Switching Power Supplies A - Z (Second Edition)

769 Pages·2012·23.892 MB·English

by  Sanjaya Maniktala

Switching Power Supplies A–Z
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Switching Power Supplies A–Z
Sanjaya Maniktala
AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON
NEWYORK(cid:129)OXFORD(cid:129)PARIS(cid:129)SANDIEGO
SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO
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Contents
Preface.................................................................................................................xv
Acknowledgments..................................................................................................xix
Chapter 1: The Principles of Switching Power Conversion.......................................1
Introduction..................................................................................................................1
Overview and Basic Terminology...............................................................................4
Efficiency.........................................................................................................4
Linear Regulators............................................................................................5
Achieving High Efficiency through Switching..............................................7
Basic Types of Semiconductor Switches........................................................8
Semiconductor Switches Are Not “Perfect”.................................................10
Achieving High Efficiency through the Use of Reactive Components.......11
Early RC-Based Switching Regulators.........................................................12
LC-Based Switching Regulators...................................................................13
The Role of Parasitics...................................................................................14
Switching at High Frequencies.....................................................................16
Reliability, Life, and Thermal Management.................................................17
Stress Derating...............................................................................................19
Advances in Technology...............................................................................20
Understanding the Inductor........................................................................................21
Capacitors/Inductors and Voltage/Current....................................................21
The Inductor and Capacitor Charging/Discharging Circuits........................22
The Law of Conservation of Energy............................................................24
The Charging Phase and the Concept of Induced Voltage..........................24
The Effect of the Series Resistance on the Time Constant..........................26
The Inductor Charging Circuit with R50 and the “Inductor Equation”....28
The Duality Principle....................................................................................29
The “Capacitor Equation”.............................................................................30
The Inductor Discharge Phase......................................................................31
Flyback Energy and Freewheeling Current..................................................32
Current Must Be Continuous, Its Slope Need Not Be.................................32
The Voltage Reversal Phenomenon..............................................................34
v
vi Contents
A Steady State in Power Conversion and the Different
Operating Modes........................................................................................35
The Voltseconds Law, Inductor Reset and Converter Duty Cycle..............39
Using and Protecting Semiconductor Switches............................................41
Evolution of Switching Topologies...........................................................................43
Controlling the Induced Voltage Spike by Diversion
through a Diode.........................................................................................43
Achieving a Steady State and Deriving Useful Energy...............................45
The Buck-Boost Converter............................................................................46
Ground-Referencing Our Circuits.................................................................48
The Buck-Boost Configurations....................................................................48
The Switching Node......................................................................................49
Analyzing the Buck-Boost............................................................................50
Properties of the Buck-Boost........................................................................50
Why Three Basic Topologies Only?.............................................................52
The Boost Topology......................................................................................53
The Buck Topology.......................................................................................57
Advanced Converter Design.........................................................................59
Chapter 2:DC(cid:1)DC Converter Design and Magnetics..........................................61
DC Transfer Functions...............................................................................................62
The DC Level and the “Swing” of the Inductor Current Waveform........................63
Defining the AC, DC, and Peak Currents.................................................................66
Understanding the AC, DC, and Peak Currents........................................................68
Defining the “Worst-Case” Input Voltage.................................................................70
The Current Ripple Ratio “r”....................................................................................72
Relating r to the Inductance.......................................................................................73
The Optimum Value of r...........................................................................................74
Do We Mean Inductor? or Inductance?.....................................................................76
How Inductance and Inductor Size Depend on Frequency..........................77
How Inductance and Inductor Size Depend on Load Current..................................77
How Vendors Specify the Current Rating of an Off-the-shelf Inductor
and How to Select It..............................................................................................78
What Is the Inductor Current Rating We Need to Consider
for a Given Application?........................................................................................79
The Spread and Tolerance of the Current Limit.......................................................83
Worked Example (1)..................................................................................................86
Current Limit Considerations in Setting r....................................................87
Continuous Conduction Mode Considerations in Fixing r...........................88
Setting r to Values Higher than 0.4 when Using
Low-ESR Capacitors..................................................................................91
Setting r to Avoid Device “Eccentricities”..................................................91
Setting r to Avoid Subharmonic Oscillations...............................................93
Contents vii
Quick Selection of Inductors Using “L3I” and
“Load Scaling” Rules.................................................................................96
Worked Examples (2, 3, and 4).................................................................................97
The Current Ripple Ratio r in Forced Continuous Conduction
Mode (“FCCM”)........................................................................................99
Basic Magnetic Definitions.........................................................................100
Worked Example (5) — When Not to Increase the Number of Turns..................103
The “Field Ripple Ratio”............................................................................104
The Voltage-Dependent Equation in Terms
of Voltseconds (MKS Units)...................................................................104
CGS Units....................................................................................................105
The Voltage-Dependent Equation in Terms
of Voltseconds (CGS Units)....................................................................105
Core Loss.....................................................................................................105
Worked Example (6) — Characterizing an Off-the-Shelf Inductor
in a Specific Application......................................................................................106
Estimating Requirements............................................................................107
Current Ripple Ratio...................................................................................109
Peak Current................................................................................................110
Flux Density................................................................................................110
Copper Loss.................................................................................................111
Core Loss.....................................................................................................112
DC(cid:1)DC Converter Design and Magnetics.................................................113
Temperature Rise.........................................................................................113
Calculating “Other” Worst-case Stresses and their Selection Criteria...................114
Worst-case Core Loss..................................................................................114
Worst-case Diode Dissipation.....................................................................115
Note that the General Diode Selection Procedure is as Follows...............116
Worst-case Switch Dissipation....................................................................116
Note that the General Switch Selection Procedure is as Follows..............118
Worst-case Output Capacitor Dissipation...................................................118
Note that the General Output Capacitor Selection Procedure
is as Follows.............................................................................................118
Worst-case Input Capacitor Dissipation.....................................................119
Note that the General Input Capacitor Selection Procedure
is as Follows.............................................................................................120
Chapter 3: Off-Line Converter Design and Magnetics........................................123
Flyback Converter Magnetics..................................................................................124
Polarity of Windings in a Transformer.......................................................124
Transformer Action in a Flyback and Its Duty Cycle................................126
The Equivalent Buck-Boost Models...........................................................129
The Current Ripple Ratio for the Flyback..................................................131
The Leakage Inductance.............................................................................132
viii Contents
Zener Clamp Dissipation.............................................................................132
Secondary-Side Leakages also Affect the Primary Side............................133
Measuring the Effective Primary-side Leakage Inductance.......................134
Worked Example (7) — Designing the Flyback Transformer...................135
Selecting the Wire Gauge and Foil Thickness...........................................142
Forward Converter Magnetics.....................................................................146
Duty Cycle...................................................................................................146
Worst-Case Input Voltage End...................................................................149
Window Utilization.....................................................................................151
Relating Core Size to Its Power Throughput..............................................152
Worked Example (8) — Designing the Forward Transformer..................154
Thermal Resistance.....................................................................................154
Chapter 4:The Topology FAQ........................................................................171
Questions and Answers............................................................................................171
Chapter 5:Advanced Magnetics: Optimal Core Selection...................................195
Part 1: Energy Transfer Principles...........................................................................195
Overview of Topologies..............................................................................195
The Energy Transfer Charts........................................................................203
Peak Energy Storage Requirements............................................................210
Calculating Inductance Based on Desired Current Ripple.........................214
Part 2: Energy to Core Sizes....................................................................................217
Magnetic Circuits and the Effective Length of Gapped Cores..................217
Stored Energy in Gapped Cores and the z-Factor......................................219
Energy of a Gapped Core in Terms of the Volume of the Core...............223
Part 3: Toroids to E-Cores.......................................................................................226
Part 4: More on AC(cid:1)DC Flyback Transformer Design.........................................229
Part 5: More on AC(cid:1)DC Forward Converter Transformer Design.......................233
Chapter 6:Component Ratings, Stresses, Reliability, and Life ............................241
Introduction..............................................................................................................241
Stresses and Derating...............................................................................................242
Part 1: Ratings and Derating in Power Converter Applications.............................245
Operating Environments..............................................................................246
Component Ratings and Stress Factors in Power Supplies........................250
Mechanical Stresses.....................................................................................263
Part 2: MTBF, Failure Rate, Warranty Costs, and Life..........................................264
MTBF...........................................................................................................264
Warranty Costs............................................................................................268
Life Expectancy and Failure Criteria..........................................................269
Reliability Prediction Methods...................................................................270
Demonstrated Reliability Testing (DRT)....................................................272
Accelerated Life Testing.............................................................................273
Part 3: Life Prediction of Aluminum Electrolytic Capacitors................................275
Contents ix
Chapter 7:Optimal Power Components Selection..............................................281
Overview..................................................................................................................281
The Key Stresses in Power Converters....................................................................281
Waveforms and Peak Voltage Stresses for Different Topologies...........................282
The Importance of RMS and Average Currents......................................................288
Calculation of RMS and Average Currents for Diode, FET, and Inductor............289
Calculation of RMS and Average Currents for Capacitors.....................................292
The Stress Spiders....................................................................................................298
Stress Reduction in AC(cid:1)DC Converters................................................................302
RCD Clamps versus RCD Snubbers........................................................................304
Chapter 8: Conduction and Switching Losses.....................................................311
Switching a Resistive Load......................................................................................312
Switching an Inductive Load...................................................................................316
Switching Losses and Conduction Loss..................................................................319
A Simplified Model of the MOSFET for Studying Inductive
Switching Losses..................................................................................................320
The Parasitic Capacitances Expressed in an Alternate System..............................322
Gate Threshold Voltage...........................................................................................324
The Turn-On Transition...........................................................................................324
The Turn-Off Transition..........................................................................................328
Gate Charge Factors.................................................................................................330
Worked Example......................................................................................................332
Turn-On.......................................................................................................333
Turn-Off.......................................................................................................335
Applying the Switching Loss Analysis to Switching Topologies...........................336
Worst-Case Input Voltage for Switching Losses....................................................337
How Switching Losses Vary with the Parasitic Capacitances................................338
Optimizing Driver Capability vis-a`-vis MOSFET Characteristics.........................340
Chapter 9: Discovering New Topologies............................................................343
Part 1: Fixed-Frequency Synchronous Buck Topology..........................................343
Using a FET (Safely) Instead of Diode......................................................343
Birth of Dead Time.....................................................................................346
CdV/dt-Induced Turn-On............................................................................347
Counting on the Body-Diode......................................................................347
External (Paralleled) Schottky Diode.........................................................348
Synchronous (Complementary) Drive........................................................350
Part 2: Fixed-Frequency Synchronous Boost Topology.........................................351
Part 3: Current-Sensing Categories and General Techniques.................................357
DCR Sensing...............................................................................................359
The Inductorless Buck Cell.........................................................................365
Lossless Droop Regulation and Dynamic Voltage Positioning.................367
Part 4: The Four-Switch Buck-Boost......................................................................369
Part 5: Auxiliary Rails and Composite Topologies.................................................375