Next-gen high-voltage low-ESR tantalum capacitorsNext-gen high-voltage low-ESR tantalum capacitors

Next-gen high-voltage low-ESR tantalum capacitors

Rated voltages of 100 V and beyond are possible with processes that allow uniformly distributed higher breakdown voltages

BY J. PETRŽÍLEK, T. ZEDNÍCEK
M. UHER, I. HORÁcCEK
J. TOMÁŠKO, and L. DJEBARA
AVX Czech Republic

Tantalum capacitors designed for high-voltage applications (above 25 V) have been used for many years in telecommunication, industrial, automotive, and other high-reliability applications. Conventional high-voltage tantalum capacitor design uses a manganese dioxide cathode that provides good reliability, stability, and robustness.

Nevertheless, there are limitations. The first is the operating voltage. Even with optimized processes of dielectric formation, rated voltages of such parts are mostly limited to 50 or 63 V.

A second limitation is equivalent series resistance (ESR). Very low ESR values have been achieved by using special anode shape designs such as multianodes, or fluted anodes, but further ESR decrease is limited by low conductivity of manganese dioxide (MnO2 ). Also a thermal runaway failure mode of the conventional MnO2 tantalum capacitors is a concern for some surge-current-intensive circuits. Thus higher voltage derating of 50% minimum is recommended in surge current intensive circuits to further limit maximum operating voltage of conventional tantalum capacitors.

Conductive polymer cathode material has been proved to provide a solution that addresses ESR reduction and reduces ignition failure mode. Nevertheless, until recently, the working voltage of tantalum conductive polymer capacitors was limited to approximately 20 V due to the maximum achievable breakdown voltage of such capacitors. Reasons for this limitation have been discussed [1], where authors suggested a hypothesis based on the reduction of effective dielectric thickness. Cracks in the dielectric can be filled by in situ produced conductive polymer that makes the final thickness of dielectric, and also the breakdown voltage, lower.

Another referenced paper [2] explains that based on the metal-insulator-semiconductor theory, where the metal is the tantalum anode, the insulator tantalum pentoxide, and the semiconductor is a conductive polymer (such as PEDOT). The authors anticipate diode-like behavior. A potential energy barrier that increases with applied voltage suppresses the current flow in an ideal state and at normal polarity. The authors also suggested that, in the case of in situ polymerized capacitor, the potential barrier at the interface between dielectric and PEDOT is deteriorated by a charge deposited on the dielectric during polymerization. This leads to elevated direct current leakage (DCL) and lower breakdown voltage (BDV).

The latest development on polymer materials and introduction of dispersed pre-polymerized intrinsically conductive polymer [1,3,4], has successfully addressed the issues with low BDV and high DCL. New application techniques, completely different compared to in situ polymerization had to also be developed in order to utilize the potentials of the new materials.

Testing results of tantalum polymer capacitors made by the advanced polymerization process can be found in reference [5]. Besides that no polymerization reactions take place during polymer application, there are fewer thermal stresses compared to MnO2 processes where high temperature is used during formation of the MnO2 electrode. As the result, the new polymer technology can offer not only low ESR and reduced ignition failure mode benefits, but also higher working voltages compared to the conventional MnO2 technologies [6]. Due to the nature of polymer capacitors surge robustness and reduced ignition failure mode, a lower derating of 20% can be used. This significantly widens the working range of tantalum capacitors to new applications such as telecommunications, LED TVs, notebook power supplies, industrial applications, etc., using higher rail voltages.

This article presents the potential and roadmap of the next generation tantalum polymer capacitors expanding their capability toward the high voltage, ultra-low ESR, opening a way for new designs with improved power capability, smaller package, higher output, and safer designs.

Ultra-low-ESR high-voltage solution

Besides the conductivity of cathode material, keeping interface resistivity as low as possible, and optimized anode design, the overall surface area of a tantalum capacitor anode is one of the most important parameters. In particular, as surface-to-volume ratio defines the capacitor’s ESR value, the higher the overall surface area, the lower the ESR [7].

The high-voltage conductive polymer series was introduced and a special focus was paid to ESR minimization. The development effort yielded a brand new category of low-ESR high-rated-voltage tantalum capacitors. Frequency characteristics of E (7343-43) case 22-µF 35-V polymer multianode capacitor (see Fig. 1 ), the ESR value at 100 kHz is approximately 15 mΩ.

Such low ESR in combination with high capacitance and small case size has not been achievable by any other technology so far. This may enable a new generation of power supplies with the same power within small dimensions or higher power in the same design as it is currently available.

Fig. 1: ESR median vs. frequency E 22 µF/35-V multianode polymer tantalum capacitors.

Increasing the limits of working voltage

The first 50-V-rated polymer capacitors have launched recently. Nevertheless, there are studies in process to evaluate the potential of the new polymer technology for development at even higher voltage ratings. The 3.3-µF 50-V conductive polymer single anodes molded in Y (7343-20) case were tested under accelerated conditions. Breakdown voltage of such capacitors in evaluation measurement was in a high range from 130 to 180 V.

Life tests were carried at elevated temperatures (105° or 125°C) and voltages 50, 75, or 100 V (at 105°C), or 33, 49.5, or 66 V (at 125°C) on 25 samples of capacitors from every group. Measurements at 50, 75, or 100 V of electrical parameters were recorded after 500, 1,000, and 2,000 hours. DCL distributions are presented in Figs. 2 and 3 .

Fig. 2: DCL leakage of 3.3- µF 50-V polymer parts after storage at 105°C and voltage 50, 75, or 100 V.

Fig. 3: DCL leakage of 3.3F 50V polymer parts after storage at 125°C and voltage 33, 49.5, or 66 V.

Values of DCL dropped during the first 500 hours, then slightly grew during the next 500 hours, but no further significant increase was observed during the consequent 1,000 hours. No failures were observed during the testing, and the DCL shift was significantly below the limit calculated on the basis of rules commonly used for tantalum polymer capacitors.

Higher spread of DCL values after 2,000 hours at 125°C, 66 V and measured at 100 V is probably connected with wider breakdown voltage distribution of original parts. Thus, under such extreme conditions, parts with lower breakdown voltage exhibit higher DCL. It is possible to make conclusions based on the life and breakdown voltage data that the tested parts could be rated as 50-, 63- , or even 75-V parts. High current surges were applied on 100 parts at 95 V and low-impedance circuit where peak current recorded reached levels of 200 A in order to test the surge robustness of the parts. None of the tested capacitors exhibited failure.

Multianode capacitors based on three 3.3-µF / 50-V rated anodes were assembled and tested consequently. Results of frequency characteristic of ESR at 100 kHz are presented in Fig. 4 . Furthermore temperature dependency of ESR at 100 kHz of multianode parts E (7343-43) case 22 µF 35 V and 10-µF 50-V polymer and manganized capacitors are compared in Fig. 5 . Very low ESR is typical for broad scale of temperatures in comparison with standard manganized product.

Fig. 4: ESR median vs. frequency E 10-µF / 50-V multianode polymer tantalum capacitors.

Fig. 5: Temperature characteristics of ESR measured at 100 kHz

Further breakdown voltage increase

Fig. 6: BDV distribution of capacitors 3.3uF prepared by standard and improved processes

The key parameter that reflects potentially improved quality and increase of working voltage is breakdown voltage. Increase and more uniform distribution of breakdown voltage were achieved by modification of anode preparation and conditions of anodization. Such distribution can lead to 100-V-rated units. ■

References

[1] U. Merker, W. Lövenich, K. Wussow, Proceedings of the 20th Passive Components Conference CARTS Europe, 2006 , p. 21-26.

[2] Y. Freeman, W. R. Arrell, I. Luzinov, B. Holman, P. Lessner, Proceedings of the Passive Components Conference CARTS USA, 2009 .

[3] U. Merker, W. Lövenich, K. Wussow, Proceedings of the 21th Passive Components Conference CARTS Europe, 2007 , pp. 293-298.

[4] U. Merker, K. Wussow, W. Lövenich, Proceedings of the 19th Passive Components Conference CARTS Europe, 2005 , p. 30-35.

[5] J. Young, J. Qiu, R. Hahn, Proceedings of the 22th Passive Components Conference CARTS Europe, 2008 , p. 49-60.

[6] J. Young, J. Qiu, R. Hahn, Proceedings of the 30th Passive Components Conference CARTS USA, 2010 , p. 347-364.

[7] I.Horacek, L.Marek, J.Tomasko, T.Zednicek, S.Zednicek, Proceedings of the 18th Passive Components Conference CARTS Europe, 2004 .

About the authors

Tomas Zednicek, Ph.D. (Tantalum Technical Marketing Manager) was awarded a master degree in Electrotechnology by the Technical University of Brno in 1993 and he joint AVX Czech Republic s.r.o. in the same year as quality engineer and from 1994 he worked on positions of failure analysis group head. He is working in technical marketing group since 1999 as worldwide technical marketing manager. In 2000 he received his PhD degree in electrotechnology — tantalum capacitors — from Technical University of Brno. He is an author of more than 50 technical papers and scientific articles about tantalum/niobium capacitors. His two CARTS papers were voted as the best and outstanding papers. AtHe was awarded in CARTS Europe 2005 he recived anby Dr. Zandman award for a great contributions to the passive component industry.

Jan Petrzilek (Conductive Polymer Technology Manager) received an M.Sc. in Organic chemistry in 1990 at the University of Pardubice (CZ) and a Ph.D. in 2002 at the field of chemical technology at the same university. R&D at the field of explosives (12 years at the Research Institute of Industrial Chemistry, Pardubice). He joined AVX in 2004 and is responsible for conductive polymer technology.

Ivan Horacek (Anode Manufacturing Process Engineering section manager) received an M.Sc. in Polymer chemistry in 1983 at the University of Pardubice (CZ) and a Ph.D. in 1992 on the same field of University. After 10 years in R&D at University of Pardubice, he joined AVX in 1997 as Process Engineer and since 1999 as the Anode Manufacturing Process Engineering section manager.

Lotfi Djebara (Conductive Polymer Development Engineer) received his MSc in solid-state physics from the University of Nantes, France. Since 2002, he was awarded a Ph.D. degree from the same university. He started as research and development Engineer for Firadec on polymer tantalum capacitors. Since 2006, he joined AVX and he was appointed as Conductive Polymer Development Engineer.

Miloslav Uher (Process Engineer) was awarded a Ing. (master) degree in Engineering by the Technical University of Brno in 2004 and he joint AVX Czech Republic s.r.o. in the same year as process engineer. He is member of conductive polymer group with focus at developing new products.

Tomasko Jaroslav (Anode Design and Assembly Manager) received his master degree in 1994 by CVUT Prague faculty of electrotechnology. He joined AVX Czech Republic s.r.o. as a section head of process and anode design in 1998 and he has been working on position of section head of anode design and assembly since 2002.