Advances in capacitor technologies may not be heralded the way semiconductor developments are, but these passive components are continually evolving nonetheless. A case in point is the pulse supercapacitor, also known as the electric double-layer capacitor (EDLC). These devices are low ESR versions of traditional supercapacitors, with better frequency characteristics, which make them behave much like a bank of electrolytic capacitors.
In only a few years, these components have experienced developments and redesigns that have produced significant improvements in key power performance parameters. Consequently, any evaluation of pulse supercapacitor technology versus other capacitor types must consider the latest generation of supercapacitors and not older parts. Specifically, this becomes even more important when you consider that these devices were really only released circa 1995 (supercapacitors) and 2000 (pulse supercapacitors).
This issue comes to mind when reading the article “Overcoming PCMCIA Power Limits in Wireless Modem Designs” in the May 2006 issue of Power Electronics Technology. The article compares the performance of the latest conformal-coated tantalum capacitors with that of supercapacitors in providing the energy storage required for GSM/GPRS transmission. Unfortunately, the article compared the performance of the very latest tantalums in the application with the performance of five-year-old EDLC technology, ignoring the most significant technology improvements made in this family of capacitors.
Current proton polymer pulse supercapacitor technology, like AVX's BestCap series, now offers a dc capacitance of 150 mF with a 4.5-V working voltage and just 65 mΩ of ESR in a 28-mm × 17-mm package. For a typical GPRS duty cycle, this device will provide up to 100 mF of capacitance at the application frequency. Another new device in this series offers 47 mF, 4.5 V and 170 mΩ in a much smaller 20-mm × 15-mm package.
If you insert these parameters into the calculations used in the aforementioned article, you will see that pulse supercapacitors can offer up to four times the efficiency of a bank of tantalums in the same, or less, real estate. Contrast that performance with the supercaps described in the article, which had a calculated figure below that of the bank of tantalums. The very “real” performance superiority of pulse supercapacitors in energy efficiency becomes increasingly important as energy budgets increase as required by the shifts from the GSM to GPRS 10 and 12 standards.
Moreover, the parametric stability and reliability of pulse supercapacitors is significantly better than represented in the wireless modem article. They have a graceful failure mode that is resistive not short circuit. Any ESR shift that may occur during the component's lifetime remains within the 3x limit at maximum-rated temperature, and is considerably less at card operating specifications (typically 60°C [140°F] max).
Concluding that previous-generation supercapacitor (EDLC) technology is inferior to the latest tantalum technology is akin to saying the combustion engine of the Model T Ford is not as efficient as the latest automotive diesel engines. New technology continues to develop across the component world and even some new polymer electrode tantalums will come closer to the pulse-supercapacitor performance when used in this application.
This comparison of different capacitor technologies underscores the old adage that it's important to compare “apples to apples” when evaluating competing technologies. Although, in this case, it may not be so much a matter of comparing two identical fruits (technologies), but making sure that both are freshly picked.
Craig Hunter is responsible for sales communications at AVX, where he has worked for 15 years.