Properties of different dielectric material - Page 1
Forum section: electronics / beginners

author: felipebalbi
author_rank: Newbie
timestamp: July 18, 2014, 05:36:50 pm
content: Hi folks,I have always been puzzled as to why different parts of a design require different capacitor types. I have been trying to figure out what's the different behavior that a design relies upon when requiring that we use e.g. an eletrolytic or ceramic capacitor.Is this something to do with the permittivity of the dielectric at different frequencies ? Can anybody point me to a good literature about the physics of dielectric materials ? It doesn't need to be deep into physics, it could be just an overview. I just really want to understand the behavior of different capacitors.I suppose I could hook up different capacitors to a signal generator and oscilloscope to see the behavior by myself. Still, I'd be very interested in a good read about the subject.Thanks

author: lgbeno
author_rank: Frequent Contributor
timestamp: July 18, 2014, 06:11:41 pm
content: You need to remember that there is no such thing as a ideal capacitor. There are all kinds of tradeoffs with the different technology. Total capacitance, accuracy, precision, series resistance, series inductance, parallel resistance (leakage), temperature coefficient of capacitance, voltage coefficient of capacitance, size, cost, reliability, lifespan.This will be a good thread if we list out the different technologies and what they are good for and also suck at...

author: felipebalbi
author_rank: Newbie
timestamp: August 04, 2014, 05:45:17 am
content: Quote from: lgbeno on July 18, 2014, 06:11:41 pmSure, I understand there are several other parameters involved.But, just for the sake of argumentation let's assume we can find the exact capacitance we want available for the same price in every dielectric material currently in use today. Why would e.g. a regulator datasheet explicitly ask us to use ceramic decoupling caps ? Why ceramic ? What's up with the dielectric material that makes it behave better in that scenario then all other choices ? Disregard cost, accuracy, precision, lifespan, reliability, size, and total capacitance.Temperature coeff probably has something to do with it. Series resistance/inductance probably as well. But I still don't get what's up with the dielectric material that makes datasheets mandate them in some designs.

author: emusan
author_rank: Newbie
timestamp: August 04, 2014, 05:57:09 am
content: Quote from: felipebalbi on August 04, 2014, 05:45:17 amYou can't really disregard all of those factors though. Those factors are exactly why these datasheets want specific types of capacitors. I know in the case of regulators the ESR of the capacitor chosen can greatly influence the efficiency of the regulator, among other factors, so they mandate something along the lines of a ceramic cap so that the efficiency you get in your circuit will be as close as possible to their own measured efficiencies.If you take a look at just the ESR alone (http://en.wikipedia.org/wiki/Equivalent_series_resistance) you'll see that there are HUGE (orders of magnitude) differences between different dielectrics. If you choose an aluminum cap over a ceramic for a circuit it could behave wildly different from the expected result. The other parameters can also have pretty huge effects as well, particularly the inductance when dealing with higher-speed systems.EDIT: as far as texts on the subject of dielectrics, I don't know of any off the top of my head right now, but I would guess most decent undergraduate electromagnetics textbooks would get you off to a good start.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2014, 07:23:44 am
content: In an idealized world, all capacitors would be vacuum dielectric, superconductor electrodes.Sounds kind of expensive, doesn't it? What, is that the sound of the real world knocking?Perhaps in a slightly more realistic world, all capacitors would be something like teflon, or polypropylene, or (low loss) ceramic or glass or porcelain. All of which are real materials, that've been used for real capacitors. But still, reality balks: they're too bulky for all purposes, and much too expensive!So it comes down to cost and size.Any cheap plastic can serve as a dielectric, and there are a lot of them out there, but they tend not to use overly cheap plastics. There are probably engineering reasons for that: it's a lot easier to wind a capacitor using very thin film, than molding it directly from a cheap resin. Not all plastics make good films, so that limits the choices a little. Among them, polyester (Mylar, etc.) is one of the cheapest, and doesn't make terrible capacitors, as it happens. So it's a good general choice.But it's not ideal, either. Polyester is still way too bulky for massive energy storage purposes (bulk filtering caps, battery backup, etc.), and it's much too lossy and unstable (the value changes under various conditions such as temperature and aging) for radio frequency tuning or applications demanding high current.So just to use a relatively brief example, there are many kinds because there are many purposes, constrained by performance, size, cost and so forth. Which ones are best for what purpose are covered in various places, or you can keep asking.Tim

author: electrophiliate
author_rank: Regular Contributor
timestamp: August 08, 2014, 12:12:15 pm
content: Quote from: T3sl4co1l on August 04, 2014, 07:23:44 amAh, just imagine the inrush current of a vacuum dielectric, superconducting, supercapacitor!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2014, 01:40:17 pm
content: Wouldn't be that bad actually...Unless you add to that idealized world, "infinite speed of light". But then a whole bunch of things go out the window.I wouldn't think you could get the equivalent transmission line impedance much below 0.1 ohms or so, assuming you could make a micro-layered vacuum capacitor. Ceramics will trump that any day, because k is so much higher.So there you go, there are still plenty of cases where even a vacuum capacitor would be unsuitable!Tim

author: G7PSK
author_rank: Super Contributor
timestamp: August 08, 2014, 02:45:46 pm
content: Why is polypropylene considered expensive it is very widely used for all sorts of things including string. As for SMD type capacitors I expect that ceramic is an easy material to use, never seen how they are made but puff pastry techniques come to mind. Just checked screen printing conductive ink onto rolls of ceramic base then stacked (puff pastry it is) cannot really be a cheaper method of production.

author: mariush
author_rank: Super Contributor
timestamp: August 08, 2014, 02:59:17 pm
content: If you want to learn more about decoupling capacitors (and why ceramic are recommended), there's a four part series on Youtube which is quite informative and explains things quite well:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2014, 03:34:58 pm
content: Quote from: G7PSK on August 08, 2014, 02:45:46 pmPolypropylene film isn't terrifically expensive or anything, but it must be at least a little more expensive, and harder to work with, than polyester (mylar films being common in all sorts of things beyond capacitors). Metallized film capacitors, in any case, require a vacuum metallization step which is simply going to make them more expensive. Although evidently not as expensive as metal foil electrodes.I wouldn't say PP is much more expensive than other films; if you want (or need) to save some HF ripple in a converter, you can throw some in by the transistors; about ten bucks worth is enough for, oh, up to a kilowatt I think.As for ceramic, it's especially well suited to SMT reflow processes. The package is leadless, refractory (not only does it withstand heat, but... what are you going to do wrong, melt it? Have fun with that!), and high capacity. The energy per dollar isn't very good, but the other characteristics trump that for the most part (and anyway, if that's what you need, there are SMT electrolytics).There are "stacked" SMT films available, but they're quite expensive, low density, and don't offer much that a beefy C0G can't deliver anyway. PPS films I think are the main exception there, if you need a good low-absorption part.Tim