Origin of Transient Voltages

Lightning, inductive load switching,

and electrostatic discharge (ESD) are

the most common sources of

electrical overstress which produce

transient voltages. Transients are

narrow spikes of voltage ranging from

less than 100 nanoseconds in

duration for ESD, to greater than a

thousand microseconds for lightning

and load switching transients.

Transient voltage magnitudes range

from tens of volts up to more than

10kV.

Direct lightning hits have typical peak

currents of 25kA but can exceed

200kA. Most damage results from the

lesser amounts of transient voltage

and current that bypasses any existing

up-front suppression. The many

parallel circuits in most distribution

systems help in sharing the transient

current, thus minimizing its effects at

any one point. Other lightning related

threats include ground potential rise

and electromagnetic coupling.

Lightning is predictably unpredictable,

so you don’t know where the next

strike will hit.

When an inductive load is switched

off, the stored energy in the inductor is

dumped into the energizing line

creating a voltage spike according to

Faraday’s law of induction: V = -L(di/

dt). These loads can be a transformer,

motor or perhaps the solenoid in a

copy machine. Poor electrical wiring

practices aggravate load switching

transients.

Inductive load switching produces the

broadest range of transient conditions:

50ns duration for electrical fast

transients (EFT, essentially high

A Primer on Transient Voltages...and their

effects on microchips

voltage noise), up to 100 plus

milliseconds for a generator load

dump, when a fully loaded heavy

vehicle generator has its load abruptly

disconnected.

Static electricity is produced when two

dissimilar materials are rubbed

together. Your shoe soles and the

floor, or just normal body movements

while sitting in a chair are typical

examples. On dry days, static

charges increase because dry

materials become good insulators.

Seven to twelve kV are typical values

of voltage buildup. When the humidity

is high, moist skin becomes conduc-

tive, continually draining off charges

and minimizing ESD effects.

Transient Entry

Transients gain entry to wiring and

circuit traces by conduction or

radiation. Examples of conduction

include a direct hit by lightning or a

resulting side-flash, inductive load

switching across a power source, and

contact by an ESD spark. Radiated

energy, transferred by electromag-

netic coupling and magnetic induction

is picked up by conductive material in

close proximity to a discharge channel

of lightning or ESD, or from nearby

wiring carrying a hefty transient of any

origin.

Power lines are prime targets for

direct lightning hits. Although surge

suppression is provided by the power

company to protect transformers, up

to 10kV can still get through to the

service entry of a building. Any other

load in close proximity to an inductive

load being switched feels the sting of

the transients produced. The energy

of an inductive source transient is

often consumed by other parallel

loads. The greater the number of

parallel loads, the lesser the effects of

the transient. Conducted ESD

normally enters a system through the

touch of a fingertip or hand held metal

tool.

Systems which are interconnected

with long wires such as telephones, oil

field and automated factory instrumen-

tation and distributed computer

systems are efficient collectors of

radiated lightning energy. Close

proximity strikes can induce voltages

of 300V or more on signal lines.

Power lines adjacent to computer data

lines have been reported to induce

both destructive and upsetting

transient voltages. Lines switching

high current inductive loads are the

most disruptive. The exceedingly fast

rise time of ESD, in the nanosecond

range, produces efficient coupling into

nearby wiring. A 7kV spark to a nearby

metal desk was observed to upset a

PC from a distance of four feet.

Effects on Microchips

Failure of silicon based electronic

equipment is manifested in several

modes, but can be can be generally

classified in one of the three following

categories: 1) hard failure, 2) upset,

and 3) latent.

Hard failures are those sustaining

permanent damage and must be

replaced to restore normal circuit

operation. If some components are

shorted, they may be vaporized when

they become part of the driving current

path. Failures resulting from latch-up

often char the component and a small

part of the underlying circuit board. At

Series DO01

the other extreme are ESD related

component failures which produce

exceedingly small failure sites, down

into the micron range. These can be

very difficult to diagnose without

sophisticated equipment.

Upset is a temporary malfunction

which may automatically reset or

require manual reset to restore the

system to normal. If a microprocessor

overwrites a memory, serious prob-

lems may occur, depending on the

computer use. Upsets are caused by

many factors including conducted and

radiated ESD, radiated EFT, and low

level conducted and radiated lightning.

Latent failures are parts that have been

zapped only once but did not fail nor

significantly degrade. These become

the “walking wounded” and without

further transient stress fail at a later,

unpredictable date. Some fail within

hours, while others may perform for

several years. These long term latent

failures may be too often blamed on

poor quality while the real culprit is

latent failure syndrome.

Failure levels

The small geometries of individual

components on integrated circuits

(ICs) make them susceptible to

transients although there is some level

of on-chip protec

tion

for most devices

in the form of a thyristor or diode

resistor network.

Nevertheless, ICs which interface with

the outside world, such as line drivers

and line receivers, still fail at 40V to

50V for 8/20us simulated lightning

pulses. Although many of these

components have been hardened to

10kV of ESD, most microchips fail

below 2kV.

Failure threshold levels vary among

vendors depending on the amount of

built-in protection. Also, survival of a

single event does not ensure against

a latent failure at some later date.

Adequate protection at signal line

entry points can ward off commonly

encountered threats.

Summary

Lightning, load switching and ESD

are sources of transient voltages

which can gain entry into sensitive

electronic equipment by conduction

or radiation. The very small geom-

etries of components on ICs makes

them vulnerable to low energy levels

of voltage spikes.

Series D001