The purpose of this document is to provide
an overview of the degradation process that can occur in metal
oxide varistors (MOVs). MOVs are variable resistors primarily
consisting of zinc oxide (ZnO) with the function of limiting
or diverting transient voltage surges. MOVs exhibit a relative
high energy absorption capability which is important to the
long term stability of the device. The growing demand of ZnO
varistors is due to the nonlinear characteristics as well as
the range of voltage and current over which they can be used.
This range is far superior to devices composed of other
materials that were used prior to the development of MOVs.1
If MOVs are used within their well-defined
specifications, degradation due to the environment is not
likely. However, the environment that MOVs are used in is not
well-defined. Low voltage ac mains are subject to lightning
strikes, switching transients, voltage swells/sags, temporary
overvoltages (TOVs) and other similar disturbances. Due to the
variety of disturbances that MOVs are exposed to, degradation
or failure are possible in many applications.
MOVs perform their intended function
reliably and experience low failure rates when applied within
their specified limits. For an MOV to operate without failure
or degradation it must quickly dissipate absorbed energy and
return to its standby operating temperature. The ability to
dissipate energy to the environment will depend on the design
of the environment itself—ambient temperature, ventilation,
heat sinking, other component population and density,
proximity of heat sources, weight of PCB conductor traces,
presence of thermal cutoff devices, etc. Degradation and
catastrophic failures may occur if an MOV is subjected to
transient surges beyond its rated values of energy and peak
current.
The life of an MOV is defined as the time
required reaching a thermal runaway condition. The
relationship between ambient temperature and the life of an
MOV can be expressed by Arrhenius rate equation,
t = t0exp[Ea-f(V)]/RT
where :
(t) = the time to thermal runaway,
t0 = constant,
R = constant,
Ea = activation energy,
T = temperature in Kelvin,
and f(V) = applied voltage.
Most Arrhenius rate models impose increased
voltage and/or elevated temperature to accelerate the reaction
rate (i.e., degradation or time to thermal runaway) and do not
adequately address the detrimental effects of surge history.2 Surge history, especially transient surges beyond rated
maximums, are perhaps the greatest single contributor to
reductions in varistor voltage, increased standby leakage
current, and ultimate thermal runaway. When increased voltage
is applied for durations longer than microseconds, physical
and chemical changes occur within the many boundary layers of
a multi-junction MOV device. As with single-junction
semiconductor devices, these changes occur on electronic and
atomic scales at rates determined by the diffusion rates of
structural defects—electrons, electron holes, and
interstitial vacancies and ions. The MOV’s joule heating
increases rapidly and exceeds the MOV’s ability to dissipate
heat causing a thermal runaway condition and ending the MOV’s
effective life.
Figure
1
Metal Oxide Varistors Description
MOVs are bipolar ceramic semiconductor
devices that operate as nonlinear resistors when the voltage
exceeds the maximum continuous operating voltage (MCOV). The
term varistor is a generic name for voltage-variable
resistor. The resistance of an MOV decreases as voltage
magnitude increases. An MOV acts as an open circuit during
normal operating voltages and conducts current during voltage
transients or an elevation in voltage above the rated MCOV.
Modern MOVs are developed using zinc oxide
due to their nonlinear characteristics and the useful range of
voltage and current is far superior to silicon carbide
varistors. The characteristic feature of zinc oxide varistors
is the exponential variation of current over a narrow range of
applied voltage. Within the useful varistor voltage range, the
voltage-current relationship is approximated by the
expression:2
where:
I = current in amperes,
V = voltage,
A = a material constant, and
a = exponent defining
the degree of nonlinearity.
MOV Failures
MOVs have a large, but limited, capacity to
absorb energy, and as a result they are subject to an
occasional failure. The significant MOV failure mechanisms
include: electrical puncture, thermal cracking, and thermal
runaway, all resulting from excessive heating, in particular,
from non-uniform heating. Non-uniform joule heating occurs in
MOVs as a result of electrical properties that originate in
either the varistor fabrication process or in the statistical
fluctuations of properties that generally occur in
polycrystalline materials.6
There are three basic failure modes for
MOVs used within surge protective devices.3
1. The MOV fails as a short circuit.
2. The MOV fails as an open circuit.
3. The MOV fails as a linear resistance.
Note: Small-diameter MOVs that initially
fail short circuit are likely to fail as an open circuit due
to the absorption of large continuous current within the MOV.
The short-circuit failure of an MOV is
usually confined to a puncture site between the two electrodes
on the disk. Large fault current can create plasma inside the
ceramic, with temperatures high enough to melt the zinc oxide
ceramic. This failure mode can be caused by long-duration
overvoltage, such as switching from a reactive load or thermal
runaway of the MOV connected to the ac mains.
Open circuit failures are possible if an
MOV is operated at steady state conditions above its voltage
rating. The exponential increase in current causes overheating
and eventual separation of the wire lead and disk at the
solder junction.
Degradation
of MOVs
It
is well-known that MOVs experience degradation due to single
and multiple current impulses. The test results documented in
Mardira, Saha and Sutton show that MOVs can be degraded from
an 8/20us surge current at 1.5 times the rated MOV surge
current. A 20 mm MOV with a 10 kA surge current rating will be
degraded if a 15 kA single pulse surge current is applied.5
When MOVs degrade they become more
conductive after they have been stressed by either continuous
current or surge current. MOVs generally experience
degradation due to excessive surges exceeding the MOV’s
rating while in operation. However, many MOVs show no signs of
degradation when operated below a specified threshold voltage.
The degradation of MOVs is primarily dependent on their
composition and fabrication, as well as their application or
duty.
Degraded MOVs were found to have smaller
average grain size and change in the diffraction peak position
compared to a new sample.5 The non-uniform
temperature distribution in the material is due to the
development of localized hot spotting during the current
impulse and the dissolving in some other phases.
In high current conditions the zinc oxide
junctions of the MOV begin to degrade resulting in a lower
measured MCOV or turn-on voltage. As the degradation
continues, and the MOV’s MCOV continues to drop until it
conducts continuously, shorting or fragmenting within several
seconds.
One of the key parameters related to
measuring degradation of a varistor is leakage current.
Leakage current in the pre-breakdown region of an MOV is
important for two reasons:
1. Leakage determines the amount of watt
loss an MOV is expected to generate upon application of a
nominal steady-state operating voltage.
2. The leakage current determines the
magnitude of the steady-state operating voltage that the MOV
can accept without generating an excessive amount of heat.
The total leakage current is composed of a
resistive current and a capacitive current. The resistive
component of current is thermally stimulated and is
significant, since it is responsible for the joule heating
within the device. The capacitive current is a function of the
MOV’s capacitance value and the applied ac voltage. If an
MOV is subjected to an elevated voltage at a specific
temperature, the internal current increases with time.
Conversely, if the MOV is subjected to an elevated temperature
at a specific applied voltage, the internal current increases
with time. This phenomenon is accelerated by higher operating
stress, and is further aggravated by elevated temperatures.
The life of an MOV is primarily determined by the magnitude of
the internal current and its increase in temperature, voltage,
and time. As the current increases, the amount of heat (if not
allowed to dissipate) can rapidly raise the temperature of the
device. This condition may result in thermal runaway that can
cause destruction of the MOV.
Tests were performed to induce thermal
runaway. Photo 1 is a 40 mm MOV with an MCOV rating of 130 volts ac. During the
test 240 V ac were applied at 15 amps and the MOV ignited.
MOVs exhibit greater power dissipation at
higher temperatures given a fixed voltage. This characteristic
can lead to thermal runaway. If the increase in power
dissipation of the MOV occurs more rapidly than the MOV can
transfer heat to the environment, the temperature of the MOV
will increase until it is destroyed.
MOVs degrade gradually when subjected to
surge currents above their rated capacity. The end-of-life is
commonly specified when the measured varistor voltage (Vn) has
changed by + 10 percent.4 MOVs usually are
functional after the end-of-life, as defined. However, if an
MOV experiences sequential surge events, each causing an
additional 10 percent reduction of Vn, the MOV may soon reach
a Vn level below the peak recurring value for the applied Vrms.
When this state is reached the MOV draws in excess of 1 mA of
current during each half-cycle of the sine wave voltage, a
condition tantamount to thermal runaway. In nearly all cases,
the value of Vn decreases with exposure to surge currents. The
degradation manifests itself as an increase in idle current at
the maximum normal operating voltage in the system. Excessive
idle current during normal, steady-state operation will cause
heating in the varistor. Because the varistor has a negative
temperature coefficient, the current will increase as the
varistor becomes hotter. Thermal runaway may occur, with
consequent failure of the varistor.
Littelfuse publishes varistor pulse rating
curves that are shown in figure
3. The pulse rating curves plot the maximum surge current
versus the impulse duration in seconds. It is noted that
stresses above the conditions may cause permanent damage to
the device.
Power
Dissipation Ratings
If
transients occur in rapid succession, the average power
dissipation is the energy (watt-seconds) per pulse times the
number of pulses per second. The power generated must be
within the specifications shown in the chart above. Operating
values must be derated at high temperatures as shown in figure
2. Note the rapid drop in rated value at temperature
greater that 85C.
Varistors can dissipate a relative small
amount of average power compared to surge power and are not
suitable for repetitive applications that involve substantial
amounts of power dissipation.
In the ANSI/IEEE C62.33 (1982) Standard for
Surge Protective Devices the following is stated: "Single
and lifetime pulse current ratings are appropriate tests of
varistor surge withstand capability. In the absence of special
requirements, energy ratings are recommended for use only as
supplements to the predominant current ratings, and for
application problems, which are more conveniently treated in
terms of energy."7
Mean
Time Before Failure (MTBF)
MTBF
is a measure of the typical number of hours that a varistor
will continuously operate, at a given temperature, before a
failure will occur. Accelerated aging test techniques are used
to understand and minimize the MOV degradation process.
To obtain MTBF value, accelerated aging
testing techniques are used to acquire the necessary data
accurately and reliably in a short period of time. The
following is a brief explanation of how an accelerated aging
test is perfomed:
1. Obtain 60-90 MOVs of the same production
run.
2. Initially test the varistor voltage @ 1
mA, and the leakage current at the rated dc working voltage.
3. Place an equal count of 20-30 varistors
in three separate temperature chambers that have the
temperature set at 85°, 105°C, and 125°C.
4. Apply rated volts ac to the devices.
5. Every 100 hours remove varistors from
testing chambers and measure the varistor voltage @ 1 mA, and
the leakage current at the rated dc working voltage.
6. If the leakage current is greater than
100 uA (an arbitrary failure point) then remove the device
from test and record the number of hours before failure.
7. Continue test until all devices have
failed, or enough data has been collected to allow an accurate
curve fit of the data.
8. Input data into a data analysis program
and extrapolate the time before failure at other temperatures.
The amount of time required to perform this
test can be long. Typically Maida tests its MOVs for 10,000
– 15,000 hours (416 – 625 days) before the test completes.
The criteria used to signify a failure or the time between
testing is arbitrary. The values shown in the procedure are
what Maida uses to run its test. Other values can be used for
these parameters if required.
Using the Arrhenius model, the data
collected is imported into a spreadsheet and then exported
into a curve-fitting program. Using the equations of the
Arrhenius model, the MTBF for a given temperature is plotted
and printed.
Accelerated testing has been used in
reliability prediction models. Accelerated testing allows
accurate reliability and failure rate estimation in a
relatively short period of time. Failure rates obtained by
subjecting electronic components to highly accelerated testing
conditions are used to estimate failure rates under normal
operating conditions.
Studies have shown that failure of many
electronic components, and varistors in particular, are due to
chemical degradation processes, which are accelerated by
elevated temperature. The Arrhenius model has found wide
application in accelerated testing technology. The Arrhenius
model is applicable if:
1. The most significant stresses are
thermal.
2. The expected mean life is
logarithmically related to the inverse of temperature.
The model is generally described by the
following equation:
ML = e A+B/T
where:
ML: Mean life
A,B: Empirically derived constants from
the life test data. The constant’s values depend on
the characteristics of the material tested and the method.
T: Absolute temperature in Kelvin
The expected mean life (ML) of a varistor
under normal operating temperatures is calculated using the
above equation. The constants A and B are calculated from the
(ML vs. temp.) graph developed during the accelerated testing
experiment. The following two equations make calculating A and
B easier:
B = (ln ML1 / ML2 ) (
1 / T1 – 1 / T2 )-1
And,
A = ln (MLI) – B / TI
T1 and T2 are high
temperatures used during the accelerated testing, and ML1 and ML2 are the corresponding mean lives obtained
from the accelerated test.
A varistor normally operates under 40°C, a
standby current value less than 50 uA and a voltage (10-15%)
less than the MCOV.
Mean life of an electronic component is the
expected mean or average life of the component. Mean life is
estimated by testing a sample of components for a period of
time, then:
The number of "varistor hours" on
test at any time can be computed by adding the lives, in
hours, of the varistors that have failed up to the moment of
estimation, to the lives, in hours, of the varistors observed
that have not failed. The greater the number of item hours
(testing time), the more confidence in the resulting estimates
of mean life.
Figure
3 is an example of the MTBF Analysis completed recently
for varistor Style D69ZOV251RA72.
The vertical axis (ML) is a label that
signifies the mean life (or the average time before failure)
of an MOV expressed in hours. The horizontal axis (1/TEMP IN
K) is a label of the temperature expressed in the reciprocal
of the temperature in Kelvin. As the reader can see from the
example the ML, at 0.00299-1 (61.5°C or 334.5°K),
equals 1e+06 or 1 million hours. The ML, at 0.0023-1 (161.8°C or 434.8°K), equals 100 hours.
Conclusion
MOVs are commonly used in a
wide variety of lightning protection systems, low voltage
surge arrestors, transient voltage surge suppressors (TVSS).
MOVs are also incorporated in general household equipment
including uninterruptible power supplies (UPS), televisions
and surround sound receivers. It is important to understand
the performance of such a widely used device since this
knowledge can help to reduce failures and increase reliability
of the power system.
Acknowledgement
The author gratefully acknowledges
the contributions of Leon Brandon Ph.D, vice president
of engineering, Maida Development Corporation.
References
1 L.M. Levinson (Editor):
"Electronic Ceramics – Properties, Devices, and
Applications," Marcel Dekker, Inc.; New York; 1988
2 Maida Development Company:
"Zinc-Oxide Varistors," 2000-2001 Catalog, p.
5-7
3 D. Birrell & R. B.
Standler: "Failures of Surge Arrestors on
Low-Voltage Mains," IEEE Transactions on Power
Delivery, 1993, Vol. 8, No. 1
4 R. B. Standler:
"Protection of Electronic Circuits from
Overvoltages," John Wiley & Sons Inc. 1989, p.
138
5 K. P. Mardira, T.K. Saha,
& R. A. Sutton : "The Effects of
Electrical Degradation on the Microstructure of Metal
Oxide Varistor," IEEE 0-7803-7285-9/01, 2001.
6 M. Bartkowiak: "Current
Localization, Non-Uniform Heating, and Failures on ZnO
Varistors", Materials Research Society, 1998,
Material Research Society Symp. Proc. Vol. 500.
7 Littelfuse: "Varistor
Products," 2002 Catalog, p 35-39
Kenneth J. Brown currently
holds the position of director of power quality
engineering at Leviton Mfg. Co. He has an undergraduate
in electronics engineering from the Ohio Institute of
Technology and an MBA from West Coast
University. He is currently chairman of the
Technical Committee at NEMA 5-VS Low Voltage Surge
Protective Devices. He has been involved in the
design of surge protective devices for 8 years and prior
to that he worked as a manager of subsystem development
on the guidance section of an air-to-ground weapon
system.
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