Polarized caps in lieu of non-polarized? - Page 1
Forum section: electronics / beginners

author: eamoex
author_rank: Contributor
timestamp: July 02, 2019, 01:48:35 pm
content: Hi All,I need to add a positive DC — positive DC converter to my circuit. Traco Power's TRS3-1250 datasheet shows a filtering scheme using non-polarized caps (fig. 1 below,Positive output cuircuit). My local supplier only has CMS versions of non-polarized caps for the specified values, and I can't do CMS. Can I use polarized, electrolytic caps here? If yes, where do I connect the negative side of those caps? Thanks for your help.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 02, 2019, 02:31:52 pm
content: The reason they show non-polarized caps, is to obtain such a low ESR value, the only caps that will consistently meet it are MLCC devices. Which are non polarized of course.Having said this, you could also parallel several polarized tantalums and achieve the required capacitance and ESR. this would be perfectly fine,Tantalums a few years ago gained a poor reputation due to the "blood tantalum" controversy. Capacitors which were certified to be "human right abuse-free" had their prices skyrocket. The unfortunate reason is that most of the World's tantalum supply comes from countries with notorious human rights abuses.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2019, 02:45:01 pm
content: The fact they have not shown the polarization marks doesn't mean the caps can't be of polarized type - it's just a generic capacitor symbol.It's not a question of being polarized vs. non-polarized, it's a much broader question to capacitor types and their pros and cons.If you want to follow this appnote blindly, an aluminum polymer electrolytic capacitor might be the most sensible choice for fig1 C1, satisfying the low ESR condition. An MLCC bank would be massive, especially if you try to guarantee even close to the specified capacitance at the actual DC bias - you'd need over 1000uF total of MLCC there!Their PI filter looks like it's simulated (worse), or prototyped (better) with some specific parts, but they fail to give the part numbers. As it stands, it's not very usable. You have a risk of creating ringing or amplifying noise at some frequencies, having no lossy elements in the filter.Don't design in an tantalum unless you know what you are doing. Ignoring the political discussion, they have two massive engineering traps and people seem to consistently overlook at least one even when they think they know the stuff. A tantalum won't work in any of the positions shown anyway.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 02, 2019, 03:36:28 pm
content: I know one of them is that they can easily fail catastrophically;What would be the other one?

author: GerryR
author_rank: Frequent Contributor
timestamp: July 02, 2019, 04:06:31 pm
content: In both figures 1 & 2, the "+" side goes toward pins 2 and 4, "-" side to pin 3. I think that was your original question.(?)

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2019, 04:18:11 pm
content: Yes, tantalum capacitors are known to fail catastrophically, shorting the power supply and burning up, but I've found them to be quite reliable. Solid tantalum capacitors don't dry up like aluminium electrolytics do.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2019, 04:18:20 pm
content: Quote from: schmitt trigger on July 02, 2019, 03:36:28 pmI meant thereasonsfor them to fail catastrophically. If they "just failed", no one would use them. They are reliable if used correctly. They fail only for specific reasons.The reasons are:1) overvoltage; mind the industry-accepted bogus* ratings of voltage (this method of rating is not seen onanyother component type AFAIK) so that the designer needs to read froma separateofficial document that the actual voltage rating is only 50-70% of the datasheet value,thenadd any derating for spikes or reliability.*) Why I call this bogus? A comparison to make the point:* an aluminium elcap datasheet truthfully tells you that if you operate the capacitor at all maximum ratings, the lifetime will be, for example, 2000 hours. A separate appnote tells you how to make this value, which is often already acceptable as is, longer if you need to. You apply normal engineering derating to compensate for any uncertaintyyouhave aboutyourdesign.* the tantalum datasheet just tells you the voltage, capacitance, hopefully ripple current ratings, no information about the lifetime. Then you have to know there's a separate document (which the datasheet does not refer to), which tells you that the voltage ratings on the datasheet are for the operating life of, for example,2 hours, completely impractical for any design. Then they go on and tell you to derate by 40%. An absolute joke. Why do they do this? Tradition, it's been like this for a long time. If any manufacturer started to rate truthfully, their products would seem inferior. Serious customers are not complaining, it's just the right amount of black magic which makes professional design engineers proud of their "secret" information. Yes, I can design with tantalums safely.2) strict current ratings, often preventing the usage as hot-pluggable input capacitors, or as switch mode converter main output capacitors. Even a single pulse event can cause a catastrophic failure.And it indeed tends to happen that when someone knows 1), they forget about 2), or vice versa. And it's always designer's fault, of course.Tantalums are not very suitable for this case, for two reasons:1) They seem to want you to provide low ESR (<0.1 ohm); tantalums exist and are used to providehighESR (~ohms),2) I can see how a DC voltage could be hot plugged to the input of these filters, causing overcurrent damage to the tantalums.Tantalums are usable when you need stable, high ESR values, or large stable capacitance values in low-current (all peaks included) circuits, like after a voltage reference or a small, current-limited linear regulator, especially when they are not stable with MLCC output caps.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 02, 2019, 04:24:05 pm
content: Absolutely agree with you, that the industry's accepted voltage rating is totally BS.Is similar to having a truck rated for two tons, ONLY IF you don't plan to drive it!And if you plan to drive it on the freeway, then it would be rated for a single ton. And if you hit a pothole, then all four wheels become detached!

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2019, 06:31:49 pm
content: To be fair, the voltage ratings for high value ceramic capacitors is also bogus, because the capacitance drops to a fraction of the specified value, at that voltage.Safety ratings are a different story. Safety critical structures typically to double the working load, so a sling designed to carry 500kg shouldn't break until at least 1000kg. A similar thing is also done with insulation, creepage and clearances, between live conductors and the user, where failure can cause electrocution, hence why they're so big.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2019, 06:49:32 pm
content: Quote from: Zero999 on July 02, 2019, 06:31:49 pmI don't think it's the same. The MLCC capacitance vs. voltage curve is a very complex thing that you just need to understand; and it's always part specific. It's a curve you need to look up. As a designer, you need to get rid of this idea that MLCC's have a capacitancevalue, they won't; it isn't a scalar.An MLCC is often used at the rated voltage just fine. For a tantalum, the idea that someone could use it at the rated voltage and expect a few hours of lifetime out of it is just totally absurd. Because they basicallyrequireyou to derate at a fixed at-least-40%, they could just rate them that way to begin with. Yes, different applications need different levels of robustness, but no one ever designs for a hours/days MTBF.For an MLCC, the issue is complex because different applications need different amounts of capacitance, and because it's a multidimensional curve of bias voltage, temperature, and time from reflow. There just isn't a way to fix the issue in a simple manner.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2019, 07:26:44 pm
content: I take your point: at least a ceramic capacitor won't blow up, but it would be easier if they just specified the rated voltage, at the rated capacitance or at least 90% of it.

author: eamoex
author_rank: Contributor
timestamp: July 02, 2019, 07:31:35 pm
content: Okay thanks. Kind of beyond my understanding.To get down to facts... I have this 12V switching power supply and I would like to power another device in parallel which takes 6V (means I could carry one power supply unit to power both devices). I know the 12V supply can deliver up to 10A. The device using 6V draws between .5 and 1.5 Amps. Is it even a good idea to use this transformer like that? And if the configuration is viable, are electrolytic caps good for filtering?

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2019, 08:37:07 pm
content: Quote from: Zero999 on July 02, 2019, 07:26:44 pmLook at the actual data: almost no X7R cap can provide 90% of rated capacitance in almost any practical condition. At least 90% is available at nearly 0V DC bias, at room temp. No one uses the capacitors in such conditions.The reality is dire, you can read on the forums how X7R is good and Y5V is bad, then you design in an X7R without looking at the curves just to find out it loses 80% of the capacitance as well, and well before the rated voltage!I've been thinking about it, how to put it in one number. For example, they could definerated voltageso that the capacitance is at least 50% of the zero-bias capacitance. But then they would likely want to have asurge voltagerating because it could be much much higher.Or, they could rate the capacitance at, say, 80% of the rated voltage, and then rate a capacitance separately at no bias. But theydoexactly that (well, at least the decent guys do; some won't, just don't use their products), in the form of the curve set.In any case, I can't figure out any practical and usable way to turn this into a single capacitance and a single rated voltage number. A bare minimum would require two voltage ratings and two capacitance ratings, and it would suck because a Digikey product engineer wouldn't figure this thing out until maybe year 2030. They could be doing it right now, from the curves, but they don't.So, like with any complex device, we are stuck with a set of curves.Completely different to tantalums, which can be simply rated with capacitance, voltage, current and temperature rating, and the simplicity is proven by the fact that the manufacturers give you a fixed voltage derating factor (typically 0.5 or 0.6), which they basically admit is just a factor between their measurement standards, and the expected reality.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 02, 2019, 08:45:14 pm
content: Quote from: eamoex on July 02, 2019, 07:31:35 pmI wouldn't bother adding the extra filtration unless you actuallyhave identifieda noise problem.If you do end up needing filtration, there is no easy answer, as filtering SMPS noise is a fairly complex subject. The appnote you posted doesn't give enough information IMHO.Often a simple, cheap electrolytic cap does fine. It has many positive aspects, like you can't accidentally create massive voltage spikes with it like you can do with large MLCC's, or it won't accidentally crack and turn into a red hot heater element, like an MLCC can. So it's almost positive that the cheap elcap won't make thingsworse.It may not be enough as is, but worth trying if you have identified an issue. The biggest minus is the instability of the ESR, so even if it seems to be filtering properly, then it might stop filtering at cold weather.

author: eamoex
author_rank: Contributor
timestamp: July 03, 2019, 05:03:16 am
content: This post I understandThanks everyone