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Protection of Electronic Circuits from Overvoltages
By Ronald B. Standler
Dover Publications, Inc.Copyright © 1989 Ronald B. Standler
All rights reserved.
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
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.
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 transient overvoltages, owing to the inability of small components to conduct large currents and to breakdown at smaller voltages.
Smaller devices make a more economical use of area on silicon wafers and decrease components cost. These smaller devices also have less parasitic capacitance and are therefore faster. However, devices often fail when the current per unit area becomes too large. The magnitude of transient currents is determined principally by external circuit parameters (e.g., nature of the source, characteristic impedance of transmission lines, resistance and inductance between the source of the transient and the vulnerable circuit, etc.). Smaller devices obviously have less area and are thus more vulnerable to damage from a current of a given level. When breakdown is considered, smaller devices that have less spacing between conductors will break down at lower voltages.
A logical approach to transient protection would be (1) to determine the threshold at which damage occurred, (2) to determine the worst-case electrical overstress that could arrive at a particular device, and (3) to design and install a protective circuit that would limit the worst-case overstress to less than the damage threshold. This simple, scientific approach has become a practical nightmare. First, as is described below there are apparently no simple criteria for determining the maximum overstress that a part can withstand without being damaged. Second, as described throughout this book, a protective circuit that can survive a worst-case overstress is often extremely expensive, massive, and bulky.
1. Damage Threshold
Failure of transformers and motors is caused by breakdown of insulation. The most important parameter in insulation breakdown is the magnitude of the peak voltage, although the rise time may also be important. Once the insulation has broken down, some of the winding is shunted with a low-impedance arc. Transformers and motors can withstand voltages that are much greater than those that cause failure in semiconductor devices. Therefore, recent concerns about damage caused by overvoltages has focused on semiconductors and has tended to ignore damage to transformers and motors.
The value of voltage, current, or power that is necessary to cause permanent damage to semiconductor devices is known as the damage threshold or failure threshold. In general, the value of the threshold is a function of the duration of the overstress. Such information is essential during the design of protective circuits, since the protection must attenuate overstresses to below the damage threshold.
The damage threshold is defined as the minimum power transfer through a terminal such that the device's characteristics are significantly and irreversibly altered. The damage threshold is a function of the waveshape and is particularly sensitive to the duration of the transient.
The most widely used model for damage threshold was presented by Wunsch and Bell (1968). They showed that the maximum power, P, that could be safely dissipated in a semiconductor junction was given by
P = kt-1/2
where t is the time duration of a pulse and k is the damage constant. Devices with a larger value of k are able to withstand larger transients. The inverse square-root dependence on the pulse duration was derived by Wunsch and Bell (1968) for adiabatic heating of the junction. This relation is approximately valid for pulse durations that satisfy
0.1 µs < t < 20 µs
This simple model is known as a thermal model, since the mechanism for damage is melting of the semiconductor by excessive energy deposited in the bulk semiconductor.
Ideally, the value of k would be a constant for a device with a particular model number. Much effort has been devoted to finding the proper value of the damage constant, k, for hundreds of different silicon diodes and transistors. Several conclusions are clear from this effort.
A relation of the form
P = At-B
fits the empirical failure data better than the form where B is one-half. Efforts to predict the values of k, A, or B from parameters on the specification sheet (e.g., thermal resistance) have not been particularly successful, so the value of k or the values of A and B must be determined by experiment.
Enlow (1981) described variations in the mean failure threshold for samples of 100 transistors from each of five manufacturers for four different 2N part numbers. Even for devices of the same model number and same manufacturer, the standard deviation for the failure threshold was often about 25% of the mean value. When failure thresholds for devices of the same model number but different manufacturers were compared, it was clear that specifying the same model number was not adequate to ensure that the two lots of devices came from the same statistical population for failure thresholds. For example, 2N718 transistors from Texas Instruments had a failure threshold of 452 ± 73 W, whereas 2N718 transistors from IDI had a failure threshold of 97 ± 7.2 W (these data are written as [bar.P] ± σ). The mean for the IDI devices is 4.86 σ from the mean of the TI devices; the mean for the TI devices is 49.3 σ from the mean of the IDI devices:
97 = 452 - (4.86 × 73)
452 = 97 + (49.3 × 7.2)
These two distributions of failure thresholds are clearly distinct. Measurement of Texas Instruments 2N718 transistors tells nothing about IDI 2N718 transistors.
Kalab (1982) investigated failure thresholds for 2N1613 and 2N4237 transistors from 30 and 29 manufacturers, respectively. One transistor of each model from each manufacturer was opened and inspected with a microscope to determine the geometry. Sixteen different geometries were used for each model–evidently some manufacturers used the same pattern as other manufacturers. For the 2N4237 transistor, the ratio of smallest to largest chip area was a factor of 20. Clearly the model number does not specify how these transistors are fabricated. Twenty samples of each model from each manufacturer were pulsed with a rectangular waveform with a duration of 1 µs and a polarity that reverse-biased one junction. The ratio of minimum to maximum failure power for the collector-base junction for these 1800 transistors of each model was about 3 x 104. Clearly the collector-base failure power cannot be specified by model number.
These damage thresholds are a statistical concept, not precise numbers that are applicable to a particular piece part. When damage thresholds are being determined, the device either fails or it does not fail. If it does fail, one has an upper bound for the damage threshold but no information about the effect of slightly smaller stresses. After the device fails, the experiment cannot be repeated for that particular piece part.
By testing a large number of devices, one can obtain a statistical distribution of damage thresholds and fit various models to these data. Such effort is expensive, and the results are not applicable to components of the same model number from a different manufacturer. Worse yet, there are apparently no discussions in the literature about whether such statistical distributions are applicable to different production lots of the same model number and same manufacturer.
Excerpted from Protection of Electronic Circuits from Overvoltages by Ronald B. Standler. Copyright © 1989 Ronald B. Standler. Excerpted by permission of Dover Publications, Inc..
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