Tantalum, MLCC, and super capacitor technologies are ideal for many energy storage applications because of their high capacitance capability. These capacitors have drastically different electrical and environmental responses that are sometimes not explicit on datasheets or requires additional knowledge of the properties of materials used, to select the best solution for a given design.
This paper compares the performance of these technologies over energy density, frequency response, ESR, leakage, size, reliability, efficiency, and ease of implementation for energy harvesting/scavenging/hold-up applications. A brief, material properties benefits and considerations of X5R, Tantalum,
Tantalum polymer, and electrochemical double-layer capacitors is provided. An example of an energy storage circuit problem is provided that has a capacitance and voltage requirement that is not achieved with a single, maximum CV capacitor for any of the relevant technologies. Capacitor banks are built with each technology that are viable solutions.
Design considerations are discussed for optimization of each capacitor bank and analyzed. Results of the analysis will show where each technology excels. This paper should be of interest to component engineers, program managers, and power electronics engineers working on energy harvesting, scavenging, and hold-up applications, due to its impact on system design and performance.
The paper was presented by Daniel West, AVX USA at the 3rd PCNS 7-10th September 2021, Milano, Italy as paper No.4.4.
Summary and Conclusions
In summary, X5R MLCC dielectrics are ideal for small loads where size and cost constraints of a design take priority.
X5R was selected for this study because of its high CV capability, low costs, and availability, but there may be other formulas that are viable for a given design. It is up to the designer to address how well the dielectric will perform in-application due to electrical, mechanical, and thermal stress; data that may or may not be explicit on manufacturer datasheets.
Tantalum (MnO2) capacitors are a good selection for long life applications, because of virtually no wear-out mechanism. These devices need to be properly derated though and polarized to make full use of its high reliability capabilities and will provide an extremely stable capacitance across high temperatures and voltage gradients. Tantalum polymers, as observed by this study, are more viable in Dying Gasp or Last Gasp applications where the power delivery
capacitors are always charged. These are small, high CV, low ESR, high voltage options, great for short burst applications that require efficient power delivery.
Finally, it is easily discernible that supercapacitors are ideal for the broadest range of energy storage applications. They easily achieve enormous capacitance values, easily implemented, have power densities much larger than batteries, and their reliability can be easily managed by proper temperature and voltage
derating. But, because of the relatively low voltage capability (per cell) of the electrochemical systems used, often series/parallel combination modules will need to be created or acquired to achieve a specific voltage rating. If modules are designed in-house, it should be known that modules typically boil down to balancing ESR and DCL performance.
References
[1] Multilayer Ceramic Capacitors Materials and Manufacture; Kahn, Manfred; https://www.avx.com/docs/techinfo/CeramicCapacitors/mlcmat.pdf
[2] High-Reliability Solid Tantalum Capacitors; Fairey, Bob; https://www.avx.com/resources/high-reliability-solidtantalum-capacitors/
[3] Conductive Polymer Capacitors Basic Guidelines; S. Zednicek, J.Petrzilek, P.Vansura, M.Weaver, C.Reynolds; https://www.avx.com/docs/techinfo/PolymerGuidelines.pdf
[4] Reliability of Supercapacitors: Unique Performance at 85°C & Self-Balancing; E.DeRose, B.Knopsnyder, B.Rawal; https://www.avx.com/docs/techinfo/whitepapers/AVX-Whitepaper-Reliability-of-SuperCapacitors-Paper1.pdf
[5] Reliability of Supercapacitors: Long-Term Reliability Test Data; E.DeRose, B.Knopsnyder, B.Rawal; https://www.avx.com/docs/techinfo/whitepapers/AVX-Whitepaper-Reliability-of-SuperCapacitors-Paper2.pdf
