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參數(shù)資料
| 型號： | EC635 |
| 廠商： | Littelfuse, Inc. |
| 英文描述： | Designing with thermally protected TMOV Varistors in TVSS Applications |
| 中文描述： | 設(shè)計與熱保護(hù)TMOV壓敏電阻的TVSS的應(yīng)用 |
| 文件頁數(shù)： | 2/4頁 |
| 文件大?。?/td> | 325K |
| 代理商： | EC635 |
Thermally Protecting
MOVs
A simple block diagram of a typical line volt-
age transient protection scheme used to
meet the sustained abnormal over-voltage,
limited current test requirements of UL1449
is shown in Figure 2. An MOV or several
MOVs in parallel are each placed across each
of the three conductive pairs; L-N, L-G, and
N-G. This offers the utmost protection for
any possible line transient. A standard fuse is
placed in series with the line to protect the
system from an over-current condition that
exceeds a predetermined level. Typically, the
current rating of this fuse is higher than the
limited current flowing through the circuit
during UL1449 testing. This requires the
addition of a TCO that is placed in series
with each MOV or Parallel combination of
MOVs to protect it from a thermal event.
Often, the MOVs used are of the radial
leaded 14mm or 20mm disk diameter vari-
ety.
TCOs are available in a variety of different
opening temperatures. The position and
orientation of the TCO is important if it is to
be effective in thermally protecting an MOV.
When subjected to a sustained over-voltage,
MOVs will short at a random point on the
disk and will rapidly begin to self-heat if a
limited current is maintained. TCOs are acti-
vated by a combination of conducted,
converted and radiated heat from the MOV,
although the majority of the heat is trans-
ferred via conduction. The position of the
TCO in relation to the heat source at this
shorting point has a considerable effect on
the speed of operation of the TCO. The
most effective heat coupling has been
observed to be via conduction through the
varistor terminal lead to the insulated termi-
nal of a metal jacket TCO. Thermal
convection and radiation processes are
effective when the heat source is immedi-
ately beside or below the TCO. Although
conduction is the most effective means of
heat transfer, the MOV and TCO are not in
full contact in most cases.The position of the
terminal leads of the TCO makes it difficult
for the TCO to be located closely enough to
the MOV for effective heat transfer. The
result is less than efficient conduction from
case to case. An example of a typical
arrangement of MOVs and TCOs is shown
in Figure 3. Note the TCO does not touch
the case of the MOV.
The response time of this arrangement can
be disproportionately increased if the TCO
is not placed in close enough proximity to
the MOV and/or the punch-through point
on the MOV occurs remotely from the
TCO’s insulated terminal. In such cases,
considerable charring of the MOV can occur
and fire is a real possibility. Shrink-wrap or
other bonding materials can aid coupling, but
in adverse circumstances they are a source
of combustible material and may actually
make things worse.
While this scheme is generally effective in
removing the MOV from the circuit during
abnormal over-voltage testing such that the
MOV does not reach critical temperatures,
the downside to this method is that TCOs
can be difficult to handle during the assem-
bly process.
Because of the low opening
temperatures,TCOs must be soldered care-
fully. When hand soldering, the iron cannot
remain in contact with the lead of the TCO
for prolonged periods. Another option is to
use clips or pliers as a heat-sink.. TCOs with
useful opening temperatures for the MOVs
typically cannot be wave soldered, as the
device will clear in the solder bath.
In
general, the use of TCOs in these types of
applications becomes largely a hand assem-
bly process.
A new technology has been developed that
will aid the designer in meeting UL1449
requirements including the sustained abnor-
mal over-voltage limited current testing,
while eliminating most of the problems asso-
ciated with other methods. This technology
is a fully integrated, thermally self-protected
MOV - TMOV
varistor Series. This new
device uses a patent pending thermal
element internal to the MOV so that it is in
direct contact with the metal oxide disk,
allowing for optimum heat transfer. Because
of the proximity of the thermal element to
the MOV body, a higher opening tempera-
ture element can be used. This allows the
thermally self-protected MOV to be wave
soldered simplifying the assembly process.
The construction method also allows the
new device to perform to standard MOV
ratings with regards to peak current, peak
energy, voltage clamp levels, etc. while
providing the safety of a thermally protected
device.
Figure 4. Illustrates the integrated
function.
Comparing Methods of
Thermally Protecting
MOVs
The internally thermally protected TMOV
varistor overcomes most the disadvantages
of the MOV/TCO combination method.
Placing the thermal element inside the epoxy
coating and close to the center of the disk
provides several benefits.
1) It optimizes
heat transfer between the MOV disk and the
thermal element by placing the thermal
element as close to the point of failure as
possible.
This greatly improves clearing
(opening) times. 2) Allows for the thermal
element to have a higher opening tempera-
ture than most TCOs used while being
protected from external heat sources. This
allows the device to be wave soldered. See
Section 6.
In order to compare the clearing times of
both methods, several standard MOVs
(Littelfuse 20mm, 130Vacrms, UltraMOV
varistors) in combination with TCOs of vari-
ous opening temperatures, Tf, were tested
Fuse
Line
TCO
MOV
Neutral
Ground
TCO
120VAC
Figure 2. Typical offline protection scheme
Figure 3. Typical Arrangement of TCOs with MOVs
** one of the MOVs has been removed for clarity
Fuse
Line
TMOV
Varistor
TMOV
Neutral
Ground
120VAC
Integrated Thermal
Element
MOV Disk
Figure 4. TMOV varistor offline protection scheme
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