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Last Updated: Apr 29th, 2008 - 13:19:20
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Voltage Polarity Effects on Polymeric PTC Current Limiting Devices
May 1, 2008
by Yanqing Du, Exponent Inc., and Albert Martin and Mario Gomez, Tyco Electronics
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Polymeric PTC current limiting devices have been used in a variety of applications. This article explores the possibilities of extending an AC-rated device to a DC application with a voltage value equal to the AC RMS value. Extensive testing data shows that there is no significant difference between the two types of voltage application. The results may be a basis for waiving lengthy life test when giving the device an equivalent AC or DC rating.
Polymeric PTC current limiting devices are composed of conductive compounds of polymer stabilizers and carbon black which results in a positive temperature coefficient (PTC) resistor. This device undergoes an abrupt change of several orders of magnitude in resistance when an overcurrent heats it above a specific temperature range.[1] The high resistance in the switched state limits the current to a small value. This residual current produces a sustained self-heating, which latches the device into the protective high resistance state. The PTC will reset only when the circuit is interrupted and the device is allowed to cool below its switching temperature.[2] This resettability provides electrical safety, and has the benefit of not requiring replacement, as would be the case for a fuse.
Due to the resettable advantage, PTC current limiters have been used to limit current in a variety of applications, such as audio and video equipment, telecommunications, battery protection, transformers and more. As a result, some AC-rated devices have had a new application in DC circuits and vice-versa. This article explores the voltage polarity effects on PTC devices, and verifies if additional tests are required for an AC-rated device to be used in an equal, DC-rated application. Extensive testing data are used to validate the hypothesis.
Performance and Reliability Test Program
Underwriters Laboratories has developed a series of test programs to evaluate the long-term performance of PTC devices under UL 1434, Standard for Safety for Thermistor-Type Devices. In particular, the series includes four types of testing that are affected by applied voltage as described in the following sections. It is important to study the difference of the PTC device under different voltage polarities to ensure the reliable and safe performance of the device. In the meantime, due to the long testing time involved, it is also beneficial to discover if there are any performance concerns with the two types of voltage, and whether the tests can be waived from a conformance perspective.
Aging Test
Under this test, the samples of PTC current limiter are energized and conditioned for 1000 hours (approximately 42 days) while in the tripped state at maximum voltage (Vmax) and carrying steady-state current. This test measures the combined long-term effect of heating and voltage stress, and most times is the most severe test among all tests.
Overload and Endurance Tests
During the overload test, samples of PTC current limiter are operated for 50 cycles while connected to Vmax and 120 percent of rated maximum current (Imax). Samples are then tested at Vmax and tripping current or greater for 6,000 cycles during the endurance test.
Cold Operational Cycling Test
This test is similar to the endurance test, except with 1000 cycles at an ambient of 0°C or lower.
Thermal Runaway Test
During the thermal runaway test, samples are subject to a voltage ramp from rated voltage (Vr) to 200 percent of Vmax. The test voltage is maintained at 200 percent of Vmax for 2 minutes.
Possible Effects at Different Voltage Polarities
This article explores the effects of voltage polarity on the polymeric PTC device, with DC voltage rating equal to the AC voltage RMS rating.
Heating Effects
For a linear resistive device, the heating effects for the AC and DC application are the same as shown in Equation 1. However, this may not be the case for a non-linear device like the PTC. Equation 1 derives the heat dissipation for the same AC RMS and DC voltage during one cycle. The resistance of the PTC device varies with temperature, which is a function of current/voltage applied; therefore, the total heat dissipated is not necessarily the same for the same AC RMS and DC voltage.
For tripped parts, the PTCs are typically constant-power devices in the tripped state. Since R = V2/P, and power dissipation is constant, R = kV2, where K is an experimentally-determined constant.
Maximum Voltage Stress
Samples are subjected to higher maximum voltage during AC application. This effect is most predominant during the aging and thermal runaway test. In addition, conductivity of the polymer might be a function of the voltage and, therefore, a difference in peak voltage may affect the device characteristics.
Phase Angle
When a device is being cycled with AC voltage, there is a phase angle when the test is initiated. Depending on the cycling rate, the DC cycling test may be a more severe test to the device than that of AC. The two tests that might be affected by this factor are the overload/endurance test and the cold operational cycling test.
Ion Migration
Due to the uni-polar electron movement under DC application, ions will likely migrate from one electrode and deposit at the other electrode after long term exposure, such as during the aging test.
Experimental Verification
Extensive testing has been performed for radial type PTC devices, LVRXXX series, with both AC and DC voltages. Two models, LVR005 and LVR040, representing the smallest and largest current ratings in the series were used in this article for discussion. The ratings of these two models are shown in Table 1.
Model
| Vr
| Imax | Ihold | Itrip | Resistance (Ω) | | LVR005 | 240 Vac | 1.0 Arms | 0.05 Arms | 0.12 Arms | 18.5~31 |
| 240 Vdc | 1.0 Adc
| 0.05 Adc | 0.12 Adc | 18.5~31 | | LVR040 | 240 Vac | 5.5 Arms | 0.40 Arms | 0.90 Arms | 0.6~0.97 |
| 240 Vdc | 5.5 Adc | 0.40 Adc | 0.90 Adc | 0.6~0.97 |
Table 1: Ratings at 20°C for Model LVR005 and Model LVR040 used for this study
A resistance versus temperature (R/T) test as shown in Figure 1, and a power dissipation test for the device under tripped state, were performed before and after each conditioning test. These two tests characterize device performance before and after each conditioning test. A unique temperature derived from the R/T curve corresponding to measured trip resistance was used to quantitatively compare the device performance change after each test.
Figure 1: Resistance vs. temperature curve for one sample of Model LVR005 for the 240 Vdc aging test.
Figure 2: Resistance vs. temperature curve for one sample of Model LVR005 for the 240 Vac aging test. (Note this is not the same sample as the sample shown in Figure 1.)
Due to a maximum interrupt voltage rating of 265 Vac for Model LVR040, the four tests discussed in this article for Model LVR040 AC rating were performed at 265 Vac instead of 240 Vac.
Aging Test Results
The percentage change in the tripped temperature for the aging test is shown in Table 2. The average percent shift is consistently larger for the AC voltage test than for the DC test. The difference is about 6%, indicating that the higher voltage stress during the AC test played a major role in device performance.
| Model LVR005 | Model LVR040 | | Sample | 240 Vac | 240 Vdc | 265 Vac | 240 Vdc | 1
2
3
4
5
6 | 7.3%
7.8%
7.9%
6.9%
6.9%
8.4% | 2.3%
0.8%
0.3%
1.4%
1.1%
3.8% | 12.9%
8.5%
13.0%
8.45%
11.6%
14.2%
| 4.6%
7.1%
7.0%
4.6%
4.4%
8.5% | | Average | 7.53% | 1.62% | 11.44% | 6.03% |
Table 2: Percent shift of tripped temperature for samples after the aging test
Note that samples used in the AC rating tests are different from those used for the DC rating tests presented in this article, even though they are labeled with the same sample number in the resulting tables.
Overload/Endurance Test Results
The percent shift of tripped temperature after the overload/endurance test is shown in Table 3. The data also shows that the percent shift is larger for the AC test than for the DC test.
| Model LVR005
| Model LVR040 | | Sample | 240 Vac | 240 Vdc | 265 Vac | 240 Vdc | 1
2
3
4
5
6 | 5.8%
7.5%
5.8%
5.5%
5.3%
7.0% | 0.7%
0.5%
0.5%
0.8%
0.2%
1.1% | 1.3%
6.8%
3.2%
6.0%
2.8%
0.7% | 1.0%
0.5%
0.7%
1.1%
0.2%
0.1% | | Average | 6.15% | 0.63% | 3.47% | 0.60% |
Table 3: Percent shift of tripped temperature for samples after the overload/endurance test
Cold Operational Test Results
The percent shift of tripped temperature after the cold operation test is shown in Table 4. The results for two models appear to be similar given the measurement accuracy and sample differences.
| Model LVR005 | Model LVR040 | | Sample | 240 Vac | 240 Vdc | 265 Vac | 240 Vdc | 1
2
3
4
5
6 | 2.6%
2.2%
1.9%
1.9%
2.1%
2.7% | 0.0%
0.8%
2.1%
2.5%
2.7%
2.0% | 0.1%
0.7%
3.6%
5.1%
0.1%
3.2% | 0.8%
5.6%
11.4%
1.4%
3.6%
0.6% | Average
| 2.23% | 1.68% | 2.13% | 3.90% |
Table 4: Percent shift of tripped temperature for samples after the cold operational test
Thermal Runway Test Results
The percentage shift in the tripped temperature for the thermal runaway test is shown in Table 5. There is no discernable difference between the AC and DC test results, possibly due to the short duration of the test (less than 5 minutes).
| Model LVR005
| Model LVR040 | | Sample | 240 Vac | 240 Vdc | 265 Vac | 240 Vdc | 1
2
3
4
5
6 | 2.4%
2.2%
0.8%
1.2%
1.8%
1.4% | 1.2%
2.8%
0.7%
0.4%
0.9%
1.2% | 0.2%
0.3%
0.1%
0.2%
0.3%
1.2%
| 5.4%
1.1%
0.5%
7.1%
0.1%
0.8% | | Average | 1.63% | 1.20% | 0.38% | 2.50% |
Table 5: Percent shift of tripped temperature for samples after the thermal runaway test
Conclusions
Based on this study, we found that there is no significant difference in performance for the same voltage but different polarities. The AC test condition appears to be more severe than the DC test condition during the aging and overload/endurance tests. These results may be used to waive long-term life tests when giving new DC ratings to a polymeric PTC current limiting device that has an equivalent AC rating. This will save the time in conformance testing and allow shorter time for the product to receive a compliance certification. If the DC performance is evaluated first and the device has a percentage shift more than 10% (above Class 2 per UL 1434 definition), it is prudent to perform the aging and overload/endurance tests in order to extend the AC rating. Further study will be needed to examine whether manufacturing techniques will affect this conclusion, and to analyze the internal structure of test samples. n
Yanqing Du was a Senior Project Engineer at Underwriters Laboratories specializing in product safety and compliance issues, and is currently a Managing Engineer at Exponent Inc. She can be reached at du@alum.mit.edu.
Albert Martin and Mario Gomez are with Tyco Electronics, and can be reached at amartin@tycoelectronics.com and mario@tycoelectronics.com.
Acknowledgment
The authors wish to thank Mr. Wayne Benns of Underwriters Laboratories Inc. for his support to the presented work. Special thanks to Professor Markus Zahn of MIT for helpful comments on this article.
References
- Narkia, M, A. Ram, and F. Flashmer. Electrical Properties of Carbon Black filled Polyethylene. Polymer Engineering Sci., vol. 18, No, 8, p. 649, June 1973
- Doljack, Frank A. PolySwitch PTC Devices-A New Low-Resistance, Conductive Polymer-Based PTC Device for Overcurrent Protection. IEEE Trans. On Components, Hybrids and Manufacturing Technology, CHMT-4, No. 4, December 1981.
- Underwriters Laboratories Inc., UL 1434, Standard for Safety for Thermistor-Type Devices, 1st Edition, April 3, 1998.
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