Power Electronics

Thermally Protected MOVs Improve Transient Suppression

Metal oxide varistors (MOVs) are commonly used for transient overvoltage suppression in many power electronics applications. Lightning, inductive load switching, capacitor bank switching, or other such events are often the sources of these transients. Under normal operating conditions, the ac line voltage applied to an MOV is not expected to exceed the MOV's maximum Vacrms rating. Overvoltage transients may occur that exceed these limits. These transients are clamped to a suitable voltage level by the MOV, provided the transient energy doesn't exceed the MOV's maximum rating. If an MOV is subjected to a sustained abnormal overvoltage, limited current condition (as required in UL1449), the MOV may go into thermal runaway, resulting in overheating, smoke, and potentially fire. For end products to comply with UL1449, the MOV must have some level of protection to prevent this failure mode. That protection has traditionally been a thermal fuse or thermal cutoff (TCO) device.

In ac line applications, neutral and ground are typically near or at the same potential. In the event of a loss of a neutral-ground connection, there is a risk that a sustained overvoltage may be applied to an MOV rated for a much lower continuous voltage. In an unlimited current condition, the MOV will first fail short. Yet, due to the high amount of energy available, it most often ruptures instantaneously. However, if there are loads tied to the ac line that limit current flow, the MOV can overheat and potentially cause the TVSS (transient voltage surge suppressor) to overheat. This potential condition is identified and addressed in the UL1449 TVSS Standard (Table 1). In many cases, it requires that end-product manufacturers include a thermal protection element for an MOV.

Table 1 defines the test voltage that you should apply to various TVSS devices, depending on the designer's desired device rating. Each test voltage is applied across each conductor pair with a short circuit current of 5A, 2.5A, 0.5A, and 0.125A respectively across each of four TVSS devices. Since this test is destructive, you need four devices to test for each of the four short circuit currents. You must energize the four devices for 7 hr, until current or temperatures within the device attain equilibrium, or until it becomes disconnected from the ac line.

For example, in a standard 120Vac line application, the requirement is for a 240Vacrms test voltage to be applied across all conductor pairs. There are three pairs: line-neutral (L-N), line-ground (L-G), and neutral-ground (N-G). In the United States, 120Vac power is usually fed from a center-tapped 240V transformer. If a break occurs at X — X in Fig. 1, then the load in the bottom phase acts as a current limiter and the line fuse may not clear. Thermally unprotected MOVs are typically rated from 130Vacrms to 150Vacrms and will heat up, outgas, and may catch fire.

Thermally Protecting MOVs

Fig. 2 shows a simple block diagram of a typical line voltage transient protection scheme used to meet the sustained abnormal overvoltage, limited current test requirements of UL1449. 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 overcurrent 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 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 14 mm or 20 mm disk diameter variety.

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 overvoltage, MOVs will short at a random point on the disk and rapidly begin to self-heat if you maintain a limited current. TCOs are activated by a combination of conducted, convected and radiated heat from the MOV, although the majority of the heat is transferred 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 terminal of a metal jacket TCO. Thermal convection and radiation processes are effective when the heat source is immediately 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 Fig. 3. Note the TCO doesn't 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 make things worse.

Although this scheme is generally effective in removing the MOV from the circuit during abnormal overvoltage testing so that the MOV does not reach critical temperatures, the downside to this method is that TCOs can be difficult to handle during the assembly process. Because of the low opening temperatures, you must carefully solder the TCOs. When hand soldering, the iron can't remain in contact with the lead of the TCO for prolonged periods. Another option is to use clips or pliers as a heat sink. Typically, you can't wave solder TCOs with useful opening temperatures for the MOVs, as the device will clear in the solder bath. In general, the use of TCOs in these types of applications becomes largely a hand assembly process.

A new technology aids the designer in meeting UL1449 requirements, including the sustained abnormal overvoltage limited current testing, while eliminating most of the problems associated with other methods. This technology is a fully integrated, thermally self-protected MOV — TMOV Varistor Series. It uses a patent pending thermal element internal to the MOV, so 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, you can use a higher opening temperature element. This allows the thermally self-protected MOV to be wave soldered, simplifying the assembly process.

Fig. 4 illustrates the basic construction of the new device. It's a leaded component with one of its two leads thermally fused. You can achieve the fusing by breaking into the copper lead and replacing it with a solder link. Under overvoltage limited current conditions the solder reflows and opens the circuit, preventing the risk of fire. Fig. 4 shows one end of the solder link has a circular insulator under it. This ensures a good insulation gap after the fusing action. The other end of the link connects to the silver electrode.

This method of construction allows the device to perform to standard MOV ratings (with regard to peak current, peak energy, voltage clamp levels, etc.) while providing the safety of a thermally protected device.

Internal Thermally Protecting MOVs

The internally, thermally protected MOV overcomes most problems of the MOV/TCO combination method. Because the thermal fuse element is inside the epoxy coating, it's in intimate contact with the ceramic disk. The thermal element is at the center of the disk, close to the heat source.

To compare the clearing times of both methods, several standard MOVs (20 mm, 130Vacrms, UltraMOV Varistors), in combination with TCOs of various opening temperatures, Tf, were tested and compared with several thermally self-protected MOVs (20 mm, 130Vacrms, TMOV Varistor). Both methods subjected the devices to a sustained abnormal overvoltage of 240V at 5A. Table 2(a), on page 52, shows the TCOs with a higher Tf take longer to clear. The 73°C TCO proved difficult to hand solder without clearing the device — despite the use of an appropriate heat sink. Table 2(b), on page 52, shows the clearing times for the internally protected MOV. Clearly, the times are shorter than for any of the MOV/TCO combinations tested.

Fig. 5, on page 52, shows the effects of applying a UL1449 abnormal overvoltage test (240Vrms, 5A) on three devices or combination of devices: 1) MOV alone (20 mm, 130Vacrms) 2) MOV/TCO combination (20 mm, 130Vacrms MOV and TCO with Tf = 94°C), and 3) thermally self-protected MOV (20 mm, 130Vacrms).

Each method involved measurement of epoxy surface temperature vs. time. As shown, the case temperature of a standard MOV rated for 130Vrms and without thermal protection will continue to rise. 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 sec. Note that the temperature 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 continues to flow outward to the epoxy for some time before finally cooling down.

Figs. 6(a), 6(b), and 6(c) 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 standard MOV or MOV/TCO combination.

Overall, the new integrated MOV-thermal fuse technology reduces part count, saves space and is UL1449 recognized to UL1449 MOV requirements and the overvoltage testing you see in Table 1, on page 48. It performs better than other methods when subjected to a limited current overvoltage condition by clearing more quickly at a lower temperature and with minimal to no outgassing or charring. It has all the performance capability of a standard MOV. You can also wave solder the new device to save on assembly costs and simplify assembly processes.

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.



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