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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： | 3/4页 |
| 文件大小： | 325K |
| 代理商： | EC635 |
and compared with several thermally self
protected
MOVs
(Littelfuse
20mm,
130Vacrms,
TMOV
varistor
–
TMOV20R130).
Both methods were
subjected to a sustained abnormal over-volt-
age of 240V at 5A. As can be seen in Table
2a and as expected, the TCOs with higher Tf
take longer to clear. The 73C TCO proved
difficult to hand solder without clearing the
device despite the use of an appropriate
heat-sink. Table 2b shows the clearing times
for the internally protected MOV.
Clearly,
the times are shorter than for any of the
MOV/TCO combinations tested.
Figure 5. shows the effects of applying a
UL1449
abnormal
over-voltage
test
(240VRMS, 5A) on three devices or combi-
nation of devices - 1) MOV alone (20mm,
130Vacrms – V20E130) 2) MOV/TCO
combination (20mm, 130Vacrms MOV –
V20E130 and TCO with Tf = 94°C), and 3)
TMOV
varistor (20mm, 130Vacrms –
TMOV20R130).
Epoxy surface temperature vs. time was
captured for each method. As can be seen,
the case temperature of a standard MOV
rated for 130VRMS will continue to rise (to
the point of combustion) if no thermal
protection is used. The MOV/TCO combo
performs better reaching temperatures of
220°C before the TCO clears. The internally
protected MOV has a faster response time,
clearing at temperatures of around 150°C in
less than 20 seconds. Note that the temper-
ature continues to rise once the thermal
fuses have cleared. Heat generated within
the zinc oxide disk is at a higher temperature
than the outer epoxy coating. Heat contin-
ues to flow outward to the epoxy for some
time before finally cooling down.
Figures 6a – 6c illustrate the effects of the
temperature rise on each MOV. As can be
seen, the new technology eliminates much of
the charring when compared with a stan-
dard MOV or MOV/TCO combination.
Wave Soldering the
TMOV Varistor
Figure 7 shows a suitable wave solder profile
that can be used for the TMOV varistor. The
profile temperatures are very typical to
those found in general wave solder methods.
In contrast, the solder profile shown for the
TCO shows temperatures much lower than
those found in a typical solder bath. In fact,
the profile shown for the TCO actually
depicts a profile at which the TCO fails
(opens) generally indicating that a TCO
(even one with a high Tf (142°C) cannot be
wave soldered.
Generally, here will be a cost benefit associ-
ated with eliminating the TCO which must
be hand soldered carefully to avoid opening
the element.
Integrity of an Open
Thermal Element
Once the thermal element of a TMOV varis-
tor opens, it is important that the element
stays open and that a reconnection does not
occur. Remember, in order for the thermal
element to have cleared (opened), the varis-
tor disk itself must have heated up due to
thermal run-away and the thermal runaway
began with a failed (shorted) varistor. It is,
therefore, undesirable to have a failed varis-
tor placed back into the circuit, electrically
speaking.
In order to ascertain the integrity of an open
thermal element within a TMOV
varistor,
devices were first subjected to an abnormal
over-voltage limited current event causing
the thermal element to clear. These devices
were then subjected to two tests. First, the
devices were subjected 6kV, 3kA 8x20sec
pulses. The TMOV
varistors were then
subjected to bias voltage and monitored for
leakage currents indicating a full or partial
reconnection.
None were noted.
Next,
1000Vrms was applied for several hours,
again with no connection as verified by the
leakage test.
Table 2a. MOV/TCO observed clearing times for 5A
limited current test
Table 2b. TMOV varistor observed clearing times for
5A limited current test
0
102030405060708090100
110
0
50
100
150
200
250
300
350
400
450
Time (seconds)
C
ase
T
emper
atur
e
(°
C)
MOV/TCO
TMOV Varistor
MOV
Figure 5. Typical surface temperature vs. Time for several
protection schemes
Figure 6a. Standard MOV
Figure 6b. MOV/TCO combination
Figure 6c. TMOV varistor
Wavesolder trials on TMOV varistor vs. TCO
142 deg TCO
14/20mm TMOV varistor
0
500
1000
1500
2000
2500
Figure 7. Wave solder profile of TMOV varistor vs. TCO
(Tf=142°C)
TCO
Tf (°C)
73
94
121
Mean
30
34
36
Range
11-52
20-46
16-56
Clearing Time (s)
Tf (°C)
TMOV
varistor
Mean
13
Range
2-25
Clearing Time (s)
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