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EC635 Datasheet(PDF) 3 Page - Littelfuse |
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EC635 Datasheet(HTML) 3 Page - Littelfuse |
3 / 4 page 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 73ºC 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 8x20µsec 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) 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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