Protection of Electronic Circuits from Overvoltages

Ronald B. Standler

• Publisher: Dover Publications
• Year: 2012
• Print ISBN: 9780486425528
• eBook ISBN: 9780486150840
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Temporary failure and permanent damage of electronic systems are often caused by electrical overstresses such as lightning, electromagnetic pulses from nuclear weapons, and switching of reactive loads. Protecting industrial, military, and consumer systems from failure is critical; and until the publication of this volume, the related literature was scattered throughout journals, patents, conference proceedings, military reports, and elsewhere. This convenient text presents practical rules and strategies for circuits designed to protect electronic systems from damage by transient overvoltages. Because many circuits operate from AC supply mains, protection of equipment operating from the mains is also discussed. The five-part treatment covers symptoms and threats, fundamental remedies, types of protective devices, applications of protective devices, and validation of protective measures. Specific topics include damage and upset, environmental threats, standard test waveforms, and properties of nonlinear transient protection devices, plus protective applications related to signal circuits, DC power supplies, and low-voltage AC mains.

Part 1

Symptoms and Threats

1

Damage and Upset

A. NATURE OF ELECTRICAL OVERSTRESS PROBLEM

Electrical overstresses (e.g. from lightning, electromagnetic pulses from nuclear weapons, and switching of reactive loads) can cause failure, permanent degradation, or temporary malfunction of electronic devices and systems. The characterization of these overstresses and the design of effective protection from them is of great importance to manufacturers and users of industrial, military, and consumer electronic equipment.

Electrical overstresses have received increasing attention during the period between 1960 and the present (1988). This trend can be expected to continue. There are several reasons for this trend: (1) devices are becoming more vulnerable; (2) vulnerable systems are becoming more common; and (3) awareness of the existence of overstresses has increased.

Modern semiconductor integrated circuits are much more vulnerable to damage by overstresses than earlier electronic circuits, which used vacuum tubes and relays. Progress in developing faster and denser integrated circuits has been accompanied by a general increase in vulnerability. At the same time that electronic circuits were becoming more vulnerable, they were also becoming more widely used. (As an example, consider desktop computers and videotape recorders: these were nonexistent items in 1960 but are quite common now.) Therefore, there are now more systems to protect from overstresses. Finally, as awareness of overstresses increases, users of vulnerable systems request appropriate protective measures.

In general, techniques for protection against transient overvoltages can be divided into three classes:

1. shielding and grounding
2. application of filters
3. application of nonlinear devices

Shielding, while important, is not sufficient protection against electromagnetic fields from lightning or nuclear weapons, because compromises in the integrity of the shield must be made (e.g., windows in aircraft; long lines must enter the shielded volume to supply electric power and carry communication signals). Various shielding and grounding techniques are covered in detail in books by Ott (1976), Morrison (1977), Ricketts et al. (1976), and Lee (1986) and in government reports by Lasitter and Clark (1970) and Sandia Laboratories (1972). The design of filters is covered in many electrical engineering text and reference books. The emphasis in this book is on the third class of techniques, nonlinear transient protection devices, although some information on filters is included in Chapters 13 and 19.

B. ORGANIZATION OF THIS BOOK

This book is divided into four parts:

1. symptoms and threats
2. nonlinear protection components
3. applications of protection components
4. validating protective measures

1. Symptoms and Threats

Transient overvoltages in electronic circuits can arise from any of the following causes: lightning, electromagnetic pulse produced by nuclear weapons (NEMP), high-power microwave weapons (HPM), electrostatic discharge (ESD), and switching of reactive loads. These sources are described in Chapter 2. These transient overvoltages can be coupled to vulnerable circuits in several different ways:

1. direct injection of current–for example, a lightning strike to an overhead conductor
2. effects of rapidly changing magnetic fields–for example, induced voltage in a conducting loop from changing magnetic fields owing to nearby lightning or NEMP
3. effects of rapidly changing electric field–for example, charging by induction from ESD
4. changes in reference (“ground”) potential due to injection of large currents in a grounding conductor that has nonzero values of resistance and inductance

A discussion of surveys of transient overvoltages in specific environments is given in Chapter 3. The propagation of transient overvoltages from their source to the vulnerable equipment is discussed in Chapter 4. Chapter 5 discusses standard overstress test waveforms that are simplifications of the environment. Chapter 6 gives a brief sketch of protective circuits and devices, which is useful in preparing the reader for the following two parts of the book.

2. Nonlinear Protection Components

Chapters 7 to 14 contain a discussion of properties of various components that are useful to protect circuits and systems from overvoltages. Spark gaps, metal oxide varistors, and avalanche diodes are emphasized. However, other components, such as semiconductor rectifier diodes, thyristors, resistors, inductors, filters, and optoisolators are also discussed. Chapter 15 explains why minimization of parasitic inductance is critical in practical transient overvoltage clamping circuits.

3. Applications of Protective Devices

Chapters 16 to 20 are concerned with application techniques to protect circuits and systems from damage by transient overvoltages. Specific applications of the components in Part 3 are discussed in the context of signal lines, dc power supplies, and low-voltage mains. Protection of circuits and systems from upset is covered in Chapter 21.

4. Validating Protective Measures

Chapters 22 to 24 discuss how to validate protective measures against damage by overstresses. Preparation of a test plan, high-voltage laboratory procedures, and safety are discussed.

C. NOMENCLATURE

There is no general agreement on a name for electrical overstress. The Institute of Electrical and Electronics Engineers (IEEE) in the United States has adopted the word surge to denote an overstress condition that has a duration of less than a few milliseconds. American engineers also use the word surge to mean something quite different: an increase in the rms voltage for a few cycles, which is called a swell by Martzloff and Gruzs (1987). To avoid misunderstanding, the author favors the use of overvoltage, which is translated from the German Überspannung. To emphasize the brief nature of the event, one may say transient overvoltage. The term electrical overstress is more general, because it includes excessive current or energy as well as voltage. To be precise, it is necessary to say electrical overstress, because there are other kinds of adverse environments–for example, extreme temperature. Because this book is only concerned with electrical overstresses, the modifier “electrical” is omitted.

Overstresses can cause two different kinds of adverse outcomes in sensitive electronic circuits and systems: damage or upset. Damage is a permanent failure of hardware. A damaged system may fail completely or partially. The only way to recover from damage is to replace defective components. Upset is a temporary malfunction of a system. Recovery from upset does not require any repair or replacement of hardware. An example of damage is a charred printed circuit board after a lightning strike. An example of upset is loss of the contents of the volatile memory in a computer when there is a brief interruption of power. A system or circuit is said to be vulnerable to damage but susceptible to upset.

Components and circuits that protect vulnerable devices and systems from damage by electrical overstresses are members of a class of devices called terminal protection devices (TPDs) or surge protective devices (SPDs). The term TPD is used by the U.S. military; SPD is used by the engineers in commercial practice. A surge protective device that is intended for electrical power systems is called an arrester.

The word mains is used in this book to refer to low-voltage ac power distribution circuits inside of buildings. In this context, low-voltage means less than 1 kV rms, and ac means a sinusoidal waveform with a frequency between 50 and 400 Hz.

Definitions of these and other specialized terms are contained in the glossary in Appendix A.

D. DAMAGE AND UPSET THRESHOLDS

Because many modern semiconductor devices (small signal transistors, integrated circuits) can be damaged by potential differences that exceed 10 V, the survivability of modern electronics is limited when exposed to transient overvoltages. Modern electronic technology has tended to produce smaller and faster semiconductor devices, particularly high-speed digital logic, microprocessors, metal oxide semiconductor (MOS) memories for computers, and GaAs FETs for microwave use. This progress has led to an increased vulnerability of modern circuits to damage by tra...

Table of contents

• DOVER SCIENCE BOOKS
• Title Page
• Copyright Page
• Preface
• Acknowledgements
• Table of Contents
• Part 1 - Symptoms and Threats
• Part 2 - Protective Devices
• Part 3 - Applications of Protective Devices
• Part 4 - Validating Protective Measures
• Appendix A - Glossary
• Appendix B - List of Manufacturers
• Appendix C - Bibliography
• BIBLIOGRAPHY
• Index
• A CATALOG OF SELECTED DOVER BOOKS IN SCIENCE AND MATHEMATICS