[sdiy] alternative to solid tantalum capacitors

Cooper Sloan mistercooper at gmail.com
Thu Mar 30 03:43:05 CEST 2017


There is now an alternative to solid tantalum capacitors

Recent advances in polymer-cathode technology and improvements in
tightening the tantalum supply-chain are allowing more designers to choose
polymer tantalum capacitors as alternatives to traditional solid tantalum
capacitors, writes Roger Tall

In specifying the ideal capacitor most designers would start with high
volumetric efficiency and then consider miniaturised packages and
capacitance stability over applied voltage, frequency, temperature and time.

A low equivalent series resistance (ESR), noise-free operation, high
ripple-current handling capability, and a decades-long life expectancy
would also feature on this technical wish-list.

Whilst conventional solid, MnO2 tantalum capacitors meet many of these
criteria, the latest polymer tantalums offer significant improvements in
performance.

Capacitance retention is improved at higher frequencies up to 500kHz and
ESR is reduced to as low as 5 mOhms. Polymer tantalums are also rated at up
to 75V and have more relaxed derating guidelines.

The inherent risk of ignition or smoke during a short-circuit failure of a
solid tantalum capacitor has also been eliminated with the polymer
tantalum’s benign failure mode.

*Capacitance stability*

[image: Figure 1]

Figure 1

The capacitance stability achieved by polymer tantalum capacitors exceeds
that of MLCCs over time, temperature and voltage. Whilst MLCCs are
susceptible to aging, polymer tantalums achieve long-term stability over an
operational lifetime of 20 years. This longevity also makes polymer
tantalums a viable alternative to aluminium electrolytic capacitors which
can dry-out in use and may require re-forming after a shelf-life of 2 years
or less.

The lack of a significant change in capacitance when polymer tantalum
capacitors operate at temperatures down to -55°C is a major advantage over
capacitors containing a liquid electrolyte.  However, their upper
temperature limit of 125°C is lower than that of MnO2 capacitors which
include versions which can operate at temperatures of up to 200°C.

*Lower ESR for higher frequency*

A relatively high equivalent series resistance (ESR) has always been a
weakness for conventional MnO2 capacitors which typically have ESR of 40 to
50mOhm. The capacitance roll-off experienced by MnO2 at frequencies above
10kHz has, therefore, limited their use in some applications.  In contrast,
polymer technology can reduce ESR to as low as 5mOhm, which ensures useful
capacitance is available at frequencies of up to 500kHz.

*Capacitance volume vs frequency*

Tantalum is not the only capacitor technology in which polymer is being
used to enhance stability. For frequencies approaching 1MHz, the 2mOhm ESR
achieved by polymer aluminium capacitors can provide slightly higher
capacitance retention without adding to design complexity. This reduced ESR
does, however, have to be traded-off against the lower maximum capacitance
of the polymer aluminium which is just 50% to 80% of a polymer tantalum in
the same package.

For applications in which miniaturisation is a priority this could be
critical. The minimum case size offered by a polymer aluminium capacitor is
7343, compared to 0805 for polymer tantalum. The voltage range of polymer
aluminium capacitors is also capped at 35V whereas polymer tantalums can be
used in applications up to 75V.

*Eliminating ethical constraints*

[image: Fig%202]The selection of tantalum capacitors is not always limited
to electrical performance, but can also be also affected by ethical and
commercial issues. Much of world’s tantalum ore is extracted in the
Democratic Republic of the Congo (DRC) where years of armed conflict have
seen militias using the tantalum and other mines to fund their activities
whilst inflicting severe human rights abuses.

The world’s response to this has been the introduction of US legislation
such as the Congo Conflict Minerals Act of 2009 and a new EU law, announced
in March 2017, which will become effective in 2021. Both of these
regulations require companies to provide the traceability necessary to
ensure that their supply-chains are free from conflict minerals.

Many companies decided that the simplest compliance strategy would be to
avoid any trade with the DRC.

By taking a more radical approach to the eradication of conflict minerals
from the supply-chain, capacitor manufacturer Kemet, has fundamentally
answered the ethical issues surrounding the sourcing of tantalum ore.
Working in partnership with a certified conflict-free mine in the DRC
allows Kemet to ensure that the mine operates in accordance with codes such
as iTSCi and that the revenues are fairly distributed to miners and the
local community.

This not only frees OEMs from the ethical constraints of using tantalum
capacitors, it also helps to eliminate supply risk, price instability and
lead-time fluctuations.

The combination of low ESR with higher voltage, stability and frequency, in
addition to more stable and ethical sourcing of the tantalum ore, means
that designers can now apply the benefits of polymer tantalum technology
across a wider range of applications.
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