Author Topic: EEVblog #626 - Ceramic Capacitor Voltage Dependency  (Read 34581 times)

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Offline T3sl4co1l

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #25 on: June 08, 2014, 07:37:23 am »
Indeed, there can be a frozen-in potential (electrets), and there can be polarization through various other physics (piezoelectricity, thermoelectricity, etc.).

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Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #26 on: June 08, 2014, 07:57:17 am »
Another problem is that even if u are aware of the problem 99% of the LDO and Switcher Datasheets and Appnotes just mention a let's say "stable with 2.2uF". I mean, is that a 2.2uF rated cap or do the mean a 'true' 2.2uF at the regulated output voltage......?

That''s the thing, you don't actually know. It's a suck it and see thing. Although if they recommend a 2.2uF ceramic, then you'd have to think they have tried a real 2.2uF ceramic. But what type? What size? Good app notes will have actual part numbers.
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #27 on: June 08, 2014, 08:00:32 am »
These fundamental Friday videos are beginning to worry me. 
There has not been a single one that taught me something new.

Well, the title says it all, fundamental. If you know your fundamentals then there is nothing to learn. That's just the way it is. Sometimes I do lesser known ones like this, but if you know those too oh well  :-//
 

Offline bitwelder

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #28 on: June 08, 2014, 08:11:22 am »
So all of this does *NOT* apply to Class I capacitors?
 

Offline JoannaK

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #29 on: June 08, 2014, 08:13:47 am »
These fundamental Friday videos are beginning to worry me. 
There has not been a single one that taught me something new.

Well, the title says it all, fundamental. If you know your fundamentals then there is nothing to learn. That's just the way it is. Sometimes I do lesser known ones like this, but if you know those too oh well  :-//

It's never possible t oplease everyone..  I liked this one, cause I have not used those particular caps and thus seen need to learn about this effect.  :-+

Afterall. There is no real component that would match the ideal (simplified) idea of the parts. Sometimes it comes and bites designer, sometimes not.

 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #30 on: June 08, 2014, 08:19:17 am »
so, how were you (Dave) first made aware of this issue? I.e., from prev. in-the-field experience, word-of-mouth, discussions/requests on this blog?

Not sure exactly when, but I first learned about non-linearity and effects like this in ceramic hydrophone sensors (they are essentially just physically large ceramic capacitors and have similar issues) at work in about 1995. And I've seen the odd effect in circuits a few times since then.
Don't recall really knowing about it before that. Of course that was pre-internet when this stuff was not easy to look up or stumble upon.
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #31 on: June 08, 2014, 08:21:11 am »
Afterall. There is no real component that would match the ideal (simplified) idea of the parts. Sometimes it comes and bites designer, sometimes not.

Yes, you could certainly go your whole career without running across this issue in practice. Depends on your "luck".
 

Offline eV1Te

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #32 on: June 08, 2014, 08:46:52 am »
I knew that ceramics were non-linear, but i would never have expected them to have this big of a non-linearity!

So why are ceramics used so often as decoupling in power rails, since that is the application where they behave the worst (small ripple voltage, large DC bias)? Wouldn't some film capacitors have better actual capacitance in this case, or are they to "slow"?
 

Offline c4757p

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #33 on: June 08, 2014, 09:00:03 am »
eV1Te - Two reasons: Ceramics perform better at high frequency, and ceramics are cheaper. If you want 1 µF at full bias, it's usually cheaper just to use 2.2 - 4.7 µF nominal in a ceramic than to use a film cap.
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Offline DJ

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #34 on: June 08, 2014, 09:16:27 am »

<snip>

 Of course that was pre-internet when this stuff was not easy to look up or stumble upon.

Very true. When I started back in the late 70's, we each had large libraries of data books and application notes.

I assembled (and still have) binders specific to various passive components.

Covers a whole wall of my lab.

Many interesting things found, often lost and then rediscovered.  Even going back to my old Philbrick apps book has yielded insight.

I donated many of my books to a local university.  Wonder if they were ever used or even survive.
 

Offline BUkitoo

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #35 on: June 08, 2014, 09:33:32 am »
Very good video!
Suprisignly I learn about this capacitor behavior just a year ago, reading some papers. That I have checked some datasheets and I have found all the data reagarding the capacitance variation with DC voltage (the good brands, of course).

I would like to mention that I think Dave`s wrong when he says that a square waveform from 0 to 5v do not add a DC bias to the capacitor.  Of course the DC bias voltage is the mean value of that. It depends on the duty cycle, low and high level voltages. I think that the capacitance test should be done with a AC voltage with no mean value and take that capacitance value as the refernce point, and then add DC bias and check the difference.

bye!

Franco
 

Offline W7NGA

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #36 on: June 08, 2014, 10:19:16 am »
> I would like to mention that I think Dave`s wrong when he says that a square waveform from 0 to 5v do not add a DC bias to the capacitor.

Joseph Fourier agrees ...

dan W7NGA
 

Online rs20

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #37 on: June 08, 2014, 10:37:24 am »
I would like to mention that I think Dave`s wrong when he says that a square waveform from 0 to 5v do not add a DC bias to the capacitor.  Of course the DC bias voltage is the mean value of that. It depends on the duty cycle, low and high level voltages. I think that the capacitance test should be done with a AC voltage with no mean value and take that capacitance value as the refernce point, and then add DC bias and check the difference.

bye!

Franco

Correct criticism; wrong solution though, I suspect. "Of course the DC bias voltage is the mean value of that"? What does this mean in engineering terms, that the capacitor is omnisciently predicting the future and thinking to itself "I think Dave is going to charge me to 5V again, so I'd better divide that by 2 and use my 2.5V capacitance..."?

AFAICT (and correct me if I'm wrong), but this DC bias effect is completely stateless. So the capacitance at time t is a function of the instantaneous voltage across the capacitor at time t, and that's all. This means that what Dave measured is a (weighted) average of the capacitance at all voltages between 0 and 3.16V. Measuring between -2.5V and 2.5V would give the average capacitance from -2.5V to 0.66V, which is arguably slightly better but hardly solves the criticism you raised.

In practice, Dave's demonstration was so completely compelling, and the effects so much stronger than any error that might be introduced this way, that it's a complete non-issue.

Pedantic, ideal solution is to use a much smaller square wave amplitude, so that the instantaneous voltage is always approximately the bias voltage. (This also gives me the interesting idea of charging a capacitor once, from 0V to maximum voltage, and seeing if these properties can be extracted from the one charging curve by looking at the gradient at each point.)
« Last Edit: June 08, 2014, 10:58:18 am by rs20 »
 

Online rs20

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #38 on: June 08, 2014, 10:51:40 am »
It is scandalous that this is not mentioned on the datasheets, since it affects a large fraction of all applications.  I guess that using a type 2 ceramic cap as a DC blocker for audio applications would be awful.  How much distortion do you get by passing a 50 Hz sine wave (2 V p/p or so) through a ceramic cap?  I don't have the tools to measure distortion, but it might be enough to see on a scope.  I'll try it.

No, there'll be absolutely negligible distortion, if whatever's after the DC blocker has a high impedance.

If your output impedance is so low, and capacitance so small, than even an ideal capacitor would experience significant attenuation at 50 Hz, then yes, that means the capacitor isn't large enough to maintain a (near-)constant voltage across itself, and you may see interesting effects. But put it this way: you can always make a DC blocker bigger. If 1u is fine, then 10u or 100u is fine too. So if you use a 100u, and it drooped to 1u during operation, isn't that 1u still fine? The capacitor still does its job of carrying AC current.

I'm thinking about inputs to amplifiers and such here. Final output to speakers, etc, could be more of an issue. But still, distortion/harmonics are only generated where significant attenuation would occur anyway, so this is hardly a "panic and give up on all ceramics in audio applications" issue.
 

Offline David_AVD

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #39 on: June 08, 2014, 10:58:19 am »
Using ceramics in audio filter sections of single supply applications could be an issue though?
 

Online rs20

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #40 on: June 08, 2014, 11:02:46 am »
Using ceramics in audio filter sections of single supply applications could be an issue though?

Why would single-supply especially be an issue? Keep in mind, if my class 2 AC coupling cap has a nominal 10u capacitance, and it's got a 2.5V bias across it that makes it really 4.7u; what I've got there is a perfectly good, well-behaved 4.7u cap as long as the voltage across it stays at 2.5V (which is precisely the job of an AC coupling capacitor). In many cases a nominal 10u ceramic cap is much more compact and long-lasting than a 4.7u nominal electrolytic cap that'll dry out/vibrate off after x years.
 

Offline BUkitoo

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #41 on: June 08, 2014, 11:05:38 am »
"Of course the DC bias voltage is the mean value of that"? What does this mean in engineering terms, that the capacitor is omnisciently predicting the future and thinking to itself "I think Dave is going to charge me to 5V again, so I'd better divide that by 2 and use my 2.5V capacitance..."?

So the capacitance at time t is a function of the instantaneous voltage across the capacitor at time t, and that's all. This means that what Dave measured is a (weighted) average of the capacitance

Lol, Ok. I think you are right. I just wanted to me mention that probably the method was not good enough. That is all.

 

Offline jesuscf

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #42 on: June 08, 2014, 11:34:20 am »
Also, don't forget to take into account the output impedance of the function generator (Ro) for these tests.  The actual resistance of the circuit is 1k + Ro, probably 1050 ohms...
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Offline Salas

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #43 on: June 08, 2014, 11:39:02 am »
Those types are supposed to be fairly microphonic too? Will they display some resonance on the scope if you will hit them with a screwdriver for instance?
 

Offline T3sl4co1l

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #44 on: June 08, 2014, 11:46:58 am »
So all of this does *NOT* apply to Class I capacitors?

Correct.  I've ran 100pF 50V C0Gs at 500V and they work just fine.

For testing I mean.  I wouldn't recommend *buying* 50V when you actually need 500V. :)

Also quite high Q (in the thousands), so they're excellent for RF and resonant applications.  About as good as silvered mica, cheaper and smaller.  Also available up to reasonable sizes (fractional uF, maybe a few up to single digit uF?), though you'll be paying for them up there.

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Offline David_AVD

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #45 on: June 08, 2014, 11:48:01 am »
Using ceramics in audio filter sections of single supply applications could be an issue though?

Why would single-supply especially be an issue?

I was thinking you're more likely to have capacitors with 0V on one side and the Vcc2 (half rail) on the other.

Like the output of a volume pot (referenced to 0V) going through a coupling cap into a Vcc2 biased op-amp section.
 

Online rs20

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #46 on: June 08, 2014, 12:01:19 pm »
I'm going to suggest something. Whenever you look at a particular capacitor in a circuit, there's two different classes you can mostly put them in:

Bulk: Times when you need a metric truckload of capacitance, because you ideally want to hold the voltage across the cap dead-steady:
 - Power supply filtering, decoupling
 - AC signal coupling/DC blocking
 
In these situations, you need the huge capacitance because you can't just choose to make your ICs consume smaller pulses of current; and it's difficult and annoying to make the input stage of your amp have an impedance of Gigohms. Much easier to just use a huge bucketload of capacitance, without being too fussed about the precise value.

Ideal solutions: tantalum/electrolytic/class 2 ceramic.
 
Precise: Times when you need a precise, stable, predictable capacitance, but you typically don't need huge values because you control the currents that'll charge/discharge the cap. In these situations, you expect and rely on the voltage across the capacitor actually changing:
 - Timing
 - RC/RL/RLC lowpass/highpass/bandpass etc
 
In these situations, you can get away with smaller capacitances because you can increase the R's and the L's in your circuit to compensate for smaller C's (up to a point).

Ideal solutions: film/all those other low-capacitance types/class 1 ceramic.

Class 1 ceramics don't go up to nearly the same capacitance values as class 2, but that's fine, because whenever you need class 1, you can design your circuit to use those smaller values.</wild generalisation>

So as an example, a single-supply (the fact that it's single supply is of no relevance, actually) audio bandpass filtering circuit might consist of input and output buffers with class 2 capacitors interfacing to the outside world. But the internal bandpass caps should probably be class 1. Just a silly little example, but I think this sort of explains why I'm fascinated, but not the least bit worried, about this DC bias effect. None of my previous designs are adversely affected by the DC bias effect, because all of the critical, precise capacitors I had just happened to be small values, and I happened to end up with C0G/NP0 for those, even though I was completely clueless about this effect.
 

Online rs20

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #47 on: June 08, 2014, 12:01:47 pm »
Using ceramics in audio filter sections of single supply applications could be an issue though?

Why would single-supply especially be an issue?

I was thinking you're more likely to have capacitors with 0V on one side and the Vcc2 (half rail) on the other.

Like the output of a volume pot (referenced to 0V) going through a coupling cap into a Vcc2 biased op-amp section.

Sure, but as I've said, this doesn't lead to distortion or harmonics or anything like that, just a different value of nominal capacitance which can be easily factored into the design.
 

Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #48 on: June 08, 2014, 01:20:14 pm »
I am baffled by some of the reactions in this topic. I can understand the you g ayers or arduino generation not knowing this, but the rest ?

This effect is limited to ceramic capacitors that use a barium titanate dielectric. Some old school through hole ceramics are different. All MLCC multilayered ceramic capacitors have this.

Dave showed dc bias dependency, mentioned temperature and frequency drift as well as case size : a 0402 is different than a 1206 all else being the same.
But, there is more : take caps with identical specs : capacitance and working voltage and class from different manufacturers and they behave totally different. It even changes inside one manufacturer depending on the series. The key is the dielectric material used.

Mlcc also have microphone effect (piezoelectric noise)

Certain vendors have specially constructed caps that are internally asymmetrically built . As frequency increases the electrons do not go to the 'far' plates in the stack. So you can buy caps where the plates are vertically placed , or offset in the body. Take these out of the tape and reel and you better not drop em or you dont know what side is up ....

One very important thing is regulators. To meet performance a regulator requires a certain output and input capacitance. This is given in the spec. You NEED to compensate the caps for dc ! The numbers in the spec are ideal values. It is up to the designer to derate the caps.

All serious cap makers have software available that lets you find behavior.

Another problem is resonance frequency. You are making a decoupling network . Well it may not work right at the frequencies your power spikes occur !

Serious design work requires characterising the power decoupling . I use a agilent 4395a network/impedance analyser. This allows me to plot the impedance over frequency with dc bias applied. Cap  values are then inserted depending on where the impedance is out of whack.
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Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #49 on: June 08, 2014, 01:25:27 pm »
Another problem is that even if u are aware of the problem 99% of the LDO and Switcher Datasheets and Appnotes just mention a let's say "stable with 2.2uF". I mean, is that a 2.2uF rated cap or do the mean a 'true' 2.2uF at the regulated output voltage......?

That''s the thing, you don't actually know. It's a suck it and see thing. Although if they recommend a 2.2uF ceramic, then you'd have to think they have tried a real 2.2uF ceramic. But what type? What size? Good app notes will have actual part numbers.

App notes will have part numbers, if the apps engineer is worth his salt...
I can't speak for everyone, not even inside one company, but, in the group where i work the cap values mandated in the datasheet are IDEAL value caps. Derating is NOT taken into account. It is up to the people building the actual circuit to compensate for the behavior of whatever they use.
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