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参数资料
| 型号： | EC635 |
| 厂商： | Littelfuse, Inc. |
| 英文描述： | Designing with thermally protected TMOV Varistors in TVSS Applications |
| 中文描述： | 设计与热保护TMOV压敏电阻的TVSS的应用 |
| 文件页数： | 4/4页 |
| 文件大小： | 325K |
| 代理商： | EC635 |
Indication of an Open
Thermal Element:
iTMOV Varistor
The benefits of the TMOV
varistor have
been thoroughly discussed, but one question
remains: How do I know when the thermal
element has cleared? By design, MOVs
exhibit a very high impedance when
subjected to voltages below its MCOV
(Maximum Continuous Operating Voltage).
So, once installed into an end product, how
do you know if the TMOV varistor is still
operational? Enter the iTMOV varistor.
The iTMOV
varistor
adds an additional
third indicator lead that provides access to
the connection between the thermal
element and the MOV electrode. Having
access to this point of the circuit makes indi-
cation of the thermal element a simple
procedure. Figures 8 shows a simple appli-
cation circuit with indication.
In Figure 8, an iTMOV varistor is used to
protect the L-N connection of a typical U.S.
120Vac line. An AC rated LED in placed
across the iTMOV varistor’s indicator lead
and the Neutral line. A series resistor, R1, is
added to limit current through the LED. A
47k
, 0.5W resistor is shown, but the
designer should review the LED’s ratings to
choose the correct value. A series diode,
D1, may be needed if the reverse voltage
rating of the LED is insufficient to handle
negative
voltages
from
the AC
line.
Additionally, a Littelfuse 3AG, 10A (313010)
fuse is shown to protect against excessive
over-current into the load, but the designer
should choose a value specific to his/her
design..
Under normal conditions, the LED is forward
biased from the line voltage through the
thermal element to Neutral. If the thermal
element opens current will be interrupted
and the LED will go off. The LED will also go
off if the Line Fuse opens indicating a loss of
power.
When paralleling TMOV
varistors, the
iTMOV varistor can be used for several or
all parallel devices. That is, one may wish to
indicate when a cer tain percentage or
TMOV varistors fail. Generally, once some
of the MOVs in parallel begin to fail, all begin
to fail.
Conclusion
The UL1449 standard includes a require-
ment that was created to protect the end
product and users from a loss of neutral situ-
ation
where
an
abnormal
over-voltage/limited current condition could
be applied to Metal Oxide Varistors. This
event would cause an MOV to have a
sustained voltage applied in excess of its
maximum working voltage, which in turn
would cause the MOV to enter a thermal
runaway condition.
Several methods exist to prevent the MOV
from reaching combustible temperatures -
the most common of which is to use TCOs.
While TCOs perform adequately in limiting
MOVs from reaching very high tempera-
tures, there are limitations. Out-gassing and
some charring are evident when the test is
applied. Additionally, the assembly process is
difficult to automate, as wave soldering is
typically not an option.
Overall, the new integrated TMOV varistor
thermal fuse technology reduces part count,
saves space and is UL1449 recognized. It
performs better than other methods of
protection when subjected to a limited
current over-voltage condition, by clearing
more quickly at a lower temperature to
reduce the potential for out-gassing or char-
ring. It has all the performance capability of
a standard MOV, including peak pulse
current capability, energy rating and clamping
voltage. The new device can also be wave
soldered which saves on assembly costs and
simplifies the assembly process by eliminating
most of the hand assembly required with
other methods.
Note:
All data was taken with a limited sample size.
Results may vary due to normal variations in
electrical
and
mechanical
parameters.
Designers are encouraged to evaluate their end
design with a large enough sample size to
ensure consistent results. In some instances
TMOV varistors may exhibit substantial heating
and venting prior to opening. Module design
should be such as to contain this possibility.
Application testing is strongly recommended.
References:
1. Transient Voltage Surge Suppressors -
UL1449, June 25, 1998
2. Littelfuse Datasheet, Thermally Protected
Metal Oxide Varistor (TMOV Varistor),
March 2001
3. Littelfuse Datasheet, High Surge Current
Radial
Lead
Metal
Oxide
Varistor
(UltraMOV Varistor Series), March 2001
4. Paul Traynham and Pat Bellew, Using
Thermally Protected MOVS in TVSS or
Power
Supply
Applications,
Power
Systems
World,
Inter tec
Exhibition
Proceedings, September 2001
1 In Table 1., “device” is defined as the end
TVSS product - example: UPS,TVSS Strip
etc.
Littelfuse, Inc.
800 E. Northwest Highway
Des Plaines, IL 60016
www.littelfuse.com
Specifications descriptions and illustrative material in this literature are as accurate as known at time of publication, but are
subject to change without notice. Please visit www.littelfuse.com for the most up-to-date product and application literature.
Copyright 2001 Littelfuse, Inc., All Rights Reserved
EC635
Fuse
Line
TMOV
Varistor
TMOV20R130M
Neutral
120VAC
D1
LED Normally On
R1
47k,0.5W
Figure 8. Indicator circuit using the iTMOV varistor (LED
normally on)
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