Properly oversizing voltage rating of Tantalum capacitors. - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: April 06, 2015, 01:14:59 pm
content: Just for a sanity check, for sizing the voltage rating for capacitors it's a typical rule of thumb to have their rated voltage to be 50% higher than the nominal. So, for a regulated 5V rail I should need the caps to be rated for at least 7.5V. But it seems the two I have to choose from are 6.3V and 16V rated.I'm using these for small SMD regulators (main logic is 5V with some chips requiring 3.3 and others 1.. I'll have a few regulators on the board so if I can get away with only using the 6.3V rated caps I would be able to reduce a part on my BOM and make a larger price break on the cheaper part.Would a 6.3V rated Tant be too low to be safe on a 5V rail. It just seems like it's skirting it, but then again the 5V tolerant chips have a maximum supply of 5.5V, I should be in the clear, right?

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2015, 01:30:54 pm
content: Provided you take care that the parts have proper ESD handling all the way through there will be little issue, just you want to place some 0R jumpers in the traces for the capacitors so that you add some series resistance to them. On the output of the 5V regulator and for the 3V3 regulator ( fed from the 5v rail) you can use the 6V3 parts direct. The jumpers are there to increase the series resistance slightly, so that ringing on the rail does not kill the capacitors. They will not be needed on the 3v3 or 1V rails as there the regulator will do the control, but on the main board on the 5V rail, fed in via a longish wire from an external 5V supply they will be needed unless your 5V supply is not removable with the power on under any circumstance.

author: wraper
author_rank: Unknown
timestamp: April 06, 2015, 01:46:33 pm
content: For tantalum capacitors to be safe, rated voltage should be at least 2x of the actual voltage applied. For best reliability 3x ratio is recommended.

author: wraper
author_rank: Unknown
timestamp: April 06, 2015, 01:50:39 pm
content: Here are derating rules from AVX:http://www.avx.com/docs/techinfo/voltaged.pdfQuote

author: tszaboo
author_rank: Super Contributor
timestamp: April 06, 2015, 02:12:35 pm
content: Just dont use tantalum. There are specialized electrolytic caps with better characteristics, which dont supply the war machine.

author: wraper
author_rank: Unknown
timestamp: April 06, 2015, 02:51:35 pm
content: Quote from: NANDBlog on April 06, 2015, 02:12:35 pmGuess then we should not solder with the tin based alloys and never use PCBs with ENIG and of course use only uncoated brass/bronze connectorsQuoteBTW where goes that oil pumped from ISIL controlled territories?

author: Pack34
author_rank: Frequent Contributor
timestamp: April 06, 2015, 02:52:59 pm
content: Quote from: SeanB on April 06, 2015, 01:30:54 pmThe power supply is not removeable. I have an on-board switching regulator rated from 7.5V - 28V. I'm going to need to rev the design anyway since the buck I'm using has since gone EOL. I'll add the 0R jumpers for the input caps on the 5V rail.Quote from: AcHmed99 on April 06, 2015, 02:01:11 pmThe datasheet for the 6.3V tant is found below. As per the datasheet (page 14), the recommend maximum application voltage at the high end temperature is 50% at lower temperatures so it's looking like my currently speced cap will not work reliably enough on the 5V rail. I am also concerned it drops to 33% on the high temperature end. I think for the initial run I"m just going to move to the 16V rated ones for both just to be sure.http://www.kemet.com/Lists/ProductCatalog/Attachments/254/KEM_T2005_T491.pdfQuote from: NANDBlog on April 06, 2015, 02:12:35 pmThe main issue I have with that is the design will be opperating in a high vibration environment. I'd like to keep it all SMD as much as possible. The tants are light and have a low profile so I don't have to worry about them. Then there's also the physical board space.Even though I'm saying this, I am most likely going to be switching from aluminum to electrolytic for the new buck I'm going to have to throw in there. The PnP machine I just got isn't going to handle the caps well and I don't like how much those aluminum ones run.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2015, 02:57:45 pm
content: What about ceramic parts, even with the horrid voltage derating you probably will come out ahead with 2 or 3 in parallel instead. 25V parts will do for everything depending on the value, and you probably do have space for some more if needed. plus no polarity and you can glue them down for vibration.

author: Niklas
author_rank: Frequent Contributor
timestamp: April 06, 2015, 04:20:32 pm
content: The 50% voltage derating comes from possible buildup of cracks during the reflow soldering. Also watch the inrush current, especially in a system that can powered up when hot (car, short stop at gas station...). I did a on-off abuse test in our lab with max input voltage switched by a relay at about 1 Hz and 100'C. The result was quite explosive for a 16V rated tantalum cap charged to 12V, just as expected.Murata has a similar derating chart for ceramic capacitors in their MLCC catalogue.

author: nctnico
author_rank: Super Contributor
timestamp: April 06, 2015, 08:56:10 pm
content: Quote from: NANDBlog on April 06, 2015, 02:12:35 pmI agree. Besides voltage rating you also need to control the current slew rate (di/dt) when using tantalum caps otherwise they may ignite. They really are crappy components if you know all the details. An MLCC capacitor is much better in every way.

author: tszaboo
author_rank: Super Contributor
timestamp: April 06, 2015, 11:02:11 pm
content: Quote from: AcHmed99 on April 06, 2015, 10:30:54 pmYou surely cannot campare the Canadian armed forces with the local warlord's gun for hire and children with Kalashnikov from Congo. 5 million died because of materials like tantalum and gold. There is blood on it. But I think it is enough world politics, everyone can decide for themselves if they can live with it or not.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2015, 05:15:39 am
content: The advantage of tantalum is you don't need an outboard ESR resistor.MLCCs and polymers frequently do.You need to go to some lengths to have a critically- or over-damped power supply network, when using very low ESR bulk caps (which look like dead shorts), when connected (via trace inductance) to discrete bypass caps (of modest size and therefore potentially resonant with the trace inductances).Regulators (of the switching or linear variety) are frequently unstable with such loads, and choices must be carefully scrutinized before going and building a design.Tim

author: Dago
author_rank: Frequent Contributor
timestamp: April 07, 2015, 06:17:36 am
content: Quote from: T3sl4co1l on April 07, 2015, 05:15:39 amModern regulators are stable just fine with ceramic caps, just don't use old crappy regulators designed for some 1 ohm electrolytic caps. There are very few (if any) applications where you should use tantalum capacitors in TYOOL 2015.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2015, 07:15:49 am
content: Quote from: Dago on April 07, 2015, 06:17:36 amIndeed!Alas, until the likes of 2N2222, 2N3055, LM741, NE555, MC34063 and many others of their vintage, have finally evaporated to the etherworld, I shan't dare leave off such advice.The most insidious part is, the oldest, and often times worst, parts are also the cheapest, sometimes by a disturbing margin. Basic example: MOSFET, IRF840. I pick this example, because I've cracked open some transistors in this range before. It has probably triple the die area of new parts of similar rating, but costs about the same. (I see there are a couple parts from AOS, Rohm, Fairchild, even On Semi, that are just a little cheaper, but many cost more.) The best performing device seems to be STD11N50M2, by a factor of about 4.5 (weighing Rds(on), Qg and price@qty=1000 all linearly). But if cost (and often, availability) wins out above all others, you'll almost always wind up with old, crusty choices.Not to mention the vast reserve of dated knowledge, that we must always, actively, teach over. New stuff is always coming, and newbies may not even be aware of it.Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 07, 2015, 04:53:40 pm
content: If derated for voltage and if you have a certain number of ohms/volt in the circuit, tantalums can be very reliable. I use them where I need a moderately large capacitance and want longer life than small electrolytics. OTOH, if every tantalum cap disappeared from the face of the earth, I wouldn't feel very deprived. There are other, often better, choices. As for new stuff, it's hard to keep up. Even people who've been in the business for decades aren't always aware of what's new and will tend to use what they know. Guilty here, anyway!

author: Howardlong
author_rank: Super Contributor
timestamp: April 07, 2015, 05:28:31 pm
content: TDK state that MLCCs need not be derated for voltage stress (as opposed to the DC bias effect on capacitance).http://product.tdk.com/capacitor/mlcc/en/faq/pdf/20_mlcc_voltage_strength.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2015, 11:51:39 pm
content: MLCCs typically fail somewhere in the 100s to 1000s of volts range (depending somewhat on dielectric thickness for very low voltage parts, but mostly on creepage distance along the body for anything higher). They're hardly useful of course, as the energy storage goes to crap once the dielectric (type 2) is saturated.C0Gs seem to keep on going, so can be used well over their ratings (if you don't need long-term reliability or production quality). They have about triple the energy density (at ratings) of any other common "high speed" type (i.e., including electrolytics and film, but excluding supercaps and batteries), but they aren't available in very large packages, not to mention they're horrendously expensive.Tim

author: Rupunzell
author_rank: Frequent Contributor
timestamp: April 08, 2015, 02:37:03 am
content: Friends don't allow their friends to use tantalum capacitors.When tantalum capacitors are used locations there they can be subjected to peak currents, they can fail by shorting which can easily bring down the power supply. When the power supply dies, the entire system often dies too.There are a host of FAR better alternatives today from low ESR 'lyrics, polymers to stacked film to TDK "Mega Cap" monolithic caps are often better choices tantalum caps.If there is concern about mechanical stress, some monolithic caps offer "soft terminations" which will help with reducing the risk of termination fractures. Before ROHS "lead free" solder was in wide usage, tin-lead alloy solders offered a degree of shock and impact resistance. That is no longer true once the solder alloys went to lead free.BerniceQuote from: nctnico on April 06, 2015, 08:56:10 pm

author: BradC
author_rank: Super Contributor
timestamp: April 08, 2015, 02:52:30 am
content: Quote from: nctnico on April 06, 2015, 08:56:10 pmI used to think that, but almost every repair I've had in recent history has been a failed MLCC, whereas the Tants are just fine. I'm even seeing more MLCC failures than Electros.

author: Monkeh
author_rank: Super Contributor
timestamp: April 08, 2015, 03:47:08 am
content: Quote from: NANDBlog on April 06, 2015, 11:02:11 pmIf you object so much I suggest you get off the internet, dispose of your computer, get off the grid, don't buy goods from shops which use technology...Wait, let's simplify that: Become Amish.

author: kjs
author_rank: Regular Contributor
timestamp: April 08, 2015, 04:25:05 am
content: Quote from: BradC on April 08, 2015, 02:52:30 amThat all depends what MLCC's you use. We meanwhile only use Taiyo Yuden, Murata, AVX and Samsung (all X5R) for 1uF+ ceramic caps with a minimum of factor 2 of voltage derating. We had horrible experience with other manufacturers during evaluation/approval of the parts. Never had any issue with the approved vendors despite lead free solder. That's the next issue: Sn96.5/Ag3/Cu0.5 seems to be more gentle on the components than the other lead free options.For tantalum we need special approval for good reasons. Unless there's no other way to get it done they aren't allowed. None of the voltage regulators we use have issues with ceramic capacitors.

author: Pjotr
author_rank: Frequent Contributor
timestamp: April 08, 2015, 11:24:35 am
content: Nothing wrong with tants. Most critical is not to go over their dV/dT rating. Apart from reverse voltage too high dV/dT is the most common failure mechanism and gave them their bad name in power supply usage. Although this was well known it was often overlooked those days by engineers. And 20 years ago they were indeed not that reliable. But modern ones have improved a lot. Just keep them them within the limits as stated in the datasheet and they last very long.MLCC's have the disadvantage of voltage ageing and voltage dependence you have to calculate in. SMD tants just give you the most uF for the size but the newer polymer aluminium (and hybrids thereof) are more and more taking over.