Switching Power Supplies A - Z (eBook)

Cover 1
Contents 6
Preface 12
Acknowledgements 18
C H A P T E R 1 - The Principles of Switching Power Conversion 20
Introduction 22
Overview and Basic Terminology 24
Understanding the Inductor 41
Evolution of Switching Topologies 62
C H A P T E R 2 - DC-DC Converter Design and Magnetics 80
DC Transfer Functions 83
The DC Level and the “Swing” of the Inductor Current Waveform 84
De.ning the AC, DC, and Peak Currents 87
Understanding the AC, DC and Peak Currents 89
Defining the “Worst-case” Input Voltage 91
The Current Ripple Ratio ‘r’ 94
Relating r to the Inductance 94
The Optimum Value of r 96
Do We Mean Inductor? Or Inductance? 98
How Inductance and Inductor Size Depend on Frequency 99
How Inductance and Inductor Size Depend on Load Current 99
How Vendors Specify the Current Rating of an Off-the-shelf Inductor and How to Select It 100
What Is the Inductor Current Rating We Need to Consider for a Given Application? 101
The Spread and Tolerance of the Current Limit 104
Worked Example (1) 107
Worked Examples (2, 3, and 4) 119
Worked Example (5)—When Not to Increase the Number of Turns 125
Worked Example (6)—Characterizing an Off-the-shelf Inductor in a Speci.c Application 129
Calculating the “Other” Worst-case Stresses 137
C H A P T E R 3 - Off-line Converter Design and Magnetics 146
Flyback Converter Magnetics 149
Forward Converter Magnetics 171
C H A P T E R 4 - The Topology FAQ 196
Questions and Answers 198
C H A P T E R 5 - Conduction and Switching Losses 222
Switching a Resistive Load 225
Switching an Inductive Load 229
Switching Losses and Conduction Loss 232
A Simplified Model of the Mosfet for Studying Inductive Switching Losses 234
The Parasitic Capacitances Expressed in an Alternate System 236
Gate Threshold Voltage 237
The Turn-on Transition 237
The Turn-off Transition 241
Gate Charge Factors 243
Worked Example 246
Applying the Switching Loss Analysis to Switching Topologies 250
Worst-case Input Voltage for Switching Losses 251
How Switching Losses Vary with the Parasitic Capacitances 252
Optimizing Driver Capability vis-à-vis Mosfet Characteristics 253
C H A P T E R 6 - Printed Circuit Board Layout 256
Introduction 258
Trace Section Analysis 258
Some Points to Keep in Mind During Layout 259
Thermal Management Concerns 266
C H A P T E R 7 - Feedback Loop Analysis and Stability 268
Transfer Functions, Time Constant and the Forcing Function 270
Understanding ‘e’ and Plotting Curves on Log Scales 271
Time Domain and Frequency Domain Analysis 274
Complex Representation 275
Nonrepetitive Stimuli 277
The s-plane 277
Laplace Transform 279
Disturbances and the Role of Feedback 281
Transfer Function of the RC Filter 283
The Integrator Op-amp (“pole-at-zero” filter) 286
Mathematics in the Log Plane 288
Transfer Function of the LC Filter 289
Summary of Transfer Functions of Passive Filters 292
Poles and Zeros 293
Interaction of Poles and Zeros 295
Closed and Open Loop Gain 296
The Voltage Divider 299
Pulse Width Modulator Transfer Function (gain) 300
Voltage Feedforward 301
Power Stage Transfer Function 302
Plant Transfer Functions of All the Topologies 303
Boost Converter 305
Feedback Stage Transfer Functions 308
Closing the Loop 310
Criteria for Loop Stability 312
Plotting the Open-loop Gain and Phase with an Integrator 312
Canceling the Double Pole of the LC Filter 314
The ESR Zero 315
Designing a Type 3 Op-amp Compensation Network 316
Optimizing the Feedback Loop 320
Input Ripple Rejection 323
Load Transients 324
Type 1 and Type 2 Compensations 325
Transconductance Op-amp Compensation 327
Simpler Transconductance Op-amp Compensation 330
Compensating with Current Mode Control 332
C H A P T E R 8 - EMI from the Ground up—Maxwell to CISPR 342
The Standards 345
Maxwell to EMI 347
Susceptibility/Immunity 352
Some Cost-related Rules-of-thumb 354
EMI for Subassemblies 354
CISPR 22 for Telecom Ports—Proposed Changes 355
C H A P T E R 9 - Measurements and Limits of Conducted EMI 358
Differential Mode and Common Mode Noise 360
How Conducted EMI Is Measured 363
The Conducted Emission Limits 367
Quasi-peak, Average, and Peak Measurements 370
C H A P T E R 10 - Practical EMI Line Filters 374
Safety Issues in EMI Filter Design 376
Practical Line Filters 378
Safety Restrictions on the Total Y-capacitance 386
Equivalent DM and CM Circuits 387
Some Notable Industry Experiences in EMI 390
C H A P T E R 11 - DM and CM Noise in Switching Power Supplies 392
Main Source of DM Noise 394
The Main Source of CM Noise 396
The Ground Choke 404
C H A P T E R 12 - Fixing EMI across the Board 406
The Role of the Transformer in EMI 408
EMI from Diodes 413
Beads, and an Industry Experience—the dV/dt of Schottky Diodes 416
Basic Layout Guidelines 417
Last-ditch Troubleshooting 418
Are We Going to Fail the Radiation Test? 421
C H A P T E R 13 - Input Capacitor and Stability Considerations in EMI Filters 422
Is the DM Choke Saturating? 424
Practical Line Filters in DC-DC Converter Modules 429
C H A P T E R 14 - The Math behind the Electromagnetic Puzzle 436
Math Background—Fourier Series 438
The Rectangular Wave 439
Analysis of the Rectangular Wave 442
The Trapezoid 443
The EMI from a Trapezoid 445
The Road to Cost-effective Filter Design 446
Practical DM Filter Design 449
Practical CM Filter Design 452
APPENDIX 1 - Focusing on Some Real-world Issues 456
Sounds Like Worst-case, But There’s Danger Lurking in the Middle 458
Loop Design Sometimes Compensates for Lower-quality Switchers 459
Re-inventing the Wheel . . . as a Square 461
The Mighty Zener 463
Better Do the Math: Ignore Transfer Functions at Your Own Peril 466
Aluminum Cap Multipliers—Why We Can’t Have Them and Eat Them Too 468
Limit Your Peak Current, Not Your Reliability 471
Reliability Is No Flash in the Pan 474
The Incredible Shrinking Core 478
Plain Lucky We Don’t Live in a PSpice World! 481
Why Does the Ef.ciency of My Flyback Nose-dive? 484
It’s Not a Straight Line: Computing the Correct Drain to Source Resistance from V-I Curves 487
Don’t Have a Scope? Use a DMM, Dummy! 489
Are We Making Light of Electronic Ballasts? 492
More on Designing Reliable Electronic Ballasts 495
The Organizational Side of Power Management: One Engineer’s Perspective 499
APPENDIX 2 - Reference Design Table 504
References 508
Index 512