Table Of ContentELEC3230 Notes - Switching Electronics
R.E. Betz
School of Electrical Engineering and Computer Science
University of Newcastle, Australia.
email: [email protected]
(cid:1)c 1999, 2000, 2001, 2002, 2003, 2004
Created: May 24, 1999
Revised: July 19, 2004
ii
Preface
The notes in this document are for a course in the School of Electrical Engi-
neering and Computer Science at the University of Newcastle, Australia. This
course covers a number of topics that can be broadly grouped under the title of
“switchingelectronics”. Electronicswitchingistheunifyingfactorthatprovides
the theme for the course. The notes were written because the subject material
covers such diverse areas as digital logic switching families, switched transmis-
sionlinesandprintedcircuitboards,switchmodepowersupplies(SMPSs),and
(to a lesser degree) converters. No single text book covers such material.
The general approach of the course is to emphasise the practical aspects of
switching and how design has to be changed to account for its effects. The
theorybehindmanyoftheseideasispresentedindetailintheappendices. This
is particularly true in relation to switching in digital systems with transmission
lines.
Thestructureofthecourseisasfollows. Thefirstpartwillconsideravariety
of issues related to switching in digital systems. This will include a review of
logic families and interfacing of different logic families. Then issues related to
interfacinglogiccomponentsonaprintedcircuitboardwillbeconsidered. This
will include noise issues, transmission line effects, terminations, cross coupling,
printed circuit board layout, decoupling issues.
The second section of the course will look at switch mode power supplies in
theirvariousforms. Themainstructuresforswitchmodepowersupplieswillbe
considered. Again practical issues will be emphasised. Design of the magnetics
for switching supplies will be considered, as well as some control issues. The
control issues are only briefly considered due to the lack of background of some
students doing the course.
The final part of the course considers high powered converter and inverter
topologies. At this stage there is only an introduction to high power switch-
ing devices, and a brief look at naturally commutate converters, mostly single
phase. Eventually there will be a reasonable treatment of three phase naturally
converters and forced commutated inverters (using thyristors as well as transis-
tors).
Robert E. Betz – Newcastle, Australia, July 19, 2004.
iv
Revision History
1999 Firstversionofthenotescreatedforthe5creditpointsubjectELEC322.
Only included the digital switching and transmission line material.
2000 A major upgrade of the material to include switch mode power supplies,
and some material on higher powered converters. This upgrade was nec-
essarybecausethesubjectchangedfromELEC322toELEC323,anddou-
bled in credit points to 10.
2001 Prior to the issue of the notes, corrections were made to the notes from
2000. Added a chapter on the practical design of switch mode power
supplies. Minor corrections and additions made to the notes through the
course of 2001.
March 2002 Minor corrections and additions made.
July 2002 Made further minor typographical corrections.
July 2003 Further typographical corrections, added section on capacitively
coupled load terminations, fixed the equivalent circuit of the home brew
probe. Added a new chapter to briefly introduce other power electronic
circuits not already considered in the notes. Added the course outline,
schedule, and Saber introductory exercise to the appendix.
August 19, 2003 Made a few typo corrections as well as a change to an in-
correct diagram.
July 19, 2004 Made typo corrections and added extra remarks in relation to
magnetic utilisation with push-pull converters. Also added an extra re-
mark in relation to the derivation of the fact that harmonics do not con-
tribute to real power. Corrected a few minor diagram errors. Added the
assignments to the appendices.
vi
Contents
List of Figures xi
List of Tables xix
I Digital Systems 1
1 Logic Families 2
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Review of Logic Family Properties . . . . . . . . . . . . . . . . . 2
1.2.1 A Brief History . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.2 The CMOS Logic Family . . . . . . . . . . . . . . . . . . 3
1.2.2.1 Logic Levels and Noise Margins . . . . . . . . . 6
1.2.2.2 Fanout . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.2.3 Specific CMOS Logic Families . . . . . . . . . . 9
1.2.3 Bipolar Logic Families . . . . . . . . . . . . . . . . . . . . 10
1.2.3.1 Bipolar Logic Noise Margins . . . . . . . . . . . 12
1.2.3.2 Fanout . . . . . . . . . . . . . . . . . . . . . . . 12
1.2.3.3 Specific TTL Logic Families . . . . . . . . . . . 13
1.3 Issues in TTL–CMOS Interfacing . . . . . . . . . . . . . . . . . . 13
2 Introduction to Digital Switching 16
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Relevant Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Propagation, Time and Distance . . . . . . . . . . . . . . . . . . 18
2.4 Lumped Versus Distributed Systems . . . . . . . . . . . . . . . . 18
2.5 Four Kinds of Reactance . . . . . . . . . . . . . . . . . . . . . . . 20
2.5.1 Ordinary Capacitance . . . . . . . . . . . . . . . . . . . . 20
2.5.2 Ordinary Inductance . . . . . . . . . . . . . . . . . . . . . 22
2.5.3 Mutual Capacitance . . . . . . . . . . . . . . . . . . . . . 28
2.5.3.1 Relationship between Mutual Capacitance and
Crosstalk . . . . . . . . . . . . . . . . . . . . . . 28
2.5.4 Mutual Inductance . . . . . . . . . . . . . . . . . . . . . . 33
2.5.4.1 Relationship Between Mutual Inductance and
Crosstalk . . . . . . . . . . . . . . . . . . . . . . 36
2.6 Speed of Digital Systems . . . . . . . . . . . . . . . . . . . . . . . 39
2.6.1 dv/dt Effects . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.6.2 di/dt Effects . . . . . . . . . . . . . . . . . . . . . . . . . 41
viii CONTENTS
2.6.3 Ground Bounce . . . . . . . . . . . . . . . . . . . . . . . . 44
2.6.3.1 Why Does Ground Bounce Occur? . . . . . . . . 44
2.6.3.2 How Does Ground Bounce Affect Circuits? . . . 45
2.6.3.3 Estimating Ground Bounce Magnitude . . . . . 47
2.6.3.4 Reducing Ground Bounce . . . . . . . . . . . . . 47
2.6.4 Lead Capacitance . . . . . . . . . . . . . . . . . . . . . . 48
2.6.5 Measurement Issues . . . . . . . . . . . . . . . . . . . . . 49
2.6.5.1 Rise Time and Bandwidth of CROs . . . . . . . 49
2.6.5.2 Self-inductance of CRO Probe Ground Clips . . 51
2.6.5.3 Mutual-inductance of CRO Probe Ground Clips 57
2.6.5.4 Loading Effect of CRO Probes . . . . . . . . . . 59
2.6.6 Better Probing Techniques . . . . . . . . . . . . . . . . . 62
2.6.6.1 Home Brew 21:1 Probe . . . . . . . . . . . . . . 62
2.6.6.2 Low Inductance with Conventional Probes . . . 66
2.6.6.3 PCB Test Points . . . . . . . . . . . . . . . . . . 66
2.6.6.4 Shield Currents and Ground Loops . . . . . . . 69
3 Point-to-Point Wiring and Transmission Lines 73
3.1 Shortcomings of Point-to-Point Wiring . . . . . . . . . . . . . . . 73
3.1.1 EMI Radiation . . . . . . . . . . . . . . . . . . . . . . . . 75
3.1.2 Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.2 Uniform Transmission Lines . . . . . . . . . . . . . . . . . . . . . 77
3.2.1 Measurement of Distributed Parameters . . . . . . . . . . 77
3.2.2 Alternative Way of Deriving Characteristic Impedance . . 79
3.2.3 Physical Explanation of Reflections . . . . . . . . . . . . . 80
3.3 Modelling of Transmission Lines . . . . . . . . . . . . . . . . . . 82
3.4 Some Practical Effects in Transmission Lines . . . . . . . . . . . 83
3.4.1 Skin Effect . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.4.2 Proximity Effect . . . . . . . . . . . . . . . . . . . . . . . 86
3.4.3 Dielectric Losses . . . . . . . . . . . . . . . . . . . . . . . 87
3.5 Termination of Transmission Lines . . . . . . . . . . . . . . . . . 87
3.5.1 General Effects of Source and Load Impedance . . . . . . 89
3.5.1.1 Load termination . . . . . . . . . . . . . . . . . 93
3.5.1.2 Source Termination . . . . . . . . . . . . . . . . 93
3.5.1.3 Very Short Line . . . . . . . . . . . . . . . . . . 95
3.5.2 Capacitive Terminations . . . . . . . . . . . . . . . . . . . 96
3.5.2.1 Equally Spaced Capacitive Loads . . . . . . . . 97
3.5.3 Multi-point Terminations . . . . . . . . . . . . . . . . . . 100
4 Ground Planes and other Printed Circuit Board Issues 108
4.1 Power Planes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.1.1 Decoupling Capacitors and Power Planes . . . . . . . . . 110
4.2 Crosstalk Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.2.1 Path of Least Inductance . . . . . . . . . . . . . . . . . . 111
4.2.1.1 Crosstalk in Ground Planes . . . . . . . . . . . . 113
4.2.1.2 Crosstalk in Slotted Ground Planes . . . . . . . 114
4.2.1.3 Crosstalk in Two Layer PCBs . . . . . . . . . . 116
4.2.1.4 CrosstalkinwithPowerandGroundFingerPCBs118
4.2.1.5 A Note on Guard Traces . . . . . . . . . . . . . 119
4.2.1.6 Distributed Cross Coupling . . . . . . . . . . . . 119
CONTENTS ix
II Switched Mode Power Supplies 125
5 Fundamental Topologies 126
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.2 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.3 Taxonomy of Switch Mode Converters . . . . . . . . . . . . . . . 127
5.3.1 Step-down or Buck Converters . . . . . . . . . . . . . . . 127
5.3.2 Step-up or Boost Converters . . . . . . . . . . . . . . . . 129
5.3.3 Buck–Boost Converters . . . . . . . . . . . . . . . . . . . 130
5.3.4 Cu´k Converters . . . . . . . . . . . . . . . . . . . . . . . . 132
5.3.5 Full Bridge Converters . . . . . . . . . . . . . . . . . . . . 134
5.4 Basic Analysis of Switch Mode Converters . . . . . . . . . . . . . 136
5.4.1 Duty Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5.4.2 Basic PWM Generator . . . . . . . . . . . . . . . . . . . . 137
5.4.3 Simplified Analysis of the Buck Converter . . . . . . . . . 139
5.4.3.1 Continuous Conduction Mode . . . . . . . . . . 139
5.4.3.2 Boundary between Continuous and Discontinu-
ous Conduction . . . . . . . . . . . . . . . . . . 141
5.4.3.2.1 Discontinuous Current with Constant
V . . . . . . . . . . . . . . . . . . . . . 142
d
5.4.3.2.2 DiscontinuousCurrentwithConstantV .145
o
5.4.3.3 Output Ripple . . . . . . . . . . . . . . . . . . . 147
5.4.3.4 Simulation . . . . . . . . . . . . . . . . . . . . . 149
5.4.4 Simplified Analysis of the Boost Converter . . . . . . . . 151
5.4.4.1 Continuous Conduction Mode . . . . . . . . . . 152
5.4.4.2 Boundary between Continuous and Discontinu-
ous Conduction . . . . . . . . . . . . . . . . . . 153
5.4.4.2.1 Discontinuous Current with Constant
V . . . . . . . . . . . . . . . . . . . . . 156
d
5.4.4.3 Simulation . . . . . . . . . . . . . . . . . . . . . 159
5.4.5 A Brief Look at the Buck-Boost Converter . . . . . . . . 159
5.4.6 A Brief Analysis of the Cu´k Converter . . . . . . . . . . . 161
5.4.7 Full Bridge dc-dc Converter . . . . . . . . . . . . . . . . . 162
5.4.7.1 Bipolar Switching . . . . . . . . . . . . . . . . . 163
5.4.7.2 Unipolar Switching . . . . . . . . . . . . . . . . 166
5.4.8 Comparison of Basic Converter Topologies. . . . . . . . . 168
5.4.8.1 Switch Utilisation . . . . . . . . . . . . . . . . . 168
5.4.8.1.1 Buck Converter . . . . . . . . . . . . . 169
5.4.8.1.2 Boost Converter . . . . . . . . . . . . . 169
5.4.8.1.3 Buck-Boost Converter . . . . . . . . . . 170
5.4.8.1.4 Full Bridge Converter . . . . . . . . . . 171
5.4.9 Synchronous Rectifiers . . . . . . . . . . . . . . . . . . . . 173
5.4.10 Resonant and Soft-Switching Converters . . . . . . . . . . 173
5.4.10.1 Why One Should Not Use Resonant Converters 176
5.4.10.2 Why One Should Use Quasi-Resonant Converters176
x CONTENTS
6 Switch Mode Power Supplies 178
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
6.2 Isolated Converter Topologies . . . . . . . . . . . . . . . . . . . . 178
6.2.1 The Forward Converter . . . . . . . . . . . . . . . . . . . 178
6.2.1.1 Other Forward Converter Topologies. . . . . . . 183
6.2.1.1.1 Two Switch Converter . . . . . . . . . . 184
6.2.1.1.2 Push-Pull Converter . . . . . . . . . . . 184
6.2.2 The Flyback Converter . . . . . . . . . . . . . . . . . . . 189
6.2.3 Utilisation of Magnetics . . . . . . . . . . . . . . . . . . . 194
6.3 Introduction to Control Techniques for Switching Power Supplies 199
6.3.1 Start-Up. . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
6.3.2 Protection Issues . . . . . . . . . . . . . . . . . . . . . . . 204
6.3.2.1 Soft Start . . . . . . . . . . . . . . . . . . . . . . 204
6.3.2.2 Voltage Protection . . . . . . . . . . . . . . . . . 204
6.3.2.3 Current Limiting . . . . . . . . . . . . . . . . . . 205
6.3.3 ControlArchitectureofaSwitchModePowerSupplySys-
tem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
6.3.3.1 Voltage Mode Control . . . . . . . . . . . . . . . 207
6.3.3.2 Voltage Feed-forward PWM Control . . . . . . . 209
6.3.3.3 Current Mode Control . . . . . . . . . . . . . . . 209
6.3.3.3.1 Slope Compensation . . . . . . . . . . . 212
7 Introduction to Practical Design of Switch Mode Power Sup-
plies 218
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
7.2 Component Selection . . . . . . . . . . . . . . . . . . . . . . . . . 218
7.2.1 Resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
7.2.1.1 Values . . . . . . . . . . . . . . . . . . . . . . . . 219
7.2.1.2 Resistor Types . . . . . . . . . . . . . . . . . . . 219
7.2.1.3 Tolerance . . . . . . . . . . . . . . . . . . . . . . 220
7.2.1.4 Selecting Values . . . . . . . . . . . . . . . . . . 220
7.2.1.5 Maximum Voltage . . . . . . . . . . . . . . . . . 220
7.2.1.6 Temperature Coefficient . . . . . . . . . . . . . . 221
7.2.1.7 Power Rating . . . . . . . . . . . . . . . . . . . . 221
7.2.1.8 Shunts . . . . . . . . . . . . . . . . . . . . . . . 222
7.2.1.9 PCB Track Resistors . . . . . . . . . . . . . . . 223
7.2.2 Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . 223
7.2.2.1 Types of Capacitors . . . . . . . . . . . . . . . . 224
7.2.2.2 Standard Values . . . . . . . . . . . . . . . . . . 224
7.2.2.3 Tolerance . . . . . . . . . . . . . . . . . . . . . . 225
7.2.2.4 ESR and Power Dissipation . . . . . . . . . . . . 225
7.2.2.5 Aging . . . . . . . . . . . . . . . . . . . . . . . . 225
7.2.2.6 dv/dt Rating . . . . . . . . . . . . . . . . . . . . 226
7.2.2.7 Series Connection of Capacitors . . . . . . . . . 226
7.2.3 Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
7.2.3.1 Schottky Diodes . . . . . . . . . . . . . . . . . . 226
7.2.3.2 PN diodes . . . . . . . . . . . . . . . . . . . . . 227
7.2.4 The BJT . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
7.2.5 The MOSFET . . . . . . . . . . . . . . . . . . . . . . . . 229
7.2.5.1 Bi-directional Conduction . . . . . . . . . . . . . 230
