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Ferromagnetic-Core Design and Applications Handbook

Electrical, Instrumentation, Electronic Design and Engineering

Ferromagnetic-Core Design and Applications Handbook
M.F. Doug DeMaw
273 pages

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Preface

The work within these chapters is dedicated expressly to engineers, technicians, and college students who arc presently or soon to be involved professionally with electronics. Nearly all modern circuits contain magnetic-core devices of one kind or another. It is not sufficient to the cause of good engineering practices Lo use inductors and transformers with magnetic cores in a casual manner. The functional parameters of toroids, rods, and pot cores are as important in a composite circuit as are the operating characteristics of the active devices being used. Therefore, more than a basic understanding of ferrites and powdered-iron components is essential today during routine design work. Among the primary considerations when dealing with fcrromagnetics are proper core selection versus operating frequency, circuit Q, power-handling capability, and physical mass. The wrong core material, or a misapplied core material, can render unusable an otherwise perfect circuit.

Emphasis has been placed on the practical aspects of magnetic core materials from low frequencies through UHF. The RF engineer or technician will find this volume invaluable in his or her daily efforts. The student will value this publication as an important textbook, and later as a standard reference for which nothing similar exists. Tedious mathematical procedures have been omitted in an effort to make comprehension more rapid and enjoyable. Equations have been used only where they a re necessary to illustrate a concept or to provide a design example.

The chapters include basic theory and practical circuit examples in which toroids, rods, slugs, and pot cores are used. All of the circuits are proven ones, based on laboratory research and development by the author. The book contains myriad examples of narrow-band and broadband transformers and inductors. Ferrite loop antennas, slug-tuned inductors, RF chokes, and ferrite beads are discussed in depth , also. Magnetic-core devices are highlighted in filters, switching types of power supplies, and impedance-matching net works. There are live appendices, which contain comprehensive design data, lists of core manufacturers, a rather lengthy bibliography, design nomographs, conversion tables, and numerous pages of component numbers and characteristics from a variety of leading manufacturers.

No credible technical author can take credit for all of the design approaches or circuits he or she commits to a text. Almost without exception the author treads unknowingly on the past work of someone else, however unintentional. Uncredited similarity between any portion of this work and that of others is unintentional and without prior knowledge on behalf of the author. Credits have been included wherever applicable.

TOC

CHAPTER 1 THE BASICS OF MAGNETIC MATERIALS
1.1 Classifying Magnetic Materials 1
1.2 Physical and Electrical Traits 2
1.2. I Powdered irons 5
1.2.2 Ferrites 6
1.3 Calculating the Coil Turns 9
1.3. I Determining inductance II
1.3.2 Circuit Q 12
1.4 Power Capability 14
1.4. I Flux density versus ue 15
1.4.2 Sample calculation 18
1.4.3 Varia1ions in the equations 21
1.5 Volt- Ampere Ratings 22
1.5.1 A sample K1 core selection 23
1.5.2 Power rating 30
1.6 Inductance versus AC/ DC Excitation 30
1.6. I Employment of the Hanna curve 31
1.6.2 Constructing a Hanna curve 33
1.6.3 Actual air gap 36

CHAPTER 2 APPLICATION OF RODS, BARS, AND SLUGS 39
2. 1 Rods and Bars 39
2.1 . I Characteristics of rods and bars 40
2. 1.2 Q and L versus coil placement 42
2.1.3 Frequency versus core material 48
2.2 Rods and Bars as Antennas 49
2.2.l Loop pattern symmetry 52
2.2.2 Noise immunity 53
2.3 Practical Loop Circuits 54
2.3. l Loop-to-mixer circuits 56
2.3.2 Loop to RF amplifier 57
2.3.3 Loop with sense antenna 59
2.3.4 Loop variations 61
2.4 Rod- Core Transformers 62
2.5 Slug Applications 63
2.5. I Performance considerations 65
2.5.2 Practical circuits 68
2.6 RF Chokes 73
2.6. I Choke applications 73

CHAPTER 3 APPLYING TOROIDAL CORES 79
3.1 Low-Level Circuits 79
3. I. I RF amplifiers 80
3.1.2 Capacitive-divider marching 83
3.1.3 Interstage coupling 85
3.1.4 Toroids in filters 87
3. 1.5 Toroids in instruments 93
3.1 .6 Other low-level applications 96
3.2 Broadband Transformers 99
3.2.l Bifilar-wound transformers JOO
3.2.2 Quadrifilar transformers IOI
3.2.3 Broadband conventional transformers 102
3.2.4 Variable impedance matching 109
3.3 Antenna Loading 11 2
3.4 Toroidal Distributed Capacitance 11 3
3.5 Power Supplies 114
3.5. I Using poll'er FETs as swi1ches 116
3.6 Core Doping 11 7
3.6. I Simple doping methods 118
3.6.2 Potting technique 120

CHAPTER 4 BEADS. SLEEVES. AND POT CORES 123
4.1 Properties of Beads 124
4.1. l Shield-bead allenuation 126
4.1.2 Practical applications for beads 130
4.1.3 Beadsas Qkillers 133
4.1.4 Beads for decoupling 135
4.1.5 Reducing incidental radit11ion 136
4.2 Ferrite Sleeves 137
4.3 Ferrite Balun Cores 141
4.4 Pot Cores 143
4.4. l Pol-core hardware 144
4.4.2 Design co11sideratio11s 146
4.4.3 POI-core designs 153
4.4.4 Tone encoder with pot core 154
4.4.5 Pot cores in filters 157

CHAPTER 5 PERMANENT-MAGNET DATA 159
5.1 The Nature of Permanent-Magnet Materials 159
5.2 Elementary Permanent-Magnet Relationships 165
5.2.1 The B-H curve 165
5.2.2 Recoil loops 166
5.2.3 Working point of a magnet (static applications) 167
5.2.4 The magnetic circuit 167
5.2.5 The permanent magnet in an external field 169
5.3 Magnetization 1 70
5.3. I Conditions for comp/ere sa111ra1ion 171
5.3.2 Magnetizing equipme111 172
5.4 Demagnetization 176
5.4. l Demagnetiza1ion techniques 176
5.5 Stability and Stabilization 177
5.5.1 M eta/lurgica/ changes 178
5.5.2 Time 178
5.5.3 Temperature 178
5.5.4 Reluctance changes 180
5.5.5 A cfrerse fields 180
5.5.6 Shock. stresss. and vibrarion
5.6 Measurements and Calibration
5.7 Handling of Permanent Magnets
5.8 Glossary of Terms
APPENDIX A REFERENCES
APPENDIX B MISCELLANEOUS DATA
APPENDIX C POT-CORE AND BOBBIN DATA
APPENDIX D CYLINDRICAL-CORE ANO COIL DATA
APPENDIX E TORROID-CORE DATA