Author Topic: EEVblog #626 - Ceramic Capacitor Voltage Dependency  (Read 37051 times)

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Offline EEVblog

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EEVblog #626 - Ceramic Capacitor Voltage Dependency
« on: June 07, 2014, 10:48:46 am »
Dave explains, shows, and measures a potentially big trap with using high value ceramic capacitors.
If your 10uF capacitor really 10uF in your circuit?
Those humble X7R caps you think are a "stable" dielectric? think again...

Class II and above ceramic capacitors can vary their capacitance drastically with DC bias voltage level and also the applied AC voltage.
Links:
http://www.murataamericas.com/murata/murata.nsf/promo_dcbias.pdf
http://www.avx.com/docs/masterpubs/mccc.pdf
http://www.ece.ucdavis.edu/vcl/asap/asap_v1/docs/X7R_C.pdf
http://psearch.murata.com/capacitor/product/GRM21BR60J106KE19%23.html
http://www.murata.com/products/design_support/simsurfing/outline.html
http://www.avx.com/SpiApps
Capacitor Tutorial - Ceramics & Impedance

The piezoelectric effect demonstrated:


 

Offline rs20

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #1 on: June 07, 2014, 12:22:34 pm »
Wow, thanks for this video -- I never would have suspected this. Makes me wonder whether I should be overspeccing my capacitances, or my capacitor voltages more (e.g., using 12V caps instead of 6.3V caps for 5V rails) as I presume the capacitance dropoff would be smaller if you reach a smaller percentage of rated voltage in general (let's just ignore the counterexample in the video with the -90% dropoff on the 100V caps).

Also, I never thought that every little decoupling cap I had could be used in a pinch as a varicap/varactor. This could make the most confusing, obfuscated schematic for a VCO ever...
 

Online VK5RC

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #2 on: June 07, 2014, 12:26:00 pm »
A great topic, Thanks
Whoah! Watch where that landed we might need it later.
 

Offline mmilejski

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #3 on: June 07, 2014, 12:35:46 pm »
Aren't some of the letter codes in the table for tolerance reversed?
I have a bag of Y5V capacitors and it says that they are -20% +80% and not +20% -80%.
And googling it gives various results:
This is similar to the one Dave used: http://m.eet.com/media/1174806/table1.jpg
and this one seems to be right: http://2.bp.blogspot.com/_c8EartLqdVs/TMmr-6Wn0WI/AAAAAAAAAkc/8A00KvYYR0g/s1600/Tolerance+code.bmp
« Last Edit: June 07, 2014, 02:25:48 pm by mmilejski »
 

Offline nathanpc

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #4 on: June 07, 2014, 02:14:07 pm »
This is incredible, I never thought a DC bias could affect the capacitance that much. I'll probably do some experiments with some capacitors myself too.
 

Offline opablo

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #5 on: June 07, 2014, 02:33:22 pm »
Is it possible that the errors come from the fact that the func gen could not be able to provide enough current to make the math work ?

Can you re-test one of the worst cases using a high current power supply and the dso triggering a single shot ?

You can see in your dso that the initial voltage is not being able to stay solid at the begining of the charge curve.


(I'm not an EE... just a hobbist... so if I'm wrong just point me wrong in a friendly way)
« Last Edit: June 07, 2014, 02:35:13 pm by opablo »
 

Offline AndreasF

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #6 on: June 07, 2014, 02:56:21 pm »
I'm not sure if I'm missing the point completely, but I don't understand why this is any way surprising, in fact I would be very surprised if a capacitor didn't behave this way. Is this really unique to ceramic caps???

Given a bias voltage (i.e. a constant voltage difference between the two pins/"plates" of the capacitor) I would think that the capacitor already holds a certain charge*. The total amount of charge it is able to hold depends on the (total) voltage differential (in a non-linear relationship). Therefore increasing the voltage differential from 0 to 1V will add a lot more charge than changing the voltage differential from 5V to 6V, which will only be able to add a smaller amount of charge thus making the time constant shorter. So, yes, in a sense the capacitance changes, but mostly because some of its capacitance is already "used up" by the bias voltage.


*This can easily be demonstrated: as soon as the bias voltage is removed, the cap will discharge.
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Offline T3sl4co1l

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #7 on: June 07, 2014, 03:38:42 pm »
I'm not sure if I'm missing the point completely, but I don't understand why this is any way surprising, in fact I would be very surprised if a capacitor didn't behave this way. Is this really unique to ceramic caps???

Yes.  Ferroelectric materials saturate in the same way that ferromagnetic materials do.  Which is better appreciated among transformer designs, but inductors frequently miss the mark on that fact.

Quote
Given a bias voltage (i.e. a constant voltage difference between the two pins/"plates" of the capacitor) I would think that the capacitor already holds a certain charge*. The total amount of charge it is able to hold depends on the (total) voltage differential (in a non-linear relationship).

By definition, if capacitance is constant, the charge is directly proportional to (linear with) voltage, Q = V*C.

If C varies with V, then Q = V * C(V), which is kind of odd.  You can imagine then trying to solve the differential equation for charging a capacitor through a resistor, but having to substitute some nonlinear function into it...

Quote
*This can easily be demonstrated: as soon as the bias voltage is removed, the cap will discharge.

Leakage is not an intrinsic property of capacitance.  It is an inevitable result of imperfect materials and their physics, but nothing at all to do with charge or capacitance or bias.

The cells in an EPROM or Flash contain very high quality capacitors (high purity SiO2 glass dielectric), which leak something on the order of single electrons per month.

Tim
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Offline DJ

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #8 on: June 07, 2014, 03:43:57 pm »
Aren't some of the letter codes in the table for tolerance reversed?
I have a bag of Y5V capacitors and it says that they are -20% +80% and not +20% -80%.
And googling it gives various results:
This is similar to the one Dave used: http://m.eet.com/media/1174806/table1.jpg
and this one seems to be right: http://2.bp.blogspot.com/_c8EartLqdVs/TMmr-6Wn0WI/AAAAAAAAAkc/8A00KvYYR0g/s1600/Tolerance+code.bmp

Y5V's are +20/-80

www.avx.com/docs/catalogs/cy5v.pdf

I avoid them and do not keep any on hand - don't want to inadvertently use one when prototyping a ckt




btw, there is also a voltage coefficient of resistance that was pronounced in old carbon composition resistors.

here's a couple articles

http://www.barthelectronics.com/pdf_files/Application%20note%201%20Voltage%20Coefficient%20Products_Pulse%20Page.pdf

www.vishaypg.com/doc?49997








Caps have all sorts of interesting characteristics,  dielectric absorption etc. Much analog fun.
« Last Edit: June 07, 2014, 05:08:04 pm by DJ »
 

Offline T3sl4co1l

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #9 on: June 07, 2014, 03:47:11 pm »
Thanks Dave!  It is an important lesson that needs to be said more often!

A good rule of thumb to choosing ceramic capacitors:
- Don't pick anything worse than X7R if you can help it,
- Look for minimum double the voltage rating you need, preferably triple,
- Design your circuit to tolerate twice the capacitor's rated tolerance, to account for manufacturing, temperature, voltage and aging (+/-30% or so for X7R?).

I did this plot some time ago, because I felt like it:



- Random "203Z" ceramic disc from the junk box, in series with a much larger cap; the common node was biased with a large resistor (>100k?) to the voltages shown.
- Capacitance was measured by resonating with a known inductor, I think it was around 100kHz (varying, of course) and 1V rms.
- The capacitor was sitting around for at least ten years, so likely had aged since its original salvage.  I annealed it by heating each leg with the soldering iron (set for 350C) for 10 seconds, then measured again.

Would it be possible to do one of these videos for magnetic components as well?  The effect is, after all, exactly the same -- ferroelectricity and ferromagnetism are two sides of the same electromagnetic coin!

My pet peeve: ferrite beads.  I ask you: what's the point of a 10A ferrite bead when it saturates at 100mA?  Nothing, that's what!  Even if you dig through their database programs (Kemet, TDK, etc. all have their things), you rarely find bias data for ferrite beads.  If you need DC filtering, don't look at ferrite beads, look at the "inductor" listing -- they make multilayer ferrite chip inductors, outwardly identical to ferrite beads, that are just as cheap and plentiful, but actually perform correctly.

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline Tek_TDS220

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #10 on: June 07, 2014, 03:57:24 pm »
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.
 

Offline NiHaoMike

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #11 on: June 07, 2014, 04:18:12 pm »
My pet peeve: ferrite beads.  I ask you: what's the point of a 10A ferrite bead when it saturates at 100mA?  Nothing, that's what!  Even if you dig through their database programs (Kemet, TDK, etc. all have their things), you rarely find bias data for ferrite beads.  If you need DC filtering, don't look at ferrite beads, look at the "inductor" listing -- they make multilayer ferrite chip inductors, outwardly identical to ferrite beads, that are just as cheap and plentiful, but actually perform correctly.
Beware that inductors are designed for low losses while chokes are designed to be lossy. Using an inductor in an application that calls for a choke can cause unexpected ringing.

I wonder if anyone has used the voltage coefficient of ceramic capacitors to an advantage. They're basically big varactors...
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Offline Alana

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #12 on: June 07, 2014, 04:27:14 pm »
Big Thx, i think one of my projects failed due to exacly this "feature".
 

Offline mariush

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #13 on: June 07, 2014, 04:38:04 pm »
A bit offtopic

Dave, when you said at around 5:30  "Come with me" i almost said out loud "Dave, you watched too much Cosmos" :D


 

Offline gxti

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #14 on: June 07, 2014, 04:40:30 pm »
All other things equal, higher voltage rating caps won't necessarily have better DC bias characteristics. Sometimes the 12V cap is just as bad as the 6.3V cap. X7R is better than Y5V for sure, but that isn't always enough either.

The best predictor for bias tolerance is the physical size of the capacitor. I stay away from 1uF caps smaller than 0805, and 10uF smaller than 1206.
 

Offline peter.mitchell

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #15 on: June 07, 2014, 05:13:41 pm »
I find a lot of comments in this thread misleading.

The effect of DC bias on capacitance is significant, however the effect of a capacitors "voltage rating" on the effect of DC bias is not significant, eg, a 50v 0.1u capacitor will have a similar drop of capacitance to a 6.3v 0.1u capacitor if all else is the same.
The thing that most closely correlated to the effect of DC bias on capacitance is physical dimensions of the capacitor, eg, a 0402 will lose more capacitance than a 0805 and a 0805 will lose more than a 1210.

So as a general rule of thumb, get the physically largest capacitor you can that is practicable for your electrical specification needs.
 

Offline DutchGert

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #16 on: June 07, 2014, 05:28:59 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......? I tend to design according to the latter but u end up using a lot more board space or more expensive caps just because u want to be sure and whack in a 10uF cap at a double or more voltage rate.
 

Offline Wytnucls

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #17 on: June 07, 2014, 06:06:47 pm »
Here is the physical process, as explained by muRata.

The mechanism of DC bias characteristic:
In the high dielectric constant capacitor type of monolithic ceramic capacitors, at present mainly BaTiO3 (barium titanate) is used as a principal component of high dielectric.
BaTiO3 has a perovskite shaped crystal structure and above the Curie temperature it becomes a cubic shape with Ba2+ ions to the vertices, O2- ion to face center and Ti4+ ion in a body centered position.
At the Curie temperature (approx 125°C) or more, it has a cubic crystal structure, and below the Curie temperature and within an ambient temperature range, one axis (axis C) stretches and other the axes shrink and turn to a tetragonal crystal structure.
In this case, polarization occurs as a result of the unit shift of axially elongated Ti4+ ion crystal. This polarization occurs without applying an external electric field or pressure, and is known as "spontaneous polarization." As explained above, a characteristic that has a spontaneous polarization and a property of changing orientation of spontaneous polarization by an external electric field to reverse is called "Ferro electricity."
The reversal of the spontaneous polarization per unit volume is equivalent to relative permittivity. Relative permittivity is observed as a capacitance.
Without a DC voltage, spontaneous polarization can happen freely. However, when a DC voltage is externally applied, spontaneous polarization is tied to the direction of the electric field in the dielectric, and independent reversal of spontaneous polarization is inhibited. As a result, the capacitance becomes lower than before applying the bias.
This is a mechanism of decrease in the capacitance after applying DC voltage.


http://www.murata.com/products/capacitor/design/faq/mlcc/property/05_more.html
 

Offline Stonent

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #18 on: June 07, 2014, 06:20:38 pm »
A bit offtopic

Dave, when you said at around 5:30  "Come with me" i almost said out loud "Dave, you watched too much Cosmos" :D

Well he did name his kid after Carl Sagan.
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Offline Skimask

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #19 on: June 07, 2014, 06:59:23 pm »
A bit offtopic

Dave, when you said at around 5:30  "Come with me" i almost said out loud "Dave, you watched too much Cosmos" :D

Well he did name his kid after Carl Sagan.

Next kid to be named Neil?  Maybe Tyson?

(P.S.  For all practical purposes, I can't stand what N.D.T. did to Cosmos)
I didn't take it apart.
I turned it on.

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Offline 13hm13

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #20 on: June 07, 2014, 07:11:58 pm »
Great episode!
Dave -- you're correct ... not a lot of experienced EEs know about this issue ... you sure know how to pick good topics ...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?
 

Offline mpep

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #21 on: June 07, 2014, 07:39:21 pm »
Fascinating topic. Not something I've ever been made aware off!
Good point about LDOs by DutchGert.
How should you design for this effect? The measured (rated) capacitance, or use a higher rated capacitor and use the de-rated value to get to the 'correct' value?
 

Offline retrolefty

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #22 on: June 07, 2014, 08:47:37 pm »
Here is the physical process, as explained by muRata.

The mechanism of DC bias characteristic:
In the high dielectric constant capacitor type of monolithic ceramic capacitors, at present mainly BaTiO3 (barium titanate) is used as a principal component of high dielectric.
BaTiO3 has a perovskite shaped crystal structure and above the Curie temperature it becomes a cubic shape with Ba2+ ions to the vertices, O2- ion to face center and Ti4+ ion in a body centered position.
At the Curie temperature (approx 125°C) or more, it has a cubic crystal structure, and below the Curie temperature and within an ambient temperature range, one axis (axis C) stretches and other the axes shrink and turn to a tetragonal crystal structure.
In this case, polarization occurs as a result of the unit shift of axially elongated Ti4+ ion crystal. This polarization occurs without applying an external electric field or pressure, and is known as "spontaneous polarization." As explained above, a characteristic that has a spontaneous polarization and a property of changing orientation of spontaneous polarization by an external electric field to reverse is called "Ferro electricity."
The reversal of the spontaneous polarization per unit volume is equivalent to relative permittivity. Relative permittivity is observed as a capacitance.
Without a DC voltage, spontaneous polarization can happen freely. However, when a DC voltage is externally applied, spontaneous polarization is tied to the direction of the electric field in the dielectric, and independent reversal of spontaneous polarization is inhibited. As a result, the capacitance becomes lower than before applying the bias.
This is a mechanism of decrease in the capacitance after applying DC voltage.


http://www.murata.com/products/capacitor/design/faq/mlcc/property/05_more.html

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Offline WarSim

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #23 on: June 07, 2014, 08:55:01 pm »
These fundamental Friday videos are beginning to worry me. 
There has not been a single one that taught me something new.  I am not an engineer.  I do not have a degree.  I have been employed in a junior engineers positions, and performed engineering duties in the past.  I wanted to use these videos to fill in the gaps because I do not have the formal degree. 
What do the people that made the electronic devices in my environment actually know.  Is it safe to to turn commercial electronics on anymore?  I know I have many things to learn in the engineering realm.  After all I am only a lowly Tecnologist.  Well that is what the idiot engineers I have met have told me.  :)
I will continue to watch these wonderful videos and wait for an introduction to something new to learn. 


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Online VK5RC

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Re: EEVblog #626 - Ceramic Capacitor Voltage Dependency
« Reply #24 on: June 07, 2014, 09:30:30 pm »
A interesting quote from muRata, to me it may imply that capacitance in one polarity may be different than the other with DC bias.
I might need to have a look myself.
Whoah! Watch where that landed we might need it later.
 


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