelve years, I have made friends with many EMI “gurus” across

the United States, but primarily in the Southwest. Many have attended my EMI

Design seminars and I have met others as an Applications engineer in the field.

My desire was to increase my knowledge of EMI filter design by learning new

and better techniques. I concluded, after some years, that very few of them had a

concrete method for the design of these low pass filters (it is black magic, isn’t

it?). I got to know most of these engineers quite well. They would give me a very

strange look when I would ask them this leading question: “What is your design

technique?” I finally concluded that in most cases it depended on what was handy

or readily available. They would have a certain capacitor or inductor on their

bench or in stock (or readily available from a supplier). At best, they would

calculate the needed inductor(s), or capacitor(s), to arrive at the desired loss.

Some would give me profound statements such as, “We design for maximum

loss!” or “We use [a certain program] to get the values needed.” To accomplish

this latter method, the “guru” would reach the desired loss by backward-engineering

the network—modifying the inductor or capacitor values or changing the number

of components to get the apparent desired loss. Some designs had so much

capacitance across the line that the EMI filter inrush current, added to the power

supply filter capacitors charging currents, would trip the circuit breaker. Others

would spend days measuring the line and load impedance using network an-

alyzers so the EMI filter could be matched to these two end sections. Unfortu-

nately, the end sections would change from application to application so that the

filter center piece would not function properly, because of the mismatch existing

in the band pass.

Another technique I have found is for the engineer to grab any core and start

winding wire around it for its inductor. Usually they could not even tell me the type

of core—MPP, HF, tape wound ferrite? “Just an available core, something off the

shelf.” This does not give a reference point from which to start. Whether the filter

works or fails, the filter inductor core must be known. The fallacious statement often

made is that a given core was used by one of the filter manufacturers and therefore

must be the correct type and about the correct number of turns.

A doctor from Sierra Vista, Arizona, wrote a very fine article in the

late 1980s on commercial filters that included a good method for calculating

the component values. The title of the article was something like “Gus, the

Electrician.” His filter component values came from “M-derived” filter design

techniques

πF

πFRd

where L is the inductance, C is the capacitance, Rd is the design impedance, and

F is the cutoff frequency. He felt that most of the original filter designers allowed

their cutoff frequencies to fall much too close to power line frequency. This

happens in the 461 specifications requiring high dB losses in the 10 to 14 kHz

region. Otherwise, few EMI filter cutoff frequencies drop as low as indicated in

the article. This is especially true for commercial filters. Gus would often switch

the ground and return wire because he thought that ground was ground. This was

a very good article and a very good design method for the component values.

He did not offer a method to calculate the cutoff frequency.

Other engineers have correctly used the energy spectrum of the switcher

frequency to determine the amount of loss required. There is trouble transforming

this information when the engineer selects the filter component values. Many

designs use very small capacitors between line and ground with very high values

of resistors in series with them. Some have used capacitors and resistors in series

such as 2000 pF in series with 470 ohms. The balanced circuit requires two of

these networks wired from both inputs to ground. One network wired between

the high, or hot, to ground and the other network wired between the neutral and

ground. So at the higher frequencies (in the case stated, above 2 MHz), the line

to line would be 940 ohms. Why? The line impedance would be somewhere

between 50 and 130 ohms, depending on the line. The energy content of the

switcher is low at these frequencies but, again, if this energy was high, why the

high series resistance?* Is there a time when this is a good technique? Yes, but

*This was written when switchers worked at 80 kHz maximum but now some are in the MHz region.

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