Switching Power Supplies A - Z (Second Edition)

Front Cover......Page 1
Switching Power Supplies A–Z......Page 4
Copyright page......Page 5
Contents......Page 6
Preface......Page 16
Acknowledgments......Page 20
Introduction......Page 22
Efficiency......Page 25
Linear Regulators......Page 26
Achieving High Efficiency through Switching......Page 28
Basic Types of Semiconductor Switches......Page 29
Semiconductor Switches Are Not “Perfect”......Page 31
Achieving High Efficiency through the Use of Reactive Components......Page 32
Early RC-Based Switching Regulators......Page 33
LC-Based Switching Regulators......Page 34
The Role of Parasitics......Page 35
Switching at High Frequencies......Page 37
Reliability, Life, and Thermal Management......Page 38
Stress Derating......Page 40
Advances in Technology......Page 41
Capacitors/Inductors and Voltage/Current......Page 42
The Inductor and Capacitor Charging/Discharging Circuits......Page 43
The Charging Phase and the Concept of Induced Voltage......Page 45
The Effect of the Series Resistance on the Time Constant......Page 47
The Inductor Charging Circuit with R = 0 and the “Inductor Equation”......Page 49
The Duality Principle......Page 50
The “Capacitor Equation”......Page 51
The Inductor Discharge Phase......Page 52
Current Must Be Continuous, Its Slope Need Not Be......Page 53
The Voltage Reversal Phenomenon......Page 55
A Steady State in Power Conversion and the Different Operating Modes......Page 56
The Voltseconds Law, Inductor Reset and Converter Duty Cycle......Page 60
Using and Protecting Semiconductor Switches......Page 62
Controlling the Induced Voltage Spike by Diversion through a Diode......Page 64
Achieving a Steady State and Deriving Useful Energy......Page 66
The Buck-Boost Converter......Page 67
The Buck-Boost Configurations......Page 69
The Switching Node......Page 70
Properties of the Buck-Boost......Page 71
Why Three Basic Topologies Only?......Page 73
The Boost Topology......Page 74
The Buck Topology......Page 78
Advanced Converter Design......Page 80
2 DC–DC Converter Design and Magnetics......Page 82
DC Transfer Functions......Page 83
The DC Level and the “Swing” of the Inductor Current Waveform......Page 84
Defining the AC, DC, and Peak Currents......Page 87
Understanding the AC, DC, and Peak Currents......Page 89
Defining the “Worst-Case” Input Voltage......Page 91
The Current Ripple Ratio “r”......Page 93
Relating r to the Inductance......Page 94
The Optimum Value of r......Page 95
Do We Mean Inductor? or Inductance?......Page 97
How Inductance and Inductor Size Depend on Load Current......Page 98
How Vendors Specify the Current Rating of an Off-the-shelf Inductor and How to Select It......Page 99
What Is the Inductor Current Rating We Need to Consider for a Given Application?......Page 100
The Spread and Tolerance of the Current Limit......Page 104
Worked Example (1)......Page 107
Current Limit Considerations in Setting r......Page 108
Continuous Conduction Mode Considerations in Fixing r......Page 109
Setting r to Avoid Device “Eccentricities”......Page 112
Setting r to Avoid Subharmonic Oscillations......Page 114
Quick Selection of Inductors Using “L×I” and “Load Scaling” Rules......Page 117
Worked Examples (2, 3, and 4)......Page 118
The Current Ripple Ratio r in Forced Continuous Conduction Mode (“FCCM”)......Page 120
Basic Magnetic Definitions......Page 121
Worked Example (5) — When Not to Increase the Number of Turns......Page 124
The Voltage-Dependent Equation in Terms of Voltseconds (MKS Units)......Page 125
Core Loss......Page 126
Worked Example (6) — Characterizing an Off-the-Shelf Inductor in a Specific Application......Page 127
Estimating Requirements......Page 128
Current Ripple Ratio......Page 130
Flux Density......Page 131
Copper Loss......Page 132
Core Loss......Page 133
Temperature Rise......Page 134
Worst-case Core Loss......Page 135
Worst-case Diode Dissipation......Page 136
Worst-case Switch Dissipation......Page 137
Note that the General Output Capacitor Selection Procedure is as Follows......Page 139
Worst-case Input Capacitor Dissipation......Page 140
Note that the General Input Capacitor Selection Procedure is as Follows......Page 141
3 Off-Line Converter Design and Magnetics......Page 144
Polarity of Windings in a Transformer......Page 145
Transformer Action in a Flyback and Its Duty Cycle......Page 147
The Equivalent Buck-Boost Models......Page 150
The Current Ripple Ratio for the Flyback......Page 152
Zener Clamp Dissipation......Page 153
Secondary-Side Leakages also Affect the Primary Side......Page 154
Measuring the Effective Primary-side Leakage Inductance......Page 155
Turns Ratio......Page 156
Effective Load Current on Primary and Secondary Sides......Page 157
Actual Center of Primary and Secondary Current Ramps......Page 158
Selecting the Core......Page 159
Number of Turns......Page 160
Actual B-Field......Page 161
Air Gap......Page 162
Selecting the Wire Gauge and Foil Thickness......Page 163
Duty Cycle......Page 167
Worst-Case Input Voltage End......Page 170
Window Utilization......Page 172
Relating Core Size to Its Power Throughput......Page 173
Thermal Resistance......Page 175
Voltµseconds......Page 176
Secondary Foil Thickness and Losses......Page 177
Primary Winding and Losses......Page 182
First Iteration......Page 186
Second Iteration......Page 187
Fourth Iteration......Page 188
Total Transformer Losses......Page 190
Questions and Answers......Page 192
Overview of Topologies......Page 216
The Energy Transfer Charts......Page 224
Peak Energy Storage Requirements......Page 231
Calculating Inductance Based on Desired Current Ripple......Page 235
Magnetic Circuits and the Effective Length of Gapped Cores......Page 238
Stored Energy in Gapped Cores and the z-Factor......Page 240
Energy of a Gapped Core in Terms of the Volume of the Core......Page 244
Part 3: Toroids to E-Cores......Page 247
Part 4: More on AC–DC Flyback Transformer Design......Page 250
Part 5: More on AC–DC Forward Converter Transformer Design......Page 254
Introduction......Page 262
Stresses and Derating......Page 263
Part 1: Ratings and Derating in Power Converter Applications......Page 266
Operating Environments......Page 267
Diodes......Page 271
MOSFETs......Page 278
Capacitors......Page 282
Mechanical Stresses......Page 284
MTBF......Page 285
Warranty Costs......Page 289
Life Expectancy and Failure Criteria......Page 290
Reliability Prediction Methods......Page 291
Demonstrated Reliability Testing (DRT)......Page 293
Accelerated Life Testing......Page 294
Part 3: Life Prediction of Aluminum Electrolytic Capacitors......Page 296
The Key Stresses in Power Converters......Page 302
Waveforms and Peak Voltage Stresses for Different Topologies......Page 303
The Importance of RMS and Average Currents......Page 309
Calculation of RMS and Average Currents for Diode, FET, and Inductor......Page 310
Calculation of RMS and Average Currents for Capacitors......Page 313
The Stress Spiders......Page 319
Stress Reduction in AC–DC Converters......Page 323
RCD Clamps versus RCD Snubbers......Page 325
8 Conduction and Switching Losses......Page 332
Switching a Resistive Load......Page 333
Switching an Inductive Load......Page 337
Switching Losses and Conduction Loss......Page 340
A Simplified Model of the MOSFET for Studying Inductive Switching Losses......Page 341
The Parasitic Capacitances Expressed in an Alternate System......Page 343
The Turn-On Transition......Page 345
The Turn-Off Transition......Page 349
Gate Charge Factors......Page 351
Worked Example......Page 353
Turn-On......Page 354
Turn-Off......Page 356
Applying the Switching Loss Analysis to Switching Topologies......Page 357
Worst-Case Input Voltage for Switching Losses......Page 358
How Switching Losses Vary with the Parasitic Capacitances......Page 359
Optimizing Driver Capability vis-à-vis MOSFET Characteristics......Page 361
Using a FET (Safely) Instead of Diode......Page 364
Birth of Dead Time......Page 367
Counting on the Body-Diode......Page 368
External (Paralleled) Schottky Diode......Page 369
Synchronous (Complementary) Drive......Page 371
Part 2: Fixed-Frequency Synchronous Boost Topology......Page 372
Part 3: Current-Sensing Categories and General Techniques......Page 378
DCR Sensing......Page 380
The Inductorless Buck Cell......Page 386
Lossless Droop Regulation and Dynamic Voltage Positioning......Page 388
Part 4: The Four-Switch Buck-Boost......Page 390
Part 5: Auxiliary Rails and Composite Topologies......Page 396
Is It a Boost or Is It a Buck-Boost?......Page 398
Understanding the Cuk, Sepic, and Zeta Topologies......Page 400
Generating the Current Waveforms of the Cuk, Sepic, and Zeta Converters......Page 404
Stresses in the Cuk, Sepic, and Zeta Topologies and Component Selection Criteria......Page 406
Part 6: Configurations and “Topology Morphology”......Page 408
Multiple Outputs and the Floating Buck Regulator......Page 412
Hysteretic Controllers......Page 415
Pulse-Skipping Mode......Page 419
Achieving Transformer Reset in Forward Converters......Page 420
Trace Section Analysis......Page 424
Some Points to Keep in Mind During Layout......Page 425
Thermal Management Concerns......Page 432
Thermal Resistance and Board Construction......Page 434
Historical Definitions......Page 437
Empirical Equations for Natural Convection......Page 438
Comparing the Two Standard Empirical Equations......Page 440
“h” from Thermodynamic Theory......Page 441
PCB Copper Area Estimate......Page 442
Sizing Copper Traces......Page 443
Forced Air Cooling......Page 444
Radiative Heat Transfer......Page 446
Miscellaneous Issues......Page 447
Transfer Functions, Time Constant, and the Forcing Function......Page 450
Understanding “e” and Plotting Curves on Log Scales......Page 452
Flashback: Complex Representation......Page 453
Repetitive and Nonrepetitive Stimuli: Time Domain and Frequency Domain Analyses......Page 454
The s-Plane......Page 456
Laplace Transform Method......Page 457
Disturbances and the Role of Feedback......Page 459
Transfer Function of the RC Filter, Gain, and the Bode Plot......Page 462
The Integrator Op-amp (“Pole-at-Zero” Filter)......Page 465
Transfer Function of the Post-LC Filter......Page 468
Summary of Transfer Functions of Passive Filters......Page 472
Poles and Zeros......Page 473
“Interactions” of Poles and Zeros......Page 475
Closed and Open-Loop Gain......Page 476
The Voltage Divider......Page 480
Voltage (Line) Feedforward......Page 481
Power Stage Transfer Function......Page 482
Plant Transfer Functions of All the Topologies......Page 483
Feedback-Stage Transfer Functions......Page 489
Closing the Loop......Page 491
Criteria and Strategy for Ensuring Loop Stability......Page 494
Plotting the Open-Loop Gain for the Three Topologies......Page 495
The ESR-Zero......Page 498
High-Frequency Pole......Page 499
Designing a Type 3 Op-Amp Compensation Network......Page 500
Optimizing the Feedback Loop......Page 504
Input Ripple Rejection......Page 506
Load Transients......Page 508
Transconductance Op-Amp Compensation......Page 509
Simpler Transconductance Op-Amp Compensation......Page 514
Compensating with Current-Mode Control......Page 516
Buck Converter Input and Output Voltage Ripple......Page 526
Overview......Page 532
Power Scaling Guidelines in Power Converters......Page 533
Concept behind Paralleling and Interleaving of Buck Converters......Page 536
Closed-Form Equations for RMS Stresses of Interleaved Buck Converter......Page 543
Interleaved Multioutput Converters......Page 546
Overview......Page 547
Passive Sharing......Page 559
Active Load Sharing......Page 564
Overview......Page 568
The Charging and Discharging Phases......Page 569
Increasing the Capacitance, Thereby Reducing tCOND, Causes High RMS Currents......Page 571
The Capacitor Voltage Trajectory and the Basic Intervals......Page 574
Tolerating High Input Voltage Ripple in AC–DC Switching Converters......Page 575
How the Bulk Capacitor Voltage Ripple Impacts the Switching Converter Design......Page 576
General Flyback Fault Protection Schemes......Page 577
The Input Current Shape and the Capacitor Current......Page 580
How to Interpret μF/W correctly......Page 582
Worked Example using either Quick Lookup Numbers or the “Arctic Analogy”......Page 583
Accounting for Capacitor Tolerances and Life......Page 585
Holdup Time Considerations......Page 586
Two Different Flyback Design Strategies for Meeting Holdup Requirements......Page 592
Overview......Page 595
How to get a Boost Topology to exhibit a Sine-wave Input Current?......Page 598
Anti-Synchronization Technique for PFC and PWM Stages......Page 603
Capacitor RMS Current Calculations With and Without Anti-Synchronization......Page 610
Interleaved Boost PFC Stages......Page 611
Practical Issues in Designing PFC Stages......Page 612
PFC Choke Design Guidelines......Page 613
Core Losses in PFC Choke......Page 615
Borderline Active PFC using Boost......Page 617
The Standards......Page 618
EMI Limits......Page 620
EMI for Subassemblies......Page 623
Electromagnetic Waves and Fields......Page 624
Extrapolation......Page 629
Quasi-Peak, Average, and Peak Measurements......Page 630
Differential Mode and Common Mode Noise......Page 631
Measuring Conducted EMI with a LISN......Page 634
Separating CM and DM Components for Conducted EMI Diagnostics......Page 637
Near-Field Sniffers for Radiated EMI Diagnostics......Page 640
Basic Safety Issues in EMI Filter Design......Page 642
Safety Restrictions on the Total Y-Capacitance......Page 645
Practical Line Filters......Page 646
Examining the Equivalent DM and CM Circuits and Filter Design Hints......Page 654
Some Notable Industry Experiences in EMI Filter Design......Page 657
The Main Source of CM Noise......Page 658
Chassis-Mounting of Semiconductors......Page 661
The CM Noise Source......Page 662
The Road to Cost-Effective Filter Design......Page 664
The Ground Plane......Page 666
The Role of the Transformer in EMI......Page 667
EMI from Diodes......Page 673
Are We Going to Fail the Radiation Test?......Page 676
Part 2: Modules and Input Instability......Page 677
Practical Line Filters in DC–DC Converter Modules......Page 678
Fourier Series in Power Supplies......Page 684
The Rectangular Wave......Page 685
The Sinc Function......Page 687
The Envelope of the Fourier Amplitudes......Page 689
Practical DM Filter Design......Page 693
ESR Estimate......Page 694
DM Calculations at High Line......Page 695
DM Calculations at Low Line......Page 697
CM Calculations at High Line......Page 699
19 Solved Examples......Page 704
Part 1: FET Selection......Page 711
Part 2: Conduction Losses in the FETs......Page 712
Part 3: FET Switching Losses......Page 714
Part 4: Inductor Loss......Page 717
Part 5: Input Capacitor Selection and Loss......Page 720
Part 6: Output Capacitor Selection and Loss......Page 721
Part 7: Total Losses and Efficiency Estimate......Page 722
Part 8: Junction Temperature Estimates......Page 723
Part 9: Control Loop Design......Page 724
Appendix......Page 730
Further Reading......Page 742
Index......Page 744