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Author Topic: EEVblog #626 - Ceramic Capacitor Voltage Dependency  (Read 26457 times)

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Offline digital

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #75 on: June 09, 2014, 11:29:51 AM »
An informative video Dave fundamental videos invariably lead to a lot of comments on the forum so it is a win win situation for all as there is a lot of knowledge here on the forum.Thanks.
 

Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #76 on: June 09, 2014, 07:06:16 PM »
The size-vs-effect aspect continues to intrigue me. If simply increasing the size of the same-chemistry cap decreases the voltage-dependency effect, are the longer PCB traces (and bigger solder joints) an effective trade-off?
 

Offline EEVblog

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #77 on: June 09, 2014, 07:27:57 PM »
The size-vs-effect aspect continues to intrigue me. If simply increasing the size of the same-chemistry cap decreases the voltage-dependency effect, are the longer PCB traces (and bigger solder joints) an effective trade-off?

In most cases, yes, if the voltage dependency is an issue for you.
 

Offline Kjelt

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #78 on: June 10, 2014, 01:41:41 AM »
Thanks for doing the request i made in september: http://www.eevblog.com/forum/suggestions/bridge-ciruits-for-fundamental-friday/msg288045/#msg288045
I think this needs to be repeated to all generations of engineers so they won't fall in this trap.
Modern engineers depending solely on simulations can get into so much trouble.
 

Offline tchicago

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #79 on: June 10, 2014, 04:39:26 AM »
Hi, David,

Thanks for a great video! Not so long ago, I personally was puzzled by those tiny 10uF capacitors so much that I had to go to a datasheet to figure out where is the catch and if there is any. And, yeah.. found those capacity vs voltage curves.

Speaking of the non-ideal capacitors and other components, I have an idea for an exciting topic for the next investigative video: component whine (also known inductor whine, coil noise, etc). Here is a thing: there is a strong belief this annoying noise in DC-DCs and SMPSs is caused not only by the inductors, but also by the capacitors that emit it via the piezoelectric effect.

In fact, long ago I was surprized by this noise coming from a board that did not have any inductors at all, only the bypass capacitors on the power rails on the 74xx-based TTL board. But the board had early Russian EEPROM chips, so I was not fully sure if it was coming from the chips or caps.

But I'm guessing the setup for this kind of experimentation is not going to be simple: probably need some good microphones and audio spectrum analyzers. And also a well though-through drive circuitry for the DUTs.
« Last Edit: June 10, 2014, 07:15:46 AM by tchicago »
 

Offline free_electron

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #80 on: June 10, 2014, 04:44:52 AM »
BaTi based MLCC caps will emitnoise if pulsed currents are sent through them.
BaTi is a piezoelectric material.

the reverse is also true. they act as microphones. they should not be used in feedback loops of regulators if the board is going to be subject to mechanical vibration !
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Online robrenz

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #81 on: June 10, 2014, 05:03:05 AM »
BaTi has been used as the sensor for fish finders.  Driven to send a pulse into the water (speaker) and then listened to for the return signal (microphone)

Offline Clear as mud

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #82 on: June 11, 2014, 07:50:05 AM »
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 ....

This is kind of off-topic, but about the symmetry of capacitors:  What about regular, symmetrical MLCCs?  Does mounting position matter?  The smaller values look the same right-side-up or up-side-down, and I have been assuming that it really doesn't matter which way you install them.  The larger values, such as an 0805 1uF capacitor, tend to be almost the same size both horizontally and vertically.  What happens if you accidentally solder one on with the plates vertical instead of horizontal?  Is it all right to do that?  Will it lead to less flexibility and thus more cracked capacitors or broken solder joints?
 

Offline T3sl4co1l

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #83 on: June 11, 2014, 12:17:45 PM »
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 ....

This is kind of off-topic, but about the symmetry of capacitors:  What about regular, symmetrical MLCCs?  Does mounting position matter?  The smaller values look the same right-side-up or up-side-down, and I have been assuming that it really doesn't matter which way you install them.  The larger values, such as an 0805 1uF capacitor, tend to be almost the same size both horizontally and vertically.  What happens if you accidentally solder one on with the plates vertical instead of horizontal?  Is it all right to do that?  Will it lead to less flexibility and thus more cracked capacitors or broken solder joints?

This is silly:

1. The impedance, as measured, is on the graph.  Period.  Is the impedance low enough for your purposes?  Great!  You don't even need to know the capacitance, damping factor, voltage rating or anything else.  (That is, assuming the impedance curve remains true in your application.)

2. The worst possible case electrical difference would be if all the plates were flat to one side, so that the effective height above the board surface ranges from nearly zero ("face down") to nearly the chip thickness.  ...So What?  That makes a loop -- an inductor.  The loop area is maybe a half a nanohenry, for say, 0805 or thereabouts.  There is no effect on the capacitance, assuming you're talking about the frequency range where it, well, capacitates...  The only difference will occur at extremely high frequencies, where it's inductive either way.

3. All capacitors exhibit skin effect, just as all conductors do.  Indeed, radiation of any given wavelength will tend to propagate within the surface of a dielectric -- conductors are not needed at all to illustrate the skin effect.  This has applications and examples ranging from dielectric antennas (microwaves) to total internal reflection (arguably, extending up into x-rays).  This is independent of construction, limited only by the dielectric constant of the material.

Wound film types are especially notable, due to the relatively large physical construction (the effective roll-off frequency is low enough to be of significance) and the effect of schoopage (end terminal metallization) shorting across the roll (thereby forcing current away from the center at high frequencies).

A good example, from my personal experience, is CDE 940 type snubber caps.  They're rated for lots of peak current.  Beefy.  But not nearly as much RMS as you would expect from their size.  (Datasheet for reference: http://www.cde.com/catalogs/940C.pdf )  Take 1uF 600V for instance.  8.9A RMS, 196A peak.  Yet the voltage curve starts rolling off at only 2kHz.  (The astute reader will notice 250VAC at 2kHz draws only 3.1A; the voltage curve drops after this, but if you calculate current, you see it rises to 6.3A at 10kHz, then around 10A at 100kHz -- presumably 8.9A from the table.  The graph might show more since it says 25C ratings, not 70C.)  Now, it's not obvious from the data if the current rating rolls off at all.  It seems to climb.  In this case, the datasheet doesn't help me illustrate my point, unfortunately!

The reason I give this example from personal experience is, I've tried them before, and they get hot.  I tried using them at more like 200-400kHz, at somewhat less than rated current, but the losses were real, disproportionately higher.  They are definitely better suited to lower frequency use (under 100kHz).  Other than the hazy foreboding of data ending at 100kHz, the datasheet doesn't tell you this.

FWIW, that application ended up with Epcos metallized MKP style caps, which worked just fine -- within ratings.  Push a smidge past and they'll heat up and melt and puke and burn -- finely engineered down to a price, without a doubt.  Other types and brands were also tried, with more and less suitable results.

A better example to illustrate my point:
http://industrial.panasonic.com/www-data/pdf/ABD0000/ABD0000CE47.pdf
Page 6, ECWFA, 250V, permissible current.  Notice how the larger uF values reach peak current at lower frequencies.

Now, regarding current ratings, let me be clear: I don't mean to assert that skin effect, or in general, eddy current effects, are limiting current capacity of large capacitors at high frequencies.  There are many possible reasons, any of which may dominate in a particular case.  (A few pages ahead, the higher voltage types show nearly flat current limit curves, at least in the smaller values.  Is skin effect a consideration for those parts?  Who knows.)  I'm no capacitor designer, so I don't know what all goes into them, I only make educated guesses.  Skin effect will vary greatly with construction, so blaming it for the unsuitability for a particular application (like the 940s in my example) is only suspicion.

TLDR: ceramic caps are hunks of ceramic with a characterized impedance over frequency.  It doesn't matter how you put them in.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline GK

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #84 on: June 11, 2014, 12:55:41 PM »
It's actually worse, in some respects, than Dave suggests, as the dielectric has 'memory' - probably what Robert Pease discussed in his 'dielectric soakage' notes.


Dielectric absorption. "Memory" is a rather misapplied term. A capacitor exhibits a memory effect no more than a still-hot light bulb remembers that it was turned on. My earliest reference for a capacitor model for describing and estimating the effects of DA is Dow Corning 1954.

I’ve posted these before: 







Offline JackOfVA

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #85 on: June 17, 2014, 08:20:14 PM »
It's actually worse, in some respects, than Dave suggests, as the dielectric has 'memory' - probably what Robert Pease discussed in his 'dielectric soakage' notes.


Dielectric absorption. "Memory" is a rather misapplied term. A capacitor exhibits a memory effect no more than a still-hot light bulb remembers that it was turned on. My earliest reference for a capacitor model for describing and estimating the effects of DA is Dow Corning 1954.


Thank you for posting the reference - well worth reading.

However, I note that Paragraph 3-9(c)(2) uses the term 'memory' as well as 'soakage.'


 


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