Varistor Products
High Reliability Varistors
Radiation Hardness
For space applications, an extremely important property
of a protection device is its response to imposed radiation
effects.
Counterclockwise rotation of the V-I characteristics is
observed in Silicon devices at high neutron irradiation
levels; in other words, increasing leakage at low current
levels and increasing clamping voltage at higher current
levels.
Electron Irradiation
A Littelfuse MOV and a Silicon transient suppression diode
were exposed to electron irradiation. The V-I curves, before
and after test, are shown below.
The solid and open circles for a given fluence represent the
high and low breakdown currents for the sample of devices
tested. Note that there is a marked decrease in current (or
energy) handling capability with increased neutron fluence.
LITTELFUSE MOV
Failure threshold of Silicon semiconductor junctions is
further reduced when high or rapidly increasing currents
are applied. Junctions develop hot spots, which enlarge
until a short occurs if current is not limited or quickly
removed.
200
SILICON
TRANSIENT
SUPPRESSION
DIODE
V
100
80
60
PRE TEST
10 RADS,
The characteristic voltage current relationship of a P– N
40
8
Junction is shown below.
18MeV ELECTRONS
I
20
SATURATION
CURRENT
8
6
4
2
10
10
10
CURRENT (A)
10
FORWARD
BIAS
FIGURE 1. RADIATION SENSITIVITY OF LITTELFUSEV130LA1
AND SILICON TRANSIENT SUPPRESSION DIODE
BREAKDOWN
VOLTAGE
It is apparent that the Littelfuse MOV was virtually
unaffected, even at the extremely high dose of 108 rads,
while the Silicon transient suppression diode showed a
dramatic increase in leakage current.
V
REDUCTION IN
FAILURE STRESSHOLD
BY RADIAL
Neutron Effects
SECONDARY
BREAKDOWN
A second MOV-Zener comparison was made in response
to neutron fluence. The selected devices were equal in
area.
REVERSE
BIAS
FIGURE 3. V-I CHARACTERISTIC OF PN-JUNCTION
Figure 2 shows the clamping voltage response of the MOV
and the Zener to neutron irradiation to as high as 1015 N/
cm2. It is apparent that in contrast to the large change in
the Zener, the MOV is unaltered. At highercurrents where
the MOV’s clamping voltage is again unchanged, the Zener
device clamping voltage increases by as much as 36%.
At low reverse voltage, the device will conduct very little
current (the saturation current). At higher reverse voltage
VBO (breakdown voltage),the current increases rapidly as
the electrons are either pulled by the electric field (Zener
effect) or knocked out by other electrons (avalanching). A
further increase in voltage causes the device to exhibit a
negative resistance characteristic leading to secondary
breakdown.
300
1.5K 200 INITIAL
VARISTOR V130A2
INITIAL AT 10
200
15
100
80
This manifests itself through the formation of hotspots,
and irreversible damage occurs. This failure threshold
decreases under neutron irradiation for Zeners, but not for
ZNO Varistors.
1.5K 200
AT 10
12
60
50
40
30
1.5K 200
AT 10
1.5K 200
AT 10
1.5K 200
AT 10
20
14
15
13
Gamma Radiation
Radiation damage studies were performed on type
V130LA2 varistors. Emission spectra and V-I characteristics
were collected before and after irradiation with 106 rads
Co60 gamma radiation. Both show no change, within
experimental error, after irradiation.
10
8
7
6
5
4
3
10
10
10
10
10
10
10
10
AMPERES
FIGURE 2. V-I CHARACTERISTIC RESPONSETO NEUTRON
IRRADIATION FOR MOV AND ZENER DIODE
DEVICES
©2008 Littelfuse, Inc.
202
High ReliabilityVaristors
Specifications are subject to change without notice.
Please refer to www.littelfuse.com/series/za hirel.html or /db hirel.html
for current information.
Revision: January 9, 2009
