As we’ve got seen in half I and half II of this sequence [1], there aren’t any inherent wear-out mechanisms in ceramic capacitors. And sure, as lined partially II, the excessive dielectric Class 2 capacitors (X7R) can lose upwards of 85% of their capacitance with use on account of utilized voltage and getting older, however even then, they don’t catastrophically fail.
Ceramic capacitors are very helpful, besides the place values bigger than the micro-farad area are required, so for bulk decoupling purposes, different varieties are wanted. Right here, we have a look at the alternate options.
Strong tantalum capacitors
Strong tantalum capacitors are broadly utilized in electronics and are fairly helpful due to their excessive capacitance and reasonably high-rated voltages of 75 VDC in frequent SMT packages. Tantalum capacitors shouldn’t have a wear-out mechanism per se, however they do have a potential catastrophic failure mechanism. When pressured underneath excessive surge voltages or currents, tantalum capacitors can fail as a brief circuit. Relying on the supply impedance, this may occasionally result in a burnt-up tantalum capacitor in your board, maybe taking your board with it.
Over the past 50 years, a 100% prevention of this drawback has escaped capacitor producers nevertheless, the final 20 years of testing exhibits that, though that is much less of a problem now, the issue stays [2].
The previous 1960’s army/NASA have tips {that a} resistor of round 3 Ω per volt must be positioned in sequence with any tantalum capacitor that’s utilized in ultra-reliable purposes like spaceflight. Just lately, the rule has been modified to round 1 Ω per volt or by limiting the present of the circuit. This, nevertheless, defeated one of many most important causes to make use of a tantalum capacitor within the first place—the low equal sequence resistance (ESR) for energy line filtering. Therefore, I’ve by no means seen anybody do that in precise “industrial” observe. Ample derating or surge limiting by design has been used as an alternative [3].
It has been discovered that not utilizing tantalum capacitors in excessive surge areas and derating the working voltage by no less than 50% yields a really dependable utilization state of affairs [2].
In different phrases, don’t use tantalums on the enter facet of a hot-plugged PCB card the place the voltage and present surge might be excessive. Alternatively, utilization on the output of a DC/DC converter is normally advantageous as a result of the comparatively gradual, and presumably present restricted turn-on conduct, limits the utmost potential voltage/present surge. One caveat could also be a non-isolated enhance converter, so think twice about your precise circuit configuration.
To mitigate the catastrophic nature of a shorted capacitor, producers have devised fusible hyperlinks which can be included in some kinds of tantalum capacitors, and for high-reliability purposes, they use surge present testing to qualify both batches of capacitors and even particular person items.
In different purposes the place the voltage/present surge is proscribed, resembling utilizing the tantalum in a frequency shaping community of a gradual management loop the place there may be normally a big sequence resistor of 100’s to 1000’s of ohms to make a pole/zero pair, the potential surge is of course restricted to a really low worth. In these instances, utilizing the next ratio of working voltage to derated voltage is feasible with excessive reliability.
As with all “giant worth” capacitors, tantalums have a tendency to seek out use in switching energy provide smoothing filter purposes. In these purposes, the root-mean-square (RMS) ripple present could be giant and, because of the tantalum’s increased ESR in comparison with a ceramic capacitor, there’s a actual hazard of damaging the capacitor on account of self-heating. Be sure you use a tantalum that’s rated for RMS present in these purposes and derate as per business, or your in-house, necessities [3].
Observe that the utmost rated voltage and the allowable RMS ripple present is normally derated with growing case temperature. There can also be an RMS ripple present and/or ripple voltage derating with frequency, so make sure you discover and examine an in depth knowledge sheet rigorously.
Most typical industrial tantalum capacitors are rated with a most case working temperature of 85oC. Increased temperature variants can be found as much as 230oC.
In the previous few a long time, varied polymer and different supplies have been more and more used to make tantalum-type capacitors. These varieties could also be higher in sure reliability features, resembling higher resistance to surge voltage failures and/or failure in an open circuit situation. Some corporations even enable for the next ratio of working voltage to rated voltage when utilizing such elements. Be sure you verify your organization’s coverage or producer’s software notes when utilizing these “newer” tantalum elements.
Lastly, molded tantalum capacitors are usually very tolerant to moisture, though once you push the utmost capacitance versus case dimension, you’ll begin to see moisture sensitivity ranges rise, it is a crucial concern with the fashionable high-temperature, lead-free soldering processes.
The one widespread reliability points with tantalum capacitors that I’ve seen are when there’s a world provide scarcity. In these cases, the prevalence of counterfeit capacitors enormously will increase. The one protected refuge from these points is to stay with manufacturer-certified distributors and provide chains.
Strong tantalum takeaway
Usually, strong tantalum capacitors are probably the most dependable of the electrolytic varieties (Determine 1). They’ve predictable efficiency, wonderful low-temperature efficiency, lengthy storage life, and when used appropriately, a protracted service life.
Determine 1 This impedance comparability of a basic objective 10µF/10V tantalum (orange curve) versus a 2.2µF/10V ceramic capacitor (blue curve) is what retains me coming again to tantalums repeatedly. That good “low Q”, “non-ringy” impedance over a number of a long time is simply “music to the eyes” of an analog engineer.
Aluminum electrolytic capacitors
Aluminum electrolytic varieties are the popular part the place very giant capacitance values are wanted. They’re ubiquitous in almost each digital machine the place absolute dimension shouldn’t be the primary design issue. Aluminum electrolytic varieties can be found with a big rated voltage on the order of a number of lots of of volts and large capacitance values on the order of hundreds of microfarads. Specialised forms of “tremendous capacitors” can be found with capacitance values exceeding a complete Farad, these are primarily used as battery substitutes, not bulk decoupling.
Aluminum electrolytic capacitors are the primary alternative in excessive ripple present purposes, like switching energy provides. They will have very low ESR values and may help many amps of RMS ripple present. The ripple present score is normally derated over temperature and frequency, so make sure you verify the detailed specs of the information sheet rigorously to your precise software, as not all producers use the identical score/derating methodology or terminology (Desk 1). ESR values on the order of tens of milliohms are generally obtainable now, which is a switching energy provide designer’s dream.
Desk 1 Typical aluminum electrolytic derating multipliers. These might be completely different for various producer’s merchandise and meant software. As an illustration, capacitors meant to be used after mains operated bridge rectifiers will usually reference the utmost RMS ripple present multiplier to 120 Hz as an alternative of the 200 kHz listed right here.
Aluminum electrolytic capacitors do have an anticipated lifetime, even when used with correct derating. Since they’re usually made with a liquid electrolyte, they may dry out over time and lose capacitance and the ESR will improve. There are lots of high-reliability varieties obtainable with outlined lifetimes of as much as a number of tens of hundreds of hours, and in case your requirement is for a long-lived product, I might advocate trying into these.
There are additionally newer polymer or natural varieties that promise decreased ESR and elevated ripple present functionality as proven in Determine 2.
Determine 2 Specialised aluminum electrolytics can be found with very low ESR values now. An ultra-low ESR comes with the benefits of increased ripple present functionality, however with the drawback of a “high-Q” impedance profile (as proven) that may trigger ringing in switching energy provide purposes.
Usually, warmth is the large killer of aluminum electrolytic capacitors as a result of warmth will are inclined to evaporate the electrolytic fluid within the capacitor, so ample temperature derating is really helpful together with ensuring to maintain self-heating with any RMS ripple present managed. Most varieties are rated to both -25 oC to 85 oC or, -55 oC to 105oC. The electrolyte tends to freeze at decrease temperatures, inflicting a increase in ESR and limiting low-temperature efficiency. Specialty varieties can be found with a -55 oC to 150 oC score.
For optimum life, voltage derating is normally really helpful to be beneath 80% of rated most voltage, and RMS present is generally derated to beneath 75% most with a restrict of 10 oC rise within the capacitor on account of self-heating [4][5]. Some corporations additionally specify a minimal working voltage of 20% of the rated most to maintain the oxide layer correctly fashioned (see beneath).
Due to the massive bodily dimension of most aluminum electrolytic’s, mounting points additionally exasperate failures, for the reason that first level of failure in any moist aluminum electrolyte is the leakage of the electrolyte and the one actual path of the electrolyte is thru the rubber finish seal, it’s crucial that each one precautions be taken to not disturb this rubber seal throughout meeting or use.
The rubber seal could be broken by bending or pulling the leads throughout product meeting. This could occur if the lead area isn’t precisely proper on a through-hole half, or by utilizing the capacitor bent over on a PCB and never supporting the leads throughout the bending operation. Determine 3 exhibits two methods to shortly shorten the lifetime of an in any other case well-designed aluminum electrolytic capacitor software.
Determine 3 Aluminum electrolytic capacitors could be broken by making use of stress to the leads within the rubber finish seal, in the end inflicting electrolyte leakage. Injury could happen in case you bend the leads flat in opposition to the physique when mounting the capacitor laying down on a PCB (left), or in case you mount the capacitor right into a footprint with the mistaken lead spacing (proper). Be sure you help the leads in a fixture or with pliers earlier than making use of ANY bending motion on the leads.
One other mounting challenge is when a big can-type electrolytic sits on a PCB. Even delivery the PCB from one location to a different could cause the capacitor to vibrate and work-harden the leads, resulting in surprisingly early lead breakage or rubber seal harm. Every time you’ve gotten the chance to have a number of tall aluminum electrolytics collectively on a PCB, you’ll be able to mount them at 90-degree angles to one another after which glue all of them along with a dab of versatile adhesive (Determine 4). Something you are able to do to assist hold the capacitor from vibrating undamped will stop the leads from work-hardening and failing.
Determine 4 Don’t neglect to make use of some adhesive on any tall aluminum electrolytic capacitors to forestall vibration harm. Making use of a versatile adhesive to teams of capacitors (left) and even on a single capacitor and a few close-by elements (proper) can considerably cut back the survivability of your product.
After I labored at Hewlett-Packard, we used to shake/vibration check each instrument throughout the design section and ALL aluminum electroytics would simply rattle off the boards even throughout the low shipping-level vibration exams. Therefore, we simply acquired used to offering mechanical help for these elements in each product on the design stage. Don’t ignore this necessary but low-cost method to instantly enhance the reliability of your merchandise. Higher but, in case you don’t have vibration testing as a part of your product qualification course of, begin it as quickly as potential.
Aluminum electrolytic cleansing
An necessary issue within the care and feeding of aluminum electrolytic capacitors is cleansing. The rubber finish seals are notably delicate to chlorinated or petrol-based cleansing solvents. This isn’t as a lot a problem because it was 30 years in the past when Freon-based degreasers have been used to wash boards, however it may well nonetheless be an issue throughout touch-up or rework in case you are not cautious. The worst half is the failure from contamination won’t present up instantly, however within the discipline. Acceptable use of water and isopropyl alcohol are usually authorised cleansing solvents. As at all times, comply with the capacitor producer’s suggestions.
Aluminum Electrolytic Lifetime & Storage
I’ve many digital devices which can be over thirty years previous and all of them use electrolytic capacitors all through, and these devices nonetheless operate advantageous. I’ve radios which can be 45 years previous, and so they all work advantageous.
I’ve additionally seen industrial and client objects utilizing off-brand capacitors fail in only a few years, though not normally catastrophically. A couple of years again I walked right into a Telcom room and checked out a rack of only a few years previous line playing cards, you may see that each one of many off-brand aluminum electrolytic capacitors on these boards confirmed indicators of ‘bulging’ indicating an overheated situation and electrolyte loss. If there ever was any fact to the saying: “You get what you pay for.” This was it.
Lastly, there’s a potential storage life challenge. After sitting unused, the oxide layer contained in the capacitor can degrade, inflicting a catastrophic improve in leakage present if turned on instantly with a low-impedance supply. This could both blow the mains fuses or harm the capacitor. The method of “re-forming” the oxide layer is required to forestall such catastrophic failures. Reforming is completed by following the producer’s advice. Determine 5 exhibits the suggestions from Vishay / BC Parts on storage and reforming [5].
Determine 5 Vishay / BC Parts’ typical storage life and reforming suggestions. Be sure you verify your producer’s precise suggestions. Supply: Vishay / BC Parts
We must always observe that as with surge failures in tantalum capacitors, storage issues in aluminum Electrolytic capacitors, in my expertise, are extra of a random incidence that normally impacts some, however not essentially all of a given inhabitants. This explains why some items of apparatus can run intermittently for 40 years or extra with none points, whereas others can have a failure after a brief interval of storage. In my expertise, you might be simply as prone to see a failure on account of lack of capacitance due to lack of electrolyte as you might be to see a failure because of the oxide layer disappearing in saved gear.
Figuring out about this challenge and having seen it firsthand, I ensure that to show my least used lab devices on no less than annually for a few hours, simply to maintain the capacitors fashioned.
Simply because the “Nice Ceramic Capacitor Scarcity of 2017” brought on a whole lot of manufacturing points for finish customers on account of half substitutions each recognized and unknown, infrequently a “Capacitor Plague” hits the aluminum electrolytic market. The final one was within the 2002 to 2005 timeframe [6]. All you are able to do to guard your self and your merchandise is to make use of respected producers who’ve secure manufacturing bases.
Aluminum Electrolytic Takeaway
I make measuring devices, and these have very lengthy anticipated lifetimes. Therefore, I’ve at all times derated my capacitors appropriately, and have used the 105oC / very long-life aluminum electrolytic kinds of recognized, respected producers, however particularly in switching energy provide purposes the place the ripple present is giant. Correct use of aluminum electrolytic capacitors can measure into many a long time of years of helpful, and business confirmed finish product lifetimes.
Bonus–capacitor paralleling suggestions
When paralleling capacitors to get a bigger ripple present score, you need to watch out that the present is balanced within the parallel capacitors to forestall selective heating and presumably early failure in one of many parallel capacitors. The primary reason behind present imbalance could be hint resistance or inductance. That is very true when making an attempt to steadiness ultra-low ESR capacitors, i.e., varieties which can be within the tens of milliohms.
The second challenge is that the ESR between any set of capacitors could also be as a lot as 2:1 from one other capacitor, particularly as they age, inflicting a ripple present imbalance between the capacitors in a parallel group (Determine 6).
Determine 6 Paralleling capacitors to extend the ripple present functionality can undergo from many issues that end in uneven present sharing between the capacitors. The ESR of the capacitors can range as a lot as 2:1 even in new capacitors, inflicting a current-sharing imbalance. In ultra-low ESR capacitors, the wiring resistance or impedance is usually a supply of general present imbalance if not rigorously thought out prematurely.
The third challenge is mounting within the closing product. I’ve seen particular person elements buried in giant teams of elements, but supposedly in a cooling airflow that was successfully blocked from that cooling air due to the upwind elements in the best way. This can trigger hotter areas relying on location. That is very true for a stack of enormous capacitors subsequent to one another. The surest method to take care of that is to be sure that the temperature rise is minimized within the capacitors within the first place.
In different phrases, in case you begin with two capacitors rated at 1 amp RMS ripple present and mix them, you’ll be able to’t anticipate to make a parallel mixture that may deal with 2 amps RMS present. Given a 2:1 ESR imbalance between many new capacitors, you’ll be able to at greatest charge the parallel mixture at 1.5 amps RMS present. Then you definitely even have to contemplate the wiring and lifelong ESR results into your calculations [4][5].
When paralleling capacitors, watch out to totally analyze the ensuing circuit and have in mind the circuit wiring, the inherent mismatch between the capacitor’s preliminary ESR, lifetime anticipated ESR, and any potential working temperature variations on account of mounting within the closing product. A thermal digicam used when qualifying any new product can assist discover these points earlier than they change into issues within the discipline.
References:
[1] Earlier assessment of Ceramic Capacitors,
[2] Tomáš Zedníček, AVX Company, “Voltage Derating Guidelines for Strong Tantalum and Niobium Capacitors”, https://www.kyocera-avx.com/docs/techinfo/VoltageDeratingRulesforSolidTantalumandNiobiumCapacitors.pdf
[3] David Mattingly, AVX Company, “Growing Reliability Of SMD Tantalum Capacitors In Low Impedance Functions”, 1994.
[3] A complete derating information: “IPC-9592 Efficiency Parameters for Energy Conversion Gadgets”, https://store.ipc.org/document-numbers/ipc-9592
[4] This previous Sprague catalog is about nearly as good an evidence of correct utilization and pitfalls of utilizing Aluminum Electrolytic as you will discover, https://archive.org/particulars/bitsavers_spragueSprors1990_119953213/web page/270/mode/2up
[5] Vishay BC elements Product Catalog, “Introduction, Primary Ideas, and Definitions: Aluminum Electrolytic Capacitors”, Doc Quantity: 28356. https://www.vishay.com/docs/28356/alucapsintrobcc.pdf
[6] The Aluminum Electrolytic Capacitor Plague of 2002-2005, https://en.wikipedia.org/wiki/Capacitor_plague
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