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May 5, 2017

Circuit Ocean Tell you When/why (not) to use Tantalum capacitors. Pros, cons, alternatives

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When/why (not) to use Tantalum capacitors. Pros, cons, alternatives

A few weeks ago I worked through the Altium SMPS design course from FEDEVEL academy and that little project left me with an unresolved question. Somehow I got the idea in the past few years that tantalum capacitors are to be avoided, never really knowing why. Yet try to find a 330uF, 6.3V capacitor with ~4mOhm ESR  and you’ll find that tantalum is basically the only option. So I wanted to start a discussion about tantalum caps with the main questions being:

–   When and why to use tantalum capacitors?
–   Why avoid using tantalum capacitors?
–   Alternatives to tantalum capacitors with pros and cons.

I did some quick research already and here’s summary of what I’ve found so far

Why and when to use tantalum capacitors?
Tantalum is used to create small sized capacitors with ‘large’ capacitance. Compared to other materials the oxide layer can be quite thin. So for all applications where pcb space is limited (e.g. mobile phones) they are the to go to type of capacitor when ceramic doesn’t cut it anymore. 

Also tantalum capacitors can be created with quite small ESR. This is why they are used a lot in (local) switched power supplies as bulk capacitor. Ceramics have even lower ESR, but in power supply regulators that might be too low for loop stability.

Why avoid using tantalum capacitors?
From what I can find these are the main reasons to avoid tantalum capacitors in your design:

Financial:
Tantalum capacitors are relatively expensive even when supply and demand are balanced. Not too long ago prices went sky high due to a shortage in raw materials. 

Reliability:
There are numerous reports of tantalum capacitors spontaneously combusting.  I haven’t read into this any further, but I take it some forum members can elaborate on this.

Moral:
Since some of the major tantalum ore (columbite-tantalite / coltan) mines are located in conflict zones (e.g. Democratic Republic of Congo) so one can argue that tantalum is a conflict mineral. I’m not sure though to what extent that still counts today as there are major mines elsewhere in the world too. 
Tantalum is a heavy metal and as such toxic and not really well for the environment. I did find a paper by AVX that discussed ‘green’ tantalum technology though.

Alternatives to tantalum capacitors and pros and cons?
Niobium caps
Apart from being mentioned I haven’t been able to find useful information yet, apart from that there is more Niobium ore available which should make Niobium caps cheaper. On the other hand Coltan is also used for mining Niobium so maybe the conflict argument could apply here too?

(Wet) Aluminum Electrolytic caps (Elco)
Specific low-ESR Elcos can be used. Larger values, larger ripple current rating, larger voltage ratings are some benefits. I must admit though that I realized that I haven’t seen SMT Elcos a lot outside consumer gear. Did I miss something?

Polymer (Solid) Aluminum caps
Used as in- or output bypass caps in SMPSs. Larger values only available in small voltage ratings. I don’t know by how much these must be derated though. Please tune in if you know more about this kind of caps.

Monolytic Ceramic caps
Larger voltage ratings, smaller derating, and higher ripple current ratings are among its benefits compared to tantalum caps. Since the ESR is much lower a small external resistance may be needed for loop stability in SMPS designs. 

Sources 
Here is some of the info I found. There is a lot to be found, and you'd have sift to get the info you want too. I included the year of publishing when available, since a few years in our industry means lots of changes.

 

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My opinion on it is there is still a strong stigma from the 1980's – 1990's where endless bits of test gear succumbed to a tantalum shorting out a supply rail or worse, with the last bit of test gear i repaired having over 30 shorted tantalum's, where they used them for decoupling where some values where low enough that ceramic could have been used. (was a current limited switch-mode that would give a negative leading edge if it was driving too big of a load, which likely resulted in blowing one after the other.)

in that case, almost all of them where replaced with an electrolytic in parallel with a ceramic, (high speed logic) or plain ceramic on its own, and though i haven't gone and tested the transient response of the board, its happily working.

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Tantalams are great. We have used them in our products for over 30 years and to date have NEVER had a field failure of one without an extreme fault otherwise happening to the unit. Yes, its a left over stigma from the days of old. When people would accidental install them backwards in low impedance circuits they will explode with a small fire ball. They actually make great electric matches. I have also NEVER seen one leak when designed with properly. Ye use about 6 on every single one of our boards and have sold from 1500 to 3000 a year of them for most of the last 30 years. For a matter of cost however we limit our use of them to applications under 35 volts and 10 uF. Above that their attributes are a law of diminishing returns anyways.

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Tantalums are basically bombs waiting to go off. Oxidizer + fuel. You'd need to control the rate of the current flowing into them or they may ignite. Reverse mounting is also asking for trouble. They will burn right through the board or explode in your face (both happened to me and it was not my fault). Besides that the raw material is found in a place where nature is to be preserved.

Nowadays there really is no reason to use tantalum caps.

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All electrolytic capacitors are have self forming and self repairing dielectric layers. 

The problem with tantalums is if too much current is allowed to flow through a fault in the dielectric the localised heating can cause more damage not repair. The damage can cause thermal runaway and rapid catastrophic failure. 

For that reason it is unwise to use tantalums in low impedance circuits which can source a lot of current and wise to substantially underrate their voltage if you must. Some manufacturers recommend adding series resistors to limit current which makes their low ESR rather pointless.

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Tantalum is really only used where you need ultra small, and the voltage rating is a thing that never ever will be exceeded, and where you never apply a reverse voltage to them, no matter how briefly. You do need to limit power put in them, but if you derate voltage enough they have a very high current capacity.

I replaced a 1000uF 16V electrolytic on my PC power supply once with a 10uF 25V wet slug tantalum unit, as I needed a capacitor and this was the only one to hand. Worked well for years afterwards, even though the current in the cap was large. the ripple was lower than what the original capacitor had. This was in a LC output filter on the 5V rail, where the original popped it's top.

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