Varistor Products
Aerospace and Military
High Reliability Varistors
Neutron Effects
I
SATURATION
CURRENT
A second MOV-zener comparison was made in response to neutron
fluence. The selected devices were equal in area.
FORWARD
BIAS
2
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 con-
trast to the large change in the zener, the MOV is unaltered. At higher-
currents where the MOV’s clamping voltage is again unchanged, the
zener device clamping voltage increases by as much as 36%.
BREAKDOWN
VOLTAGE
V
300
REDUCTION IN
FAILURE STRESSHOLD
BY RADIAL
1.5K 200 INITIAL
VARISTOR V130A2
200
15
INITIAL AT 10
SECONDARY
BREAKDOWN
100
80
1.5K 200
AT 10
REVERSE
BIAS
12
60
50
40
FIGURE 3. V-I CHARACTERISTIC OF PN-JUNCTION
30
1.5K 200
AT 10
At low reverse voltage, the device will conduct very little current (the
saturation current). At higher reverse voltage V (breakdown
1.5K 200
AT 10
1.5K 200
AT 10
20
14
15
13
BO
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.
10
10
8
7
6
5
4
3
10
10
10
10
10
10
10
AMPERES
FIGURE 2. V-I CHARACTERISTIC RESPONSETO NEUTRON
IRRADIATION FOR MOV AND ZENER DIODE
DEVICES
This manifests itself through the formation of hotspots, and irreversible
damage occurs. This failure threshold decreases under neutron irradia-
tion for zeners, but not for Zinc Oxide Varistors.
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.
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.
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.
Both show no change, within experimental error, after irradiation.
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.
The characteristic voltage current relationship of a PN-Junction is shown
in Figure 3.
147
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