Some thoughts on DC/DC converters, part one

EDN will be publishing Chapter 4 of Linear Technology's book "Analog Circuit Design, Volume II". This chapter is an extremely informative tutorial for anyone that will be designing with DC/DC switching converters.

The following is Chapter 4 "Some thoughts on DC/DC converters" by the late Jim Williams and Brian Huffman from Linear Technology's Volume II book entitled, "Analog Circuit Design-- Immersion in the Black Art of analog design" by Bob Dobkin and the late-Jim Williams published by Elsevier/Newnes.

This book is a great companion volume to Volume I with informative application notes and a full complement of reference designs. The chapters are not just every day application notes and reference designs, but give insights to problem-solving, design decision-making the thought process that goes along with a robust, successful design. That's why I love this book. The authors, Bob Dobkin and the late Jim Williams (Along with the research and writings of Carl Nelson and Bob Widlar), have a rich history and depth of experience that they share with the readers. This book is a keeper that needs to be on every designer's bookshelf, right next to Volume I.

I have designed many power management solutions in my career in the past 40 years. I wish that I had such a book like this on my shelf because it would have made my design efforts and decisions so much easier. A chapter such as Chapter 4 is a "golden nugget" of information for all levels of designers that need to develop a power management system.

This chapter will be published on EDN Power Management Design Center in five parts since it encompasses a great deal of information. You will certainly benefit from these ideas and designers will have added insights into selection of the proper architecture and implementation of a robust design in a power management system. Please enjoy part one.

Introduction

Many systems require that the primary source of DC power be converted to other voltages. Battery driven circuitry is an obvious candidate. The 6V or 12V cell in a laptop computer must be converted to different potentials needed for memory, disc drives, display and operating logic. In theory, AC line powered systems should not need DC/DC converters because the implied power transformer can be equipped with multiple secondaries. In practice, economics, noise requirements, supply bus distribution problems and other constraints often make DC/DC conversion preferable. A common example is logic dominated, 5V powered systems utilizing ±15V driven analog components.

The range of applications for DC/DC converters is large, with many variations. Interest in converters is commensurately quite high. Increased use of single supply powered systems, stiffening performance requirements and battery operation have increased converter usage.

Historically, efficiency and size have received heavy emphasis. In fact, these parameters can be significant, but often are of secondary importance. A possible reason behind the continued and overwhelming attention to size and efficiency in converters proves surprising. Simply put, these parameters are (within limits) relatively easy to achieve! Size and efficiency advantages have their place, but other system-oriented problems also need treatment.

Low quiescent current, wide ranges of allowable inputs, substantial reductions in wideband output noise and cost effectiveness are important issues. One very important converter class, the 5V to ±15V type, stresses size and efficiency with little emphasis towards parameters such as output noise. This is particularly significant because wideband output noise is a frequently encountered problem with this type of converter. In the best case, the output noise mandates careful board layout and grounding schemes.

In the worst case, the noise precludes analog circuitry from achieving desired performance levels (for further discussion see Appendix A, “The 5V to ±15V Converter — A Special Case”). The 5V to ±15V DC/DC conversion requirement is ubiquitous, and presents a good starting point for a study of DC/DC converters.