Ray Perez
Associate Editor
In the arena of EMI measurements, conducted emission from power supplies and converters rank very much at the top of noise sources that EMC engineers spend most time in suppressing. No only can you measure this noise in conductive tests, but also in radiated tests since a lot of these conducted noise sources also act as unintentional common mode voltage sources capable of causing radiated noise in unintentional antennas (e.g power cables, I/O cables,...etc). I’d like to spend some time in the next issues of the EMCS Newsletter (only when my turn arrives) reviewing a series of switching mode power supply (SMPS) design books that I’ve found useful in the past. These are non-EMC books from the point of view that EMI is addressed only as a chapter or as several sections in the books. However, though these are basically design books, if you are an EMC engineer you are certainly aware that a lot of what EMI does to your hardware depends mostly on how you designed your power supply boards. It is to the benefit of all EMC engineers that they also be well versed in design issues so that they can contribute primarily in the early design process of power subsystems rather than doing “patch work” in containing EMI at the end of the product cycle.
The first of these books is a paperback version written by John Lenk who is a well-known writer for the prestigious electronic design magazine known as EDN. John has written quite a few books, of which some have become best sellers (a difficult thing for an engineering book). This is really a simplified “hands-on” SMPS book which I consider useful for those who want to acquire a practical, yet not detailed design background in these types of power supply design. The book provides sufficient information to design and build switching power supplies from scratch. There are five chapters in the book. The first four chapters provide the basics for all phases of practical design, including test and troubleshooting for switching supplies. The final chapter includes about 100 worked-out design examples, using the techniques described in the first four chapters. In the design examples, each example starts with several approximations or guidelines for choosing the components on a trial basis assuming a set of design goals and initial conditions. Using then these approximate values in experimental circuits, the desired results are produced by varying the test component values.
The first chapter covers basic switching power supply design circuits. The emphasis is on switching regulators which are available these days in integrated circuits (IC). The data sheets for IC switching regulators often show the connections and provide all the necessary design parameters to convert the IC to a complete supply by just adding the needed external components. Chapter 1 describes the functions and operations of switching mode regulators, such as basic switching regulator functions and typical switching regulator circuits. Switching regulator theory is described in good detail for the five typical kinds of switching regulator circuits such as: booster or step-up, buck-boost or inverting, buck or step down, fly back, and forward.
Chapter 2 covers the interesting subject of heat sinks for SMPS. It is often assumed that switching regulators do not require heat sink. Although this is sometimes true in many cases, it may be necessary to use heat sinks for high current switching regulators. Switching supplies contain at least one shunt or series transistor that must pass the load current. The power dissipated can be significant and the need for proper design of heat sinks is necessary. Would you believe that temperature problems and bad design of heat sinks can affect the EMI noise in SMPS? Believe it, and here is why. Most data sheets specify components parameters (transistors, diodes, rectifiers, ICs, etc.) at a given temperature. However, most of these parameters change with temperature. Because components rarely operate at the exact temperature shown on the data sheets (usually 25C is chosen), it is important to know the parameters at the actual operating temperature. For example, in the case of transistors, the critical parameters that change with temperature are current gain, collector leakage, and power dissipation. A temperature increase, for example, can also increase the current gain of a transistor and hence the possibility of more switching noise being generated. Furthermore, heat sinks introduce parasitic effects into the overall circuit design and such parasitics can affect the current distribution paths of common mode currents which are the major generator of noise. Therefore, not only is it important to properly design heat sinks to offset the effects of temperature, but also to diminish the effect of parasitics. Chapter 2 covers such topics as thermal resistance, thermal runaway, heat sink ratings, a commercial heat sink selection guide, and calculating heat sink capabilities (e.g power dissipation).
Chapter 3 is devoted to the inductors and transformers used in SMPS. The magnetic components are the greatest source of design problems and design failures in switching supplies. Inductors design basics are covered in the chapter, such as inductor value basics and the procedures to use the correct inductance value, optimum inductance, saturation effects, core material trade-off, high frequency core losses and high flux MPP cores. The solution to EMI problems usually depend on application trade-off (such as usage of shielded inductors if EMI must be kept to a minimum). The chapter goes through the design principles in selecting the inductors for step-up regulators, step-down regulators, and inverting regulators. Transformer design and selection is also addressed in the chapter with an example of a transformer design.
Troubleshooting and testing is discussed in chapter 4. The chapter is devoted to testing and troubleshooting for switching supplies in general. The first sections of the chapter cover testing as the first step in troubleshooting is to test the circuit. The testing sections start by describing test procedures that are generally sufficient for most practical applications. The troubleshooting section provides a series of notes to localize problems if the circuits are failing to perform as expected. These notes are included primarily for those readers who are not familiar with switching supplies. Among the tests discussed are: output tests, load regulation tests, basic line-regulation tests, efficiency tests, ripple tests (conducted emissions), transformer characteristics, transformer impedance ration, and transformer winding balance. Switching power supplies present particular problems when they are being tested, some of which are quite unique. Ground loops is one of the most common.
The last chapter of the book, Chapter 5, occupies about half of the book. All of the general design information in the previous four chapters will be used in the last chapter. However, each IC has special design requirements, all of which are discussed in great detail. The circuits in this chapter can be used as is or modified by altering component values. Among the IC regulator design considered in detail are those using Raytheon, Harris, and Linear Technology IC for switching regulation. For each of the design processes, the following is covered: basic design approach, step-up design, step-down design, inductor selection, low battery detector, bias current shutdown, buck boost application, feedback compensation, short circuit protection, output filter capacitor, controlling output EMI, input and output filtering, switching diodes selection, and others.
I would recommend this book as a primer for SMPS design. The book is also filled with manufacturer data sheets and many specifications typically found in SMPS design IC. In my next review, we will visit one more of these books before we address more detailed books.