GE-MOV® Varistor – The Super Alpha Varistor
J.D. Harnden, F.D. Martzloff, and W.G. Morris
General Electric Company
Reprinted from unclassified General Electric TIS Report 72CRD260, December 1972
Significance
Part 7 – Mitigation techniques
A public-domain (unclassified) General Electric report aimed at the engineering community to provide
technical information in support of the trade announcement published in Electronics (see MOV Announce
in Part 6.
Note – 2005 perspective:
With hindsight, this report reflecting the enthusiasm at General Electric for the answer to the quest for an
effective and economical transient suppressor in consumer products (see Evaluate 1963 SPDs in Part 7),
is probably at the source of the unfortunate selection of device ratings that made the electronics-oriented
“GE-MOV®” select a disc thickness for its 130-V rating according to what became a perception of “lower
is better” for surge protection (200 V at 1 mA DC, see Figure 12 in the report). That position was abetted
by some now-forgotten trade publications emphasizing the need to limit transient overvoltages to less
than 100 V above peak line voltage, hence the citing of a low level 330 V for the “clamping voltage” of
130-V TVSSs in the fist edition of UL Std 1449.
G e n e r a l Electric Company
C o r p o r a t e Research a n d Development
S c h e n e c t a d y , N e w York
TECHNICAL lNFORMATlOlV
SERIES
NO
SUBJECT
AUTHORHarnden,
JD, Jr. ,
72CRD260
Martzloff, F D
hm
varistor
M o r r i s , WGt, Golden, F B+
3ecember 1972
""'GE-MOV+ V a r i s t o r - - T h e Super Alpha "
1
N O PAGES
Varistor
14
CORPORATE
ORG
lU
l ATN
l GPhy s i c s and E l e c t r i c a l
-
R E S E A R C H A N D DEVELOPMEN1
Engineering L a b o r a t o r y
S C H E N E C T A D Y , N.Y.
SUMMARY
Today t h e r e i s considerable interest and development activity in
a new c l a s s of metal oxide varistor which exhibits a high degree of nonlinearity a s compared with previous devices. Because of the metal
oxide, the device has been named a metal oxide v a r i s t o r and has been
trademarked MOV. * This quest forTsuper" o r higher performance
i s seemingly a continuous one - - whichfor example, in the case of
t r a n s i s t o r s has seen the emergence of the Darlington and super beta
transistor.
The voltage a c r o s s a varistor and the current through it a r e related
by a power law I = k Vn. The exponent n will typically have values 2 5
to 50 o r more, leading to a characteristic very s i m i l a r to that of a
Zener diode. Over a wide current range, the voltage remains within
a very narrow band f o r a specific device, and can be r e f e r r e d to a s the
"varistor voltage" for that device. The nonlinear electrical charact e r i s t i c makes the device useful in voltage regulation applications, and
in particular for limiting surges and transient voltages that may appear
on power lines.
Polycrystalline devices using zinc oxide have been the subject of
much r e s e a r c h over the past dozen o r so years. Recent work on the
zinc oxide-bismuth oxide system has shown many improvements in
properties compared to other devices based on silicon carbide,
selenium, etc.
:*Trademark of General E l e c t r i c Company.
tGE Semiconductor Products Dept., Schenectady. N. li
$GE Semiconductor Products Dept., Auburn, N. Y.
K E Y WORDS
v a r i s t o r , nonlinear v a r i s t o r , m e t a l oxide
v a r i s t o r , MOV, GE-MOV
INFORMATION PREPARED FOR
Additional Hard Copies Available From
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P.O. Box 43 B l d g 5 , Schenectady , N.Y., 12301
Microfiche Copies Available From
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P.O. Box 4 3 Bldg. 5 , Schenectady , N.Y., 12301
R D-54 ( 10170)
-
G E MOV:;: VARISTOR - - T h e Super Alpha V a r i s t o r
J. D. Harnden, J r . ,
F. D. Martzloff,
W. G. M o r r i s ? and F. B. Golden*
INTRODUCTION
V a r i s t o r s a r e p e r h a p s among t h e oldest of t h e
e l e c t r i c a l and electronic devices. Yet today t h e r e i s
considerable i n t e r e s t and development activity in a
new c l a s s of m e t a l oxide v a r i s t o r which exhibits a
high d e g r e e of nonlinearity a s compared with previous
devices. B e c a u s e of t h e m e t a l oxide, t h e device h a s
been named a m e t a l oxide v a r i s t o r and h a s been t r a d e T h i s quest f o r "super" o r higher
m a r k e d MOV::.
p e r f o r m a n c e is seemingly a continuous one - - which,
f o r example, in t h e c a s e of t r a n s i s t o r s h a s s e e n t h e
e m e r g e n c e of t h e Darlington and s u p e r beta t r a n s i s t o r .
T h e voltage a c r o s s a v a r i s t o r and t h e c u r r e n t
through i t a r e related b y a power law I = k Vn. T h e
exponent n will typically have values 25 t o 50 o r m o r e ,
leading to a c h a r a c t e r i s t i c v e r y s i m i l a r t o that of a
Z e n e r diode. O v e r a wide c u r r e n t range, t h e voltage
r e m a i n s within a v e r y n a r r o w band f o r a specific
device, and c a n b e r e f e r r e d t o a s t h e " v a r i s t o r voltage"
f o r that device. T h e nonlinear e l e c t r i c a l c h a r a c t e r i s t i c
m a k e s the device useful in voltage regulation applications, and in p a r t i c u l a r f o r limiting s u r g e s and t r a n sient voltages that m a y a p p e a r on power lines.
N
ELECTRODE
CURRENT FLOW
Bl STABLE INTERGRANULAR
BISMUTH OXIDE
(INSUMTOR AT LOW VOLTAGE
CONDUCTOR AT HIGH VOLTAGE)
CONDUCTIVE Z I N C OXIDE
1 1 1 1 1 1 1 1 1
EPOXY ENCAP SUIANT
V
WIRE LEADS
Fig. 1 S t r u c t u r e of m e t a l oxide v a r i s t o r .
Polycrystalline devices using zinc oxide have
been the subject of much r e s e a r c h o v e r t h e past
dozen o r s o y e a r s . (1) Recent work on t h e zinc oxidebismuth oxide s y s t e m h a s shown many improvements
i n p r o p e r t i e s compared t o o t h e r devices based on s i l i con carbide, selenium, etc. (2-4)
STRUCTURE AND PROCESSING
A p r e f e r r e d type of m e t a l oxide v a r i s t o r c o n s i s t s
of a polycrystalline c e r a m i c body, with m e t a l contacts
and w i r e l e a d s applied, and suitably encapsulated
( F i g . 1). Zinc oxide and bismuth oxide, d e s i r a b l e
ingredients f o r a high exponent v a r i s t o r , a r e mixed
with o t h e r ( p r o p r i e t a r y ) additives in powder f o r m ,
p r e s s e d into d i s k s and s i n t e r e d above 1200°C. T h e
bismuth oxide is molten above 825OC, a s s i s t i n g t h e
formation of a d e n s e polycrystalline c e r a m i c through
liquid-phase sintering. During cooling, t h e liquid
phase f o r m s a rigid, amorphous coating around each
ZnO g r a i n , giving a m i c r o s t r u c t u r e of ZnO g r a i n s
isolated f r o m one another by a thin, continuous i n t e r g r a n u l a r phase. T h i s complex, two-phase m i c r o s t r u c t u r e is responsible f o r the nonlinear c h a r a c t e r i s t i c
MICROSTRUCTURE AND INTERGRANULAR PHASE
T h e n a t u r e of t h e continuous i n t e r g r a n u l a r phase
c a n be s e e n in Fig. 2. T h i s is a scanning e l e c t r o n
::Trademark of t h e G e n e r a l E l e c t r i c Company.
t G E Semiconductor P r o d u c t s Dept. Schenectady, N. Y.
$GE SemiconductorProducts Dept.. Auburn, N. Y.
.
Manuscript Received 9 / 14 / 7 2
Fig. 2 Scanning e l e c t r o n m i c r o g r a p h (860x1.
m i c r o g r a p h a t 2000X of a s i n t e r e d c e r a m i c , containing
zinc oxide and bismuth oxide i n a r a t i o of 50: 1. T h e
zinc oxide g r a i n s have been dissolved in a n ammonium
chloride solution, revealing t h e amorphous phase.
The m o s t prominent f e a t u r e is t h e t r i a n g u l a r r o d s
which a p p e a r a t t h e intersection of t h r e e g r a i n edges.
although t h e r e a r e s e v e r a l regions w h e r e t h e v e r y
thin m e m b r a n e ( l e s s than l000A) which s e p a r a t e d
g r a i n f a c e s r e m a i n s intact. T o understand t h e
e l e c t r i c a l p r o p e r t i e s of t h e MOV v a r i s t o r , one m u s t
c o n s i d e r not only t h e doping of ZnO c r y s t a l s by
bismuth ions, but a l s o t h e doping of the bismuth
oxide i n t e r g r a n u l a r p h a s e by zinc ions, and t h e geom e t r i c a l effects of t h e two-phase s t r u c t u r e .
indicate conduction limited by hopping o r tunneling in
t h e thin l a y e r s , with relatively e a s y conduction through
t h e n-type ZnO g r a i n s .
PROPERTIES O F ZINC OXIDE GRAINS
E x p e r i m e n t s on evaporated f i l m s of B i g , on ZnO
show that t h e i n t e r g r a n u l a r l a y e r i s capable of nonl i n e a r c h a r a c t e r i s t i c alone. F i g u r e 3 i l l u s t r a t e s the
f a c t that a ZnO single c r y s t a l is l i n e a r , w h e r e a s t h e
s a m e c r y s t a l with an evaporated Bi,O, f i l m shows
nonlinearity s i m i l a r t o t h e m e t a l oxide v a r i s t o r .
T h i s , together with t h e m i c r o s t r u c t u r e shown in Fig.
2, e m p h a s i z e s t h e importance of t h e i n t e r g r a n u l a r
phase in o r d e r to achieve good v a r i s t o r p r o p e r t i e s .
T h e r a t h e r low resistivity, about 1 ohm-cm a t
25'32, of semiconducting zinc oxide is attributed t o
conduction e l e c t r o n s f r o m ionized zinc i n t e r s t i t i a l
atoms. T h e addition of monovalent ions, such a s
Na+, introduces t r a p s , and t h e r e s i s t i v i t y i n c r e a s e s .
Conversely, trivalent ions such a s ~ r contribute
+ ~
additional e l e c t r o n s with a f u r t h e r reduction in r e sistivity. Bismuth doping of t h e zinc oxide g r a i n s is
l e s s than 100 ppm a s determined by e l e c t r o n m i c r o p r o b e analysis. T r i v a l e n t bismuth ions would be expected t o d e c r e a s e t h e r e s i s t i v i t y of zinc oxide, but
in t h i s concentration they probably have a negligible
effect. It i s sufficiently a c c u r a t e t o d e s c r i b e t h e zinc
oxide g r a i n s in t h e MOV v a r i s t o r a s being n-type,
1 o h m - c m semiconductor.
EVAPORATED THIN-FILM EXPERIMENT
'
OPERATION
PROPERTIES O F THE B i 2 0 3 INTERGRANULAR
PHASE
Unlike the abundance of m a t e r i a l available on
zinc oxide, v e r y l i t t l e is known about t h e e l e c t r i c a l
behavior of Bi,O,.
It h a s a much h i g h e r resistivity,
about 1011 o h m - c m a t 25OC. but virtually nothing is
known about t h e n a t u r e of t h e c a r r i e r s and t h e defect
s t r u c t u r e . T h e melting point is 825°C with the liquid
dissolving a n appreciable amount of ZnO. According
t o a limited phase d i a g r a m , it f o r m s a 6Bi20,- ZnO
cubic compound below 800°C. A c c u r a t e a n a l y s i s of
t h e i n t e r g r a n u l a r p h a s e i n t h e Z n O - B i p , c e r a m i c is
difficult, but m o s t evidence points t o w a r d s a n a m o r phous s t r u c t u r e , containing up t o p e r h a p s 20 p e r c e n t
zinc, with a r e s i s t i v i t y much g r e a t e r than t h e zinc
oxide g r a i n s .
F i g . 3 V-I c h a r a c t e r i s t i c of zinc oxide and ZnOB i ,O,
.
PROPOSED MECHANISM
--
HOPPING
RELATING PROPERTIES TO MICROSTRUCTURE
Many p r o p e r t i e s of t h e MOV v a r i s t o r can b e
d i r e c t l y related t o t h e m i c r o s t r u c t u r e , and t h e prope r t i e s of t h e two phases. At low applied voltages,
t h e resulting e l e c t r i c a l c h a r a c t e r i s t i c s is p r i m a r i l y
that attributable t o t h e leakage c u r r e n t through t h e
i n t e r g r a n u l a r phase. T h e behavior is approximately
that of a n insulator a t t h e low r a n g e s of applied voltage,
and t h e applied voltage thus r e s u l t s in a v e r y high field
a c r o s s the i n t e r g r a n u l a r p h a s e and a low field i n t h e
g r a i n s . In the v a r i s t o r range, t h e i n t e r g r a n u l a r phase
b e c o m e s nonlinear and c u r r e n t through i t i n c r e a s e s
v e r y rapidly a s t h e voltage i s slowly i n c r e a s e d . The
conduction m e c h a n i s m which r e s u l t s i n t h e nonlinear
c h a r a c t e r i s t i c is a s o u r c e of intense investigation and
is probably s p a c e c h a r g e limited c u r r e n t s o r tunneling. In the v e r y high c u r r e n t range, t h e r e s i s t a n c e of
t h e i n t e r g r a n u l a r l a y e r b e c o m e s l e s s than t h e ZnO
g r a i n s , causing t h e V-I c h a r a c t e r i s t i c t o once again
tend towards linearity. Capacitance-frequency and
capacitance-voltage c u r v e s f o r t h e c e r a m i c body
By comparing a l l the e l e c t r i c a l p r o p e r t i e s m e a s u r e d in t h i s work, it is proposed that the m e c h a n i s m
is a combination of e l e c t r o n hopping and tunneling in
t h e amorphous thin l a y e r s . T h e specific a r e a s of
a g r e e m e n t with t h e hopping model a r e ( 1 ) a v e r y
slight d e c r e a s e in polarizability with frequency, ( 2 )
a d e c r e a s e in AC r e s i s t i v i t y with frequency, approaching l / f 2 n e a r 1 0 5 H z , and ( 3 ) a low activation
energy value of l e s s than 0. 01 e v i s calculated.
PROPOSED MECHANISM - - HOPPING AND
TUNNELING
L i n e a r conduction is one f e a t u r e of t h e hopping
model which d o e s not a g r e e with t h e m e a s u r e d c h a r a c t e r i s t i c s . E l e c t r o n tunneling is a nonlinear conduction
m e c h a n i s m usually associated with a s m a l l t e m p e r a t u r e coefficient which m a y be considered a s a s e r i e s
s t e p in t h e conduction p r o c e s s . T h e estimated thickn e s s of approximately 600A for t h e i n t e r g r a n u l a r
p h a s e is, however, r a t h e r l a r g e f o r tunneling through
i t t o t a k e place; but if t h e p r o c e s s is a s s u m e d t o cons i s t of a s e r i e s of tunneling and hopping steps, t h e
highly nonlinear conduction f e a t u r e of tunneling c a n
b e introduced. T h e f i e l d s calculated in t h e i n t e r g r a n u l a r p h a s e a r e approximately 10' V / c m s o t h a t
one might expect local field concentrations sufficient
t o initiate tunneling.
COMPARISON O F THIN-FILM PROPERTIES WITH
SINTERED PROPERTIES
In t h e evaporated thin-film experiment, it is r e quired that t h e ZnO single c r y s t a l s u b s t r a t e t e m p e r a t u r e b e above 20OoC, o r that a subsequent annealing b e
done, in o r d e r t o get a n insulating l a y e r . Thus, it is
likely that diffusion of Zn into t h e f i l m will r e s u l t in a
doping l e v e l comparable t o t h e i n t e r g r a n u l a r phase.
T h e inherent s u r f a c e roughness even o n a polished
s u r f a c e r e s u l t s in thin regions which a r e m o r e s u s ceptible t o e l e c t r i c breakdown. T h e thin-film s a m ples show s i m i l a r V-I c h a r a c t e r i s t i c s t o t h e s i n t e r e d
c e r a m i c s in t h e low c u r r e n t region, but g e n e r a l l y
f a i l a s high localized c u r r e n t d e n s i t i e s develop.
T h e s e types of defects a r e l e s s likely t o o c c u r i n t h e
liquid-phase s i n t e r e d bodies. An important f e a t u r e
of liquid-phase s i n t e r i n g is t h e i n c r e a s e d solubility
i n t h e liquid phase of regions of s m a l l r a d i u s of c u r v a t u r e o n t h e s u r f a c e of t h e g r a i n s , and t h e t r a n s p o r t of
t h i s m a t e r i a l t o regions of negative c u r v a t u r e o r p r e f e r r e d low e n e r g y f a c e s . T h u s the i n t e r f a c e s a r e
n a t u r a l l y smooth and flat and m o r e uniform conduct i o n is possible. Liquid-phase s i n t e r i n g is a r a t h e r
unsophisticated technique by m o d e r n semiconductor
p r o c e s s i n g s t a n d a r d s . It t e n d s t o automatically
develop t h e atomically smooth i n t e r f a c e s and uniform
i n t e r g r a n u l a r p h a s e r e q u i r e d f o r nonlinear conduction.
T h i s , together with t h e redundancy of a l a r g e t h r e e dimensional, s e r i e s - p a r a l l e l network, r e s u l t s in the
MOV v a r i s t o r being a device which h a s outstanding
nonlinear p r o p e r t i e s and r e m a r k a b l e stability.
VOLTAGE RANGE, VOLTS P E R MIL CONCEPT,
AND LOWER VOLTAGE LIMIT
Fig. 4 Idealized V-I c h a r a c t e r i s t i c .
useful c h a r a c t e r i s t i c of t h e new m e t a l oxide v a r i s t o r s .
T h e low c u r r e n t l e v e l r e s i s t a n c e is due basically t o
t h e r e s i s t i v i t y of t h e i n t e r g r a n u l a r phase. T h e capacit a n c e is due t o v e r y thin d i e l e c t r i c of t h e i n n e r granul a r phase and is a f a i r l y significant f a c t o r in t h e dyn a m i c c h a r a c t e r i s t i c of t h e m a t e r i a l . T h e high c u r r e n t c h a r a c t e r i s t i c is due t o t h e b a s i c r e s i s t i v i t y of
t h e zinc oxide g r a i n s and r e p r e s e n t s t h e limiting
r e s i s t a n c e of t h e device.
ELECTRICAL CHARACTERIZATION
T h e following t r e a t m e n t s e r v e s a s a n introduction r a t h e r than exhaustive t r e a t m e n t of t h e e l e c t r i c a l p r o p e r t i e s . F o r m o r e d e t a i l s s e e Refs. 5 and 6.
F i g u r e 5 provides an i n t e r e s t i n g composite view
of t h e voltage-current relationship f o r silicon c a r b i d e
v a r i s t o r s and MOV v a r i s t o r s and p a r a m e t r i c a l l y with
l i n e a r r e s i s t a n c e s . T h e p r e s e n c e of l i n e a r r e s i s t o r
c u r v e s helps t o highlight t h e g r e a t r a n g e of v a r i s t o r
operation. As pointed out e a r l i e r , t h e zinc oxide
m a t e r i a l by itself is e s s e n t i a l l y a l i n e a r r e s i s t o r
highly conducting.
sic
Devices of various v a r i s t o r voltages a r e f e a s i b l e
f r o m approximately 30 volts t o m a n y kilovolts. F o r
a given composition and processing, it is convenient
t o talk about volts p e r mil; t h e v a r i s t o r voltage is
proportional t o t h e t h i c k n e s s of t h e m a t e r i a l between
t h e contacts. Indeed, it is possible t o mechanically
s l i c e t h e v a r i s t o r m a t e r i a l into s m a l l pieces, and
calculate t h e voltage f r o m the thickness. T h e r e is,
of c o u r s e , a minimum divisibility which is p r a c t i c a l
in t e r m s of p r e s e r v a t i o n of t h e high exponent c h a r a c t e r i s t i c . T h i s limitation begins t o show itself when
t h e r e a r e insufficient g r a i n boundaries within t h e
s t a t i s t i c a l e l e c t r i c a l path r e m a i n i n g f o r t h e thin c r o s s
section.
IDEALIZED V - I CHARACTERISTICS
F i g u r e 4 shows t h e idealized voltage v e r s u s c u r r e n t c h a r a c t e r i s t i c . T h e wide r a n g e of c u r r e n t cove r i n g approximately 10 o r d e r s of magnitude is a highly
Fig. 5 V a r i s t o r and r e s i s t o r log-log comparison.
The important e l e c t r i c a l significance of t h e alpha
o r exponent upon clamping r a t i o is shown in F i g . 6.
Thus it will b e seen, f o r example, with a typical alpha
of 30 that t h e c u r r e n t c a n v a r y o v e r a range of 10,000
t o one with a voltage change in t h e o r d e r of approxim a t e l y 1. 36.
1.2
1.1
I
-
I.3
VOLTAGE R A T I O
1.4
RATINGS
Fig. 6 Clamping r a t i o s f o r different c u r r e n t r a t i o s a s
a function of alpha (a ).
As in t h e c a s e with any new electronic device,
questions regarding effects of t e m p e r a t u r e aging
under s t o r a g e and s t r e s s , plus overload conditions,
and high-frequency p r o p e r t i e s a l l need t o b e examined
and c h a r a c t e r i z e d in detail.
F i g u r e 7 shows t h e t e m p e r a t u r e effect f o r t h e
typical VP s e r i e s of MOV v a r i s t o r s . T h e m a t e r i a l
exhibits a slight negative voltage t e m p e r a t u r e coefficient in t h e o r d e r of 0. 05% t o 0. 1% p e r "C. T h i s
I
100
MICROAMPERES
I
1
I
I
I
I
10
100
1
10
M I ? [ IAMPERES
L o n g - t e r m stability t e s t s under DC and AC conditions have well exceeded 10,000 h o u r s under various
s t r e s s l e v e l s with encouraging r e s u l t s . S e v e r a l important stability a r e a s being investigated include long
t e r m with DC b i a s at m o d e r a t e power dissipation and
stability with repetitive short-duration, high-energy
pulses. T h e DC stability under m o d e r a t e power d i s s i pation ( 1 watt f o r 14 mm disk) h a s been established
with a t r e n d t o a polarization-like m e c h a n i s m whereby
a s m a l l shift in the device voltage o c c u r s with t i m e .
Under a condition of constant c u r r e n t in t h e device,
t h e voltage a c r o s s t h e device i n c r e a s e s by a few p e r cent and tends toward t h e stable asymptote.
Under
a condition of constant voltage applied t o t h e device,
t h e c u r r e n t d r a w n by t h e device d e c r e a s e s with time.
B e c a u s e of t h e high exponent c h a r a c t e r i s t i c t h e constant voltage t e s t with d e c r e a s i n g c u r r e n t r e s u l t s in
substantial d e c r e a s e in t h e power dissipation i n t h e
device. Hence, even l e s s c u r r e n t and g r e a t e r
stability.
I
100
AMPERES
F i g . 7 Typical V P s e r i e s t e m p e r a t u r e effect.
coefficient m a y change with composition. At v e r y
low c u r r e n t l e v e l s t h e coefficient i s substantially
l a r g e r , but t h i s d o e s not affect t h e n o r m a l operating
r a n g e of t h e device. Storage a t t e m p e r a t u r e s of
125°C h a s shown no degradation (the p r o c e s s involves
s i n t e r i n g above 1200°C and t e m p e r i n g at s e v e r a l
hundred degrees). T h e r e is a relationship between
m a x i m u m s t e a d y s t a t e dissipation and ambient t e m p e r a t u r e . Typical values indicate 1 watt f o r a d i s k
with 3 c m s q u a r e t o t a l s u r f a c e a r e a and a 70°C f r e e
convection ambient.
Repetitive p u l s e s of high energy (up to 60 joules
in a 20 rnm disk) have a l s o been applied, producing a
detectable shift in t h e V-I c h a r a c t e r i s t i c with asymptotic
t r e n d s . Most of the changes take place in the f i r s t 1 to
10 pulses, while 90 additional p u l s e s produce the shift
to d e c r e a s i n g amplitude. The low power r a t i n g s r e flect a package t h e r m a l t r a n s f e r limitation r a t h e r than
a b a s i c limitation on t h e p a r t of t h e v a r i s t o r .
P e a k c u r r e n t r a t i n g s in t h e c a s e of t h e l a r g e r
d i a m e t e r s i z e s have been a r b i t r a r i l y limited due t o
l a c k of rating equipment capability. T h e l a r g e r
d i a m e t e r devices have twice t h e active a r e a of t h e i r
s m a l l e r counterparts. L i n e a r l y extrapolating t h e c u r r e n t rating would yield a f i g u r e of 2000 a m p in place
of t h e 1250 a m p value of t h e data sheet. Although
localized a r e a s of nonuniformity m a y prevent t h e u s e
of a l i n e a r r a t i o it a p p e a r s t h e 1250 a m p value is
ultraconservative and will be upgraded in t h e n e a r
future. Likewise, t h e 7 y s e c pulse b a s e width is an
a r b i t r a r y value limited by t h e f i r s t generation of
rating t e s t equipment. T h e r e a r e s t r o n g indications
that t h e device m a y b e capable of sustaining r a t e d
peak c u r r e n t s f o r b a s e widths in t h e 50 t o 100 p s e c
range. T h e equivalent c i r c u i t f o r t h e MOV v a r i s t o r
i s approximately indicated in F i g . 8. The typical
value of capacitance is broken out and c h a r a c t e r i z e d
in Fig. 9. In addition, t h e high-frequency p r o p e r t i e s
a r e of i n t e r e s t and a r e shown f o r two production dev i c e s in Fig. 10, w h e r e r e s o n a n c e with t h e inductance
of t h e l e a d s becomes apparent. A s a point of c o m p a r ison a fixed point 0. 005 p F capacitor is included.
SWITCHING SPEED
T e s t s have been performed with c u r r e n t p u l s e s
of 0. 5 n s e c r i s e t i m e , 2 t o 250 n s e c pulse widths up
t o 6 5 a m p on devices with 1 . 4 c m d i a m e t e r . To p e r f o r m t h i s t e s t , a disk of MOV m a t e r i a l w a s placed
in a low inductance t e s t c i r c u i t and a c u r r e n t pulse
w a s applied. C u r r e n t in t h e device and voltage a c r o s s
it w e r e r e c o r d e d with a high-speed oscilloscope. T h e r e
d o e s not appear t o b e a "switching time" n o r a d v / d t
effect involved. When a f a s t front voltage s u r g e i s
applied t o t h e device, i t s capacitance will immediately
m a k e it a p p e a r a s low impedance and, when t h e capacit a n c e is fully charged, t h e device will m e r e l y o p e r a t e
a t t h e point corresponding t o i t s V - I c h a r a c t e r i s t i c .
T h i s is t o b e compared with s o m e o t h e r s u p p r e s s i o n
devices such a s s p a r k gaps which p o s s e s s considerable
variation in t h e i r protection l e v e l depending upon t h e
r a t e a t which t h e voltage is applied t o them.
APPLICATION T O DISTRIBUTION SYSTEM PROBLEMS
Fig. 8 MOV v a r i s t o r e l e m e n t a r y equivalent circuit.
MOV CAPACITANCE (PICOFARADS)
---
Fig. 9 Typical capacitance.
T r a n s i e n t s u r g e s originate f r o m a v a r i e t y of
s o u r c e s . R e g a r d l e s s of whether they a r e operating
o n AC o r DC voltages, t h e e l e c t r i c a l c i r c u i t s a r e
often plagued by voltage t r a n s i e n t s t h a t a r e either
generated within t h e individual c i r c u i t o r t r a n s m i t t e d
into t h e c i r c u i t through e x t e r n a l s o u r c e s . With inc r e a s i n g sensitivity of electronic equipment and i t s
widespread application, it is important t o understand
t h e n a t u r e of s u c h d i s t u r b a n c e s , t h e i r s o u r c e and effective methods of coping with t h e m i n o r d e r t o achieve
s a t i s f a c t o r y l e v e l s of reliability consistent with t h e
inherent p r o p e r t i e s of s o l i d - s t a t e components. T h e
m o s t common s o u r c e of t r a n s i e n t s in a distribution
s y s t e m i s t h e L ( d i / d t ) t r a n s i e n t that r e s u l t s f r o m
t r a n s f o r m e r magnetizing c u r r e n t s a s they a r e
switched, e i t h e r within t h e f e e d e r utility s y s t e m o r
within an i n d u s t r i a l p l a n t ' s own distribution s y s t e m .
Another s o u r c e of c o n c e r n is due to r e s i d u a l lightning
s u r g e s . T h e s e r e s i d u a l s u r g e s can b e thought of a s
t h e overflow o n t h e m a i n lightning a r r e s t e r s that most
c o n s u m e r , c o m m e r c i a l , and industrial distribution
s y s t e m s a r e protected with a t t h e Interface between
t h e utility and t h e u s e r ' s distribution s y s t e m s . Still
o t h e r s o u r c e s will b e found in h o m e s t h e m s e l v e s r e sulting f r o m equipment that i s connected t o t h e utility
s y s t e m . Field exposure o v e r many y e a r s and millions
of device operating h o u r s produced t h e following typic a l d a t a f o r s m a l l synchronous l i n e c o r d clocks.
R e f e r t o F i g . 11. With t h e development of high-speed
continuously monitoring oscilloscopes, p i c t u r e s of
s o m e of t h i s e l e c t r i c a l pollution s h e d s f u r t h e r light
on t h e e n e r g y content and duration of s u c h s u r g e s .
F i g u r e 12(a) shows t h e r e s u l t s on a log photo of an
OBJECTIONABLE
ELECTRIC CLOCK FIELD HISTORY
...
...
.:. ...
3 YEAR PERIOD
24 HOUR PER DAY
CONTINUOUS
MARGINAL
W
V)
LL
_I
3
MINIMAL
2
I
I
I
2 KV
6 KV
INSULATION L E V E L
Fig. 10 Typical V P s e r i e s capacitance.
F i g . 11 E l e c t r i c clock (line operated) field history.
F i g . 1 2 T r a n s i e n t voltages in homes.
automatic r e c o r d i n g c a m e r a in a home installation.
Notice that t h e r e a r e a number of s u r g e s within t h e
t i m e span of t h i s recording, i. e.. 24 hours. T h e
b r i g h t s t r i p e i s t h e 117 volts which a r e p r e s e n t a t
a l l t i m e s , while t h e t r a n s i e n t s a r e a composite of
a l l t r a n s i e n t s photographed during a 24-hour period.
In (b) a single t r a n s i e n t i s isolated in o r d e r t o give
a c l e a r e r view of t h e o s c i l l a t o r y n a t u r e and length of
such a disturbance. In ( c ) , r e f e r e n c e is made t o
s e v e r a l levels of e l e c t r i c a l voltage level s t r e s s . T h e
dotted line indicates the t y p i c a l p r a c t i c e of insulation
design in which twice r a t e d voltage i s added t o a
thousand. It will b e noticed that t h i s i s r e a l l y insufficient in view of t h e o s c i l l o g r a m s and data presented
previously in F i g . 11 if adequate l o n g - t e r m stability
is t o b e achieved. By incorporating a MOV device,
t r a n s i e n t . In c a s e 1 t h e assumption i s m a d e that the
v a r i s t o r shunted a c r o s s the load in t h e p r e s e n c e of
t h e 2000-volt t r a n s i e n t with a s u r g e impedance of 50
o h m s will be protecting a t 400-volt level. T h i s will
r e q u i r e a s t e a d y - s t a t e power dissipation in t h e v a r i s t o r
s i n c e it is connected contmuously t o t h e 1 1 7 volt r m s
line. F o r t h e silicon c a r b i d e v a r i s t o r with a typical
alpha of 3, t h i s would r e s u l t in 600 watts of steadys t a t e dissipation. In many applications, this, of
c o u r s e , would b e a n unacceptable limitation and
penalty on t h e total equipment. Notice that in t h e
c a s e of t h e m e t a l oxide v a r i s t o r with a n exponent of
-30, t h e dissipation i s many o r d e r s of magnitude
lower and completely s a t i s f a c t o r y f o r such applications. T h e lower t h e exponent, t h e higher will b e t h e
power dissipation under s t e a d y - s t a t e conditions; and
conversely t h e higher t h e exponent, t h e lower will bec o m e t h e standby l o s s e s . In c a s e 2, each example
i s shown f o r s t e a d y - s t a t e power dissipation of 1 watt
i n t h e v a r i s t o r bodies and t h e r e s u l t i n g clamping peak
voltages.
it might b e possible to successfully o p e r a t e a t a supp r e s s i o n o r clamping level a s exhibited b y the dashed
c h a r a c t e r i s t i c . Thus, r a t h e r than i n c r e a s i n g the
o v e r - a l l insulation r e q u i r e m e n t s t o a higher l e v e l it
would be possible through a coordinated s y s t e m s design t o achieve a n improved product.
T h e s u r g e s presented in Fig. 12 a r e not a t a l l
atypical. Indeed, with m o r e automatic switching
functions and complex e l e c t r i c a l equipment in the
h o m e they will b e found r a t h e r frequently. In fact.
t h e b e t t e r and newer t h e wiring and installation, t h e
l e s s protection is inherently provided b y spillover
o c c u r r i n g i n poorly wired outlets, switches, e t c . ,
s o that t h e r e will b e a n i n c r e a s e i n t h e r e s u l t i n g
i m p r e s s e d level o n connected equipment. As m o r e
and m o r e automatic c o n t r o l s begin t o m a k e t h e i r way
into home applications with the upgrading of heating
s y s t e m s , cooking equipment, a i r conditioning--not t o
n a m e t h e sophisticated electronic entertainment equipment t o b e found in a typical home. T h i s gives r i s e t o
c o n c e r n on t h e p a r t of t h e manufacturer t o adequately
i n s u r e f o r t h e u s e r t h e longevity and satisfaction t h a t
h e is entitled to.
APPLICATION T O EQUIPMENT PROBLEMS
A r c l e s s Switching
Thus, t h e challenge of how t o protect and i m p r o v e
"environment" c e r t a i n l y m u s t include t h e e l e c t r i c a l
s p e c t r u m . T h e following Fig. 1 3 v e r y graphically
i l l u s t r a t e s t h e importance of t h e high alpha in o r d e r
to provide varying d e g r e e s of protection. Again, a s
a m a t t e r of convenience, t h e dotted and dashed bands a r e
superimposed upon the 60 Hz wave with i t s i m p r e s s e d
CASE I
S I C VARISTOR
I
CASE 2
10 = 3 1
Fig. 1 3 V a r i s t o r steady s t a t e l o s s e s .
A r c l e s s o r black switching commutation c a n b e
approached b y placing t h e v a r i s t o r d i r e c t l y a c r o s s
switch contacts. W h e r e explosive environments m a y
b e encountered such a s in t h e o i l r e f i n e r y industry,
c o n s i d e r a b l e savings c a n b e achieved b y eliminating
t h e t h r e a t of a n arc-induced explosion. Applications
r e q u i r i n g high reliability and long life will want t o
u s e t h e v a r i s t o r a c r o s s mechanical contacts f o r p u r p o s e s of reducing contact e r o s i o n inherent with a r c i n g
contacts. T h i s p r o b l e m i s especially acute when
operating on DC. T h e o s c i l l o g r a m s i n Fig. 14 show
I
Fig. 14 Snap action contact voltage-time o s c i l l o g r a m s
( a ) no v a r i s t o r (b) with v a r i s t o r .
r e c o r d i n g s of t h e voltage generated b y a snap-acting
contact operating f r o m a 90 volt DC supply interrupting
a n inductive load. With a MOV v a r i s t o r attached ext e r n a l l y t o t h e contact s t r u c t u r e , the voltage i s
virtually t h a t of t h e v a r i s t o r , and r e s t r i k i n g is g r e a t l y
reduced, a s i s of c o u r s e t h e magnitude, s i n c e it is
limited in a v e r y effective manner. T h i s i s in d i r e c t
c o n t r a s t t o t h e n o r m a l situation in which t h e e n e r g y
a p p e a r s a s heating of contacts and blades.
T h i s application will a l s o s e r v e t o introduce a
new concept of integrated design. Unlike o t h e r supp r e s s o r components, t h i s new v a r i s t o r can a l s o
s e r v e in a s t r u c t u r a l mode. T h u s , by removing a
typical s p a c e r f r o m t h e switch contacts, it is possible
t o provide both functions a s shown in Fig. 15.
p + 7 7
FROM DRIVER A M P L I F I E R
E IN
1
(UNREGULATED)
"T
I
E OUT
(REGULATED)
1
Fig. 17 S t a r t up t r a n s i e n t voltage s u r g e protection.
Fig. 15 Integral u s e of MOV v a r i s t o r in contact
assembly.
Start-up T r a n s i e n t s
B e c a u s e of interwinding capacitance, step-down
t r a n s f o r m e r s when switched on can i m p r e s s s e v e r e
t r a n s i e n t s o n a n y connected s e c o n d a r y a s shown in
F i g . 16. Another s o m e t i m e s overlooked s o u r c e of
enables t h e c i r c u i t d e s i g n e r to p r o g r a m a soft c u r r e n t
r i s e through the s e r i e s r e g u l a t o r while avoiding t h e
accompanying voltage s u r g e that would b e p r e s e n t
during slow charging of Clwlthout t h e v a r i s t o r .
Another example of semiconductor voltage protection
shows us t h e simplification of component count r e sulting f r o m t h e improved e l e c t r i c a l p r o p e r t i e s of
t h e m e t a l oxide varistor. T h e s m a l l 117 volt lineoperated r a d i o c h a s s i s (Fig. 18) r e q u i r e s t h e u s e of
r e s i s t o r s and c a p a c i t o r s and high-voltage t r a n s i s t o r s
i n t h e output s t a g e in o r d e r t o s u c c e s s f u l l y stand t h e
voltage s p i k e s which r e s u l t f r o m distortion under o v e r loading conditions. T h e o s c i l l o g r a m f o r the t r a n s i s t o r
s t r e s s with n o r m a l r e s i s t o r and capacitor s u p p r e s s i o n
should b e compared with t h e simplified v e r s i o n and
i t s r e s u l t i n g o s c i l l o g r a m when t h e MOV alone is used
f o r suppression. Note t h a t the r e s u l t i n g voltage s t r e s s
on t h e t r a n s i s t o r is reduced t o a significantly low value
allowing a re-optimization of t h e t r a n s i s t o r and i t s
c i r c u i t components. Resulting c u r r e n t through t h e
m e t a l oxide v a r i s t o r is a l s o indicated.
P r o t e c t i o n f r o m Color TV Kinescope Interelement
Arcing
Metal oxide v a r i s t o r s can b e m a d e r a t h e r thick
and thus r e a d i l y applied to high-voltage c i r c u i t s .
Kilovolt ( p e r device) applications should become
common i n t h e n e a r f u t u r e . Such a possible application might b e in television s e t s . At t h e p r e s e n t time,
s p a r k g a p s together with diode c l a m p c i r c u i t s a r e often
used f o r video d r i v e r t r a n s i s t o r protection. T h e s e ,
however, s u f f e r f r o m sensitivity t o m o l s t u r e and
dust and limited life. It i s believed that by replacing
t h e gap with a m e t a l oxide v a r i s t o r improved reliability
will r e s u l t . T h i s i s shown schematically i n F i g . 19.
Fig. 16 Voltage t r a n s i e n t due t o energizing s t e p down t r a n s f o r m e r s .
component f a i l u r e c a n o c c u r in t h e t r a n s i s t o r s e r i e s
p a s s regulation a s shown in F i g . 17. Upon turn-on.
t h e capacitor a p p e a r s a s a s h o r t and t h e t r a n s i s t o r
is exposed t o t h e full unregulated bus voltage. B y
placing t h e MOV v a r i s t o r a c r o s s t h e t r a n s i s t o r , it
Other potential high-voltage applications which
c a n be achieved with i n c r e a s e d power handling m e t a l
oxide v a r i s t o r packages include high-voltage stabilization c i r c u i t s , television v e r t i c a l and horizontal
output limiting and d r i v e r protection, microwave oven
magnetron protection and x - r a y generation control,
and high-voltage AC clamping.
-m
7
VOLTAGE
VOLTAGE
CURRENT
Fig. 18.
Application of v a r i s t o r s in A - M receiver.
Step #5: Check s t e a d y - s t a t e power dissipation in
t h e c a s e of DC applications. D e r a t e e n e r g y r a t i n g
w h e r e power dissipation demands elevate the v a r i s t o r
c a s e t e m p e r a t u r e beyond 8 5 O C .
COLOR
KINESCOPE
Step #6: W h e r e r e p e t i t i v e t r a n s i e n t s m a y b e
encountered, calculate watt-seconds p e r pulse and
multiply by repetition r a t e to d e t e r m i n e added power
device m u s t dissipate. Add t o steady s t a t e idling
power of previous s t e p #5.
Step #7: Review ambient environment f a c t o r s
s u c h as operating and s t o r a g e t e m p e r a t u r e s t o i n s u r e
compliance within specifications.
HOW DO METAL OXIDE VARISTORS COMPARE
WITH OTHER DEVICES?
i
FROM Y VIDEO DRIVER
In Fig. 20 a c o m p a r i s o n i s presented f o r selenium
and m e t a l oxide v a r i s t o r s . F i g u r e 2 1 indicates t h e
peak c u r r e n t capability f o r s e l e n i u m and m e t a l oxide
F i g . 1 9 Color RGB video d r i v e r protection f r o m
a r c i n g kinescope.
Typical Steps in Selecting a Metal Oxide V a r i s t o r
At t h e p r e s e n t t i m e , devices a r e available c o v e r ing a r a n g e of voltage f r o m 140 t o 1400 and in continuous power r a t i n g s f r o m 0. 5 t o 1. 3 watts. T h e
following guides will help to i l l u s t r a t e t h e m a j o r cons i d e r a t i o n s in t h e p r o p e r device selection:
Step #1: Consider t h e n o r m a l high-lineAC peak
t e r m i n a l voltage t o b e applied t o t h e MOV v a r i s t o r .
Find t h e model with t h e c l o s e s t higher peak r e c u r r e n t
voltage rating.
Step #2: Determine o r e s t i m a t e t h e e n e r g y level
of t h e t r a n s i e n t t o b e s u p p r e s s e d . T h i s e n e r g y level
is usually determined by 112 L12 w h e r e switching
t r a n s i e n t s a r e involved. "I" is t h e peak of t h e magnetizing c u r r e n t flowing in t h e inductance "L, " t h e
field of which s t o r e s t h e t r a n s i e n t energy. In t h e
c a s e of t r a n s f o r m e r s , "I" m a y be considered a s t h e
peak of t h e exciting c u r r e n t .
0
200
RUS
VOLTAGE
400
600
RATING -VOLTS
Fig. 20 S u p p r e s s o r volume r e q u i r e m e n t s .
Step # 3 : D e t e r m i n e o r e s t i m a t e t h e c u r r e n t level
of t h e t r a n s i e n t t o b e s u p p r e s s e d . F r o m s t e p # 2 t h e
peak magnetizing c u r r e n t of f e e d e r t r a n s f o r m e r s ,
reflected to t h e secondary, i s often used to e s t i m a t e
t h e peak t r a n s i e n t c u r r e n t . F r o m t h e specification
sheet, d e t e r m i n e t h e maximum clamping voltage t h a t
c a n b e obtained. If t h i s value i s above t h e r e q u i r e d
clamping voltage, a s p e c i a l selection will be r e q u i r e d .
Step #4: Select t h e model f r o m t h e specification
sheet that provides t h e r e q u i r e d peak r e c u r r e n t
voltage, maximum clamping voltage, and e n e r g y
rating. T h e l a r g e r v a r i s t o r s will b e needed f o r t h e
v e r y high e n e r g y applications.
F i g . 2 1 S u p p r e s s o r peak c u r r e n t capability.
v a r i s t o r s , while Fig. 22 indicates t h e s u r g e e n e r g y
dissipation capability. F i g u r e 23 c o m p a r e s a number
of v a r i s t o r devices including silicon, selenium, silicon carbide, and m e t a l oxide v a r i s t o r , and a s a point
of r e f e r e n c e a l i n e a r ohmic r e s i s t o r . F i g u r e 24 i s a
l i n e a r plot of s o m e of t h e s a m e c h a r a c t e r i s t i c s .
While t h e log plots a r e t o b e p r e f e r r e d in m o s t applications, it does r e p r e s e n t a somewhat new f o r m of
presentation f o r electronic component u s e r s who m o s t
SELENIUM
single plate voltages, w h e r e a s t h e o t h e r data a r e
t r u e l y f o r single units. S e r i e s operation of MOV
v a r i s t o r s is v e r y practical.
-,
I
I N S T A N T A N E O U S VOLTS
-
F i g . 24 L i n e a r v o l t / a m p e r e c h a r a c t e r i s t i c s of common voltage s u p p r e s s o r s .
RMS VOLTAGE RATING
- vous
'8
Fig. 22 S u p p r e s s o r e n e r g y dissipation capability.
nn,
D.O.1
* bC r I %
'
,"
30
- lO.000
%.*Iepm=",
F i g . 2 5 Comparison of t r a n s i e n t voltage s u p p r e s s o r s .
F U T U R E DEVICE TRENDS
I N S T A N T A N E O U S CURRENT ( A M P S I
F i g . 23 Log-log v o l t / a m p e r e s c h a r a c t e r i s t i c s of
common voltage s u p p r e s s o r s .
usually in the past have been accustomed t o using
linear scales.
F i g u r e 2 5 a t t e m p t s t o s u m m a r i z e in tabular f o r m
a v a r i e t y of c h a r a c t e r i s t i c s f o r s o m e of the m o r e
competitive t r a n s i e n t voltage s u p p r e s s o r s . T h e
voltage r a t i n g shown p e r device is t h e peak voltage.
s o t h a t f o r r m s operation t h e values a r e 1 . 4 1 lower.
T h i s portion of t h e table i s not e x t i r e l y consistent
s i n c e many plates are used i n a s e r i e s connection f o r
selenium and packaged a s one t o achieve higher than
T h e flexibility of polycrystalline m a t e r i a l should
allow g r e a t v e r s a t i l i t y in meeting t h e g r e a t needs of
t h e e l e c t r o n i c and e l e c t r i c a l industries. Mechanical
adjustment of t h i c k n e s s b y the manufacturer to p r o vide exact voltage i s a n example. T h e ability t o s l i c e
and d i c e f r o m l a r g e r pieces should provide a flexibility in s a m p l e and prototype production while
machining and shaping open up wide possibilities.
A f u r t h e r development of a number of p r o c e s s e s
beyond t h e s i m p l e and conventional pill p r e s s p r o c e s s
i s v e r y encouraging.
F i g u r e 26 indicates a r a n g e of two leaded encapsulated devices covering a r a n g e of voltages up
t o 1400 s o m e of which a r e typical of t h e V P s e r i e s
c o m m e r c i a l l y available. Also included i n t h i s
photograph a r e s a m p l e s of higher power and voltage
devices which c a n be achieved through simply inc r e a s i n g t h e volume of the m a t e r i a l .
T h e next m a j o r developments will r e v o l v e
around package changes f o r enhanced application
v e r s a t i l i t y and improved heat t r a n s f e r purposes.
P a c k a g e s will b e c o m e available t h a t will allow a
v a r i e t y of mounting methods f o r t h e equipment
As mentioned e a r l i e r , i n c r e a s i n g t h e heat d i s s i pating capability of the m e t a l oxide v a r i s t o r substantially improve the effective e l e c t r i c a l c h a r a c t e r i s t i c s
by changing the effective clamping r a t i o . T h i s is
accomplished b y i n c r e a s i n g t h e design maximum
peak idle c u r r e n t . When the idle c u r r e n t i s i n c r e a s e d
t h e clamping r a t i o is d e c r e a s e d . F o r a single device
t h e exact value of the d e c r e a s e in clamping r a t i o f o r
a fixed l e v e l of peak s u p p r e s s i o n c u r r e n t can b e
determined by r e f e r r i n g t o F i g . 6. F o r example.
a s s u m i n g a n alpha of 25 and a 1 m a peak idling c u r r e n t , t h e clamping r a t i o of 10 a m p is l. 45. Changing
t h e idling c u r r e n t by a decade t o 10 m a r e s u l t s in a
clamping r a t i o of 1. 32 a t t h e s a m e peak c u r r e n t of
10 amp.
Fig. 26 Lead mounted MOV v a r i s t o r s and bulk
varistor material.
d e s i g n e r . Bolt-down capability with and without heat
t r a n s f e r capability is envisioned. T h i s will allow
d i r e c t fastening t o c h a s s i s , b r a c k e t s , bus b a r s , and
heat exchangers. F u r t h e r m o r e , low inductance pillshaped packages that a r e compatible with p r e s s u r e mounted t h y r i s t o r packaging a r e being investigated.
Finned design f o r d i r e c t a i r convection and
forced cooling is v e r y feasible. Initial prototypes
a few being shown in F i g . 27 indicate 5 watt d i s s i p a tion capability a t 25OC ambient convection cooled
using 1 1 / 2 - i n c h - s q u a r e plates and up t o 50 watts f o r
stud-mounted, water-cooled units ( a s with s e m i conductors e l e c t r i c a l isolation can a l s o b e introduced
into t h e package). T h e d e g r e e to which t h e application r e q u i r e s improved t r a n s i e n t r a t i n g v e r s u s steadys t a t e r a t i n g will, of c o u r s e , influence t h e s u c c e s s of
a n individual approach.
Beyond package improvements, t h e f u t u r e holds
p r o m i s e of having v a r i s t o r s with alphas exceeding 35.
Low-voltage p r o c e s s e s (under 100 volts) should make
t h e m a t t r a c t i v e f o r t r a n s p o r t a t i o n and signal c i r c u i t r y
applications.
CONCLUSIONS
T h e s e developments coupled with t h e d e v i c e ' s
e x t r a o r d i n a r y speed should now allow t h e solution of
many of t h e difficult p r o b l e m s of voltage control in
t h e nanosecond and kilowatt a r e a . A few applications
have been cited by way of illustration, but knowingly
t o t h e exclusion of many o t h e r s of i n t e r e s t such a s
semiconductor snubber networks, and s t o r a g e r e c o v e r y t r a n s i e n t s , i n d u s t r i a l control c i r c u i t s , instrumentation, and communications. T h e appended
bibiography should b e of keen i n t e r e s t i n t h i s r e g a r d .
It i s becoming increasingly c l e a r that t h e new
m e t a l oxide v a r i s t o r i s a valuable addition t o t h e
power conditioning design e n g i n e e r s tool kit. I t s
high-energy absorption capability coupled with tight
clamping r a t i o s m a k e it a s u p e r i o r t r a n s i e n t supp r e s s o r . High-voltage stabilization r e q u i r e m e n t s
m a y a l s o b e met, using t h e m e t a l oxide v a r i s t o r , a s
f u t u r e packaging and higher alphas evolve. Commercially, it is expected that t h e m a r k e t f o r t h e s e
new polycrystalline s e m i c o n d u c t o r s will amount t o
between $20 t o $30 million within 5 y e a r s .
ACKNOWLEDGMENTS
T h e a u t h o r s wish t o acknowledge the invaluable
support and encouragement of t h e i r c o - w o r k e r s ,
T. E. Anderson, K. L. Benson, D. P. Shattuck, and
D. M. T a s c a , a s well a s many o t h e r colleagues i n
support of t h e s e developments.
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Fig. 27
E x p e r i m e n t a l higher power finned d e v i c e s .
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