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Mark Fortunato
Rebob
Thanks for a very informative article Mark, but now I'm wondering how I snuck by ...
Temp and voltage variation of ceramic caps, or why your 4.7-uF part becomes 0.33 uF
Mark Fortunato
11/26/2012 10:00 PM EST
Not All X7Rs Are Created Equal
Since my RC time-constant problem was far greater than would be explained by the specified temperature variation, I had to dig deeper. Looking at the data for capacitance variation versus applied voltage for my capacitor, I was surprised to see how much the capacitance changed with the conditions that I set. I had chosen a 16V capacitor to operate with a 12V bias. The data sheet indicated that my 4.7 μF capacitor would typically provide 1.5 μF of capacitance under these conditions! Now this explains the problem that my RC circuit was having.
The data sheet then showed that if I just increased the size of my capacitor from 0805 to 1206, the typical capacitance under these conditions would be 3.4 μF. This called for more investigation.
I found that the Murata® and TDK® websites have nifty tools that allow one to plot the variations of capacitors over different environmental conditions. I investigated 4.7 μF capacitors of various sizes and voltage ratings. Figure 1 graphs the data that I extracted from the Murata tool for several different 4.7 μF ceramic capacitors. I looked at both X5R and X7R types in package sizes from 0603 to 1812 and with voltage ratings from 6.3VDC to 25VDC.
Figure 1. This is a graphic representation of temperature variation vs. DC voltage for select 4.7 μF capacitors.
Note, first, that as the package size increases, the capacitance variation with applied DC voltage decreases, and substantially.
A second interesting point is that, within a package size and ceramic type, the voltage rating of the capacitors seem often to have no effect. I would have expected that using a 25V-rated capacitor at 12V would have less variation than a 16V-rated capacitor under the same bias. Looking at the traces for X5Rs in the 1206 package, we see that the 6.3V-rated part does indeed perform better than its siblings with higher voltage ratings.
If we had looked over a broader range of capacitors, we would have found this behavior to be common. The sample set of capacitors that I was considering do not exhibit this behavior as much as the general population of ceramic capacitors.
A third observation is that, for the same package, the X7Rs always have better temperature sensitivity than X5Rs. I do not know if this holds true universally, but it did seem so in my investigation.
Using the data from this graph, Table 2 shows how much the X7R capacitances decreased with a 12V bias.
We see a steady improvement as we progress to larger capacitor sizes, until we reach the 1210 size. Going beyond that size yields no improvement.
In my case, I had chosen the smallest available package for a 4.7 μF X7R because size was a concern for my project. In my ignorance I had assumed that any X7R was as effective as any other X7R—clearly, not the case. To get the proper performance for my application, I had to use a larger size package.
Choosing the Right Capacitor
I really did not want to go to a 1210 package. Fortunately, I had the freedom to increase the values of the resistors involved by about 5x and, thus, decrease the capacitance to 1.0 μF. Figure 2 graphs the voltage behavior of several 16V, 1.0 μF X7R caps versus their 4.7 μF, 16V, X7R cousins.
The 0603 1.0 μF capacitor behaves about the same as the 0805 4.7 μF device. Both the 0805 and 1206 1.0 μF capacitors perform slightly better than the 1210 4.7 μF size. By using the 0805 1.0 μF device, I was thus able to keep the capacitor size unchanged while getting a capacitor that only dropped to about 85% of nominal and not to about 30% of nominal under bias.
But there was more to be learned. I was still confused. I had been under the impression that all X7R caps should have similar voltage coefficients since the dielectric used was the same, namely X7R. I contacted a colleague and expert on ceramic capacitors1. He explained that there are many materials that qualify as “X7R.” In fact, any material that allows a device to meet or exceed the X7R temperature characteristics, ±15% over a temperature range of -55ºC to +125ºC, can be called X7R. He also explained that there are no voltage coefficient specifications for X7R or any other types.
This is a very important point, so I will repeat it. A vendor can call a capacitor X7R (or X5R or any other type) as long as it meets the temperature coefficient specs, regardless of how bad the voltage coefficient is.
As an applications engineer, this fact simply reinforces the old maxim (pun intended) that any experienced apps engineer knows, "Read the data sheet!”
As the capacitor vendors have made smaller and smaller components, they have had to compromise on the materials used. To get the needed volumetric efficiencies in the smaller sizes, they have had to accept worse voltage coefficients. Of course, the more reputable manufacturers do their best to minimize the adverse affects of this trade-off.
Consequently, when using ceramic capacitors in small packages, or indeed any components, it is extremely important to read the data sheet. Regrettably, often the commonly available data sheets are abbreviated and will have very little of this kind of information, so you may have to request more detailed information from the manufacturer.
What about those Y5Vs that I summarily rejected? For kicks, let’s examine a common Y5V capacitor. I will not identify the vendor of this part, as it is no worse than any other vendor’s Y5V. I chose a 4.7 μF capacitor rated at 6.3V in an 0603 package and looked at the specs at 5V and +85ºC. At 5V the typical capacitance is 92.9% below nominal, or 0.33 μF. That’s right. Biasing this 6.3V-rated capacitor with 5 volts will result in a capacitance that is 14 times smaller than nominal.
At +85ºC with 0V bias the capacitance decreases by 68.14%, from 4.7 μF to 1.5 μF. Now you might expect this to reduce the capacitance under 5V bias from 0.33 μFto 0.11 μF. Fortunately, these two effects do not combine in this way. In this particular case the change in capacitance with 5V bias is worse at room temperature than at +85ºC.
To be clear, with this part under 0V bias we see the capacitance drop from 4.7 μF at room temperature to 1.5 μF at +85ºC, while under 5V bias the capacitance increases with temperature from 0.33 μF at room temperature to 0.39 μF at +85ºC. This should convince you that you really need to check component specifications carefully.
Next: Conclusion
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carlyaz
11/27/2012 3:41 PM EST
I once used a 4.7uf 0402 (had to be small) X7R capacitor in a timing circuit (relatively non stringent) and found that only over time the capacitance dropped (50% or so). It was 6.3v rated and my circuit circuit was run a on 1.55v silver oxide battery. The pulse circuit duty cycle, essentially, only impressed an average of about 0.1 volt DC bias. Funny thing is that the capacitor retained the lower capacitance when power was removed and it could be refreshed to the original capacitance by applying soldering iron heat. Its been a while since I stumbled over this and I do believe there is literature (maybe panasonic) regarding this particular affect. BTW: 50 % did not meet my "non-stringent" requirement so I switched to the AVX tancerum type. Good article and warning to all.
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Alan60
11/27/2012 3:45 PM EST
I like Kemet and AVX for that reason. There are other good vendors as you mentioned, but stick with the devil you know. At some point the capacitance tracks the voltage, causing nonlinearities from the delta-C. Any idea about the time frame of the voltage effects? At what frequency would the voltage coefficient kick in? Hz? Sub-Hz?
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mtripoli
11/27/2012 4:33 PM EST
Thank you Mark for this article. We tend to forget these things, and as you say, as the demand for "smaller and smaller" goes up these things become more critical than just a cursory glance at the data sheet. A "shout out" too to the manufacturers like Murata, TDK, AVX (who provide Spice models and 3D STEP files for some parts!) and Kemet (who always come through in a pinch with samples) that make these tools available; anytime a manufacturer provides these kinds of tools they go straight to the top of my list for "check here first".
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Mark Fortunato
11/28/2012 11:28 AM EST
mtripoli, good point. a corellary to your last statement; those who do not provide these kinds of tools (or other means to find this data) should go to the bottom of the list.
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LostInSpace2010
11/27/2012 7:36 PM EST
This gets everyone sooner or later. I once worked on a PLL circuit that worked fine until a capacitor change to a "Better" part made the loop go unstable. It turns out that with the original capacitor in the integrator was loosing capacitance at higher DC bias and this was almost perfectly compensating the increasing loop gain of the PLL loop! The "Better" Capacitor did not loose capacitance with DC bias and the loop broke out into oscillation. This will get everyone sooner or later also - the seemingly small change that can't go wrong ends up making the circuit fail. Humbling.... At least it didn't fail in our customers hands, just on our test benches.
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PeterKay
11/27/2012 10:40 PM EST
On the positive side, a large capacitance variation (4uF). This could be used to tune a low frequency filter or oscillator merely by altering the DC bias conditions. Will keep this in mind if an application ever comes up. Great article.
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T.ORR
11/28/2012 2:43 AM EST
Tim Orr
It is the case that surface mount ceramic capacitors have a lot of hidden problems. I have never measured a capacitance that was above the claimed value on the reel, for X5R and X7R. Also, if you use them to decouple power supplies you may only get 20% of their value.
Q: Why use Y5V capacitors? A: Y5V capacitors are as cheap as chips. However, I think that a bag of chips may make a better capacitor.
Products made in the 'East' may have X7R in the BOM, and something cheaper on the PCB.
C0G and NP0 capacitors are the nearest to a stable SM capacitor that you can purchase, if you are very rich and want high value parts.
When is a capacitor a resistor? I have had two units returned because they stopped working. It turns out that an AC coupling capacitor (100uF X5R) developed a DC leakage current that made it look like a 4k resistor! As far as I find out, mechanical shock can cause micro cracks in the ceramic material, which conduct. I removed the capacitor and looked at it under a microscope. It looked perfect, but the 1/2 kg metal box which housed the product was twisted and its front panel BNC was bent!
If you want to make a microphone or a loudspeaker, put ceramic capacitors on your PCB.
And so it goes on! Film capacitors still work well.
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Mark Fortunato
11/28/2012 11:36 AM EST
Tim, good points. It is interesting to note that one of my customers for the design I was writing about had arly failures that I tracked down to a (physically) large ceramic cap. This particular cap was cracking on half the boards and I could measure a resistance on these caps from a few hundred ohms to a few 10's of k. They had not use my recommended sources. This particular cap was rated for 250V and was typically biased by about 170VDC (120VAC rectified). Rather than use caps from my recommended realiable sources they bought them from on of those companies in the "East" as you mentioned. These were not mechnically designed to handle the stress. The guy from TDK whom I reference in the article showed how they and other reputable manufacturers have designed in mechanical features to handle the stress from the piezo-electic effects that cause the stresses.
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Brian Frost
11/29/2012 2:23 AM EST
This is a great article with a lot of insight. Hardly any engineers know about the voltage coefficient of ceramic caps and indeed most cap measurement LCR meters or cap 'bridges' can only apply a DC bias voltage of a few volts to the cap wen measuring, so the effect is rather hidden. To this end we manufacture and supply an external bias unit which sits between an LCR meter and the capacitor under test allowing up to 5kV to be applied to investigate and confirm application suitability. It can be rather revealing!
http://www.appliedrelaytesting.co.uk/products/asy0360.html
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tiorbinist
11/30/2012 1:45 PM EST
Back I the day (as it were: about 1996) I was working on a 70Mhz amplifier with an outrageous gain (over 60dB.) I was worried enough about it that I was using some serious decoupling on the powersupply lines: 10uF distributed approximately 1" intervals along the main power and ground bus, 1u and .1u at the PS pins to the very-close-ground, using microstrip construction for signal lines to the amp and from there to the high-speed comparator which created the 70Mhz clock for the timing chain, proper transmission line impedance matching, the works.
The circuit oscillated at 900Mhz, almost exactly. As in your case, I tested everything repeatedly, but it wasn't until I checked the .1uF AVX ceramic disk caps that I discovered the source: these little buggers would oscillate on 5v bias whether they were connected to anything else or not. After testing 10 at random out of the bag of 100, I relabeled the bag "900Mhz Oscillators" with a schematic (+5vdc to cap terminal || other terminal to ground with any load from 5v to ground) and put them aside. I got some different .1uF ceramic caps, which didn't self-oscillate and carried on.
I never did have a need for 900Mhz one-element oscillators, nor did I have the luxury of finding out why they did that.
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3drob
11/30/2012 2:18 PM EST
Good article.
I also ran into this issue early in my career. I was asked to investigate why a batch of battery chargers weren't working. I traced it down to an RC oscillator with the wrong frequency (too high). Like you, I checked the parts (correct), checked their values (also correct). When I scoped the waveform, it wasn't what I was expecting (if I remember, the standard RC waveform was inverted). Turns out a bad (wrong) batch of caps was used, and their capacitance varied widely over voltage. So as the voltage ramped up, the capacitance dropped, and the time constant dropped making the frequency too high.
Long story short, don't just rely on good manufacturers, but make sure your distributers are providing the correct parts.
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Dennis82
11/30/2012 3:01 PM EST
If you absolutely positively must have a ceramic capacitor that does not have temperature and voltage degradation issues, call Novacap and ask them about type H dielectric capacitors.
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John.McCorkle
12/14/2012 3:34 PM EST
Why your 220uF cap is really only 7uF EVEN AT ROOM TEMP.
Download the Kemet file KEM_T2009_T495.pdf and look at the plot on page 4 how the capacitance declines with frequency -- 220 uF up to about 20kHz and by 5 MHz it is down to 7 uF. No wonder so many switchers get made that make so much noise even after swamping them with ton's of Farads. Picking a cost effective and minimum size cap set is a pain. While this Kemet data sheet is pretty good, (it covers C vs Freq, C vs Temp, Voltage vs Temp, Dissipation factor, DC Leakage, ESR) it still misses specifications for microphonics (both piezoelectric and capacitance modulation seriously affect low noise circuits like a ref for an ADC or a resonant tank circuit for RF.
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CCarpenter
12/18/2012 9:52 PM EST
Any idea why I can't read page two of this article?
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Sicaps
12/20/2012 8:20 AM EST
Dear Mark,
i fully agree to all your comments on capacitance value decrease vs (temp°/DCbias).
You also forgot to mention that the Type II MLCC's also show a capacitance drop vs lifetime ( and extra -5% to take into account, and just after 1000hours operating).
As of today, Type I MLCC had the utmost performances, but very small capacitance density, so if you wanted stable high caps, that would use a lot of board space...
Now check this one out :
After 15 years working in capacitor world, i've joined a SpinOff of Philips semiconductor, called IPDiA.
We produce High density Silicon capacitors.
In few words, we have same performances as NPO's ( capa stability, failure rates, no piezzo effect), but we have same density as X7R.
We can indeed do for example 100nF in a 0402 ( iso a 1206 in NPo's).
And that is not all...
We specify our Sicaps up to 250°C, and we can offer thickness below 100µm thin.
Last but not least, our leakage current is rated in pAmps !
Imagine : NPO performances, but with 1µF 1206 250°C 100µm thin. Not bad hé ?
As the product director at IPDiA, i am glad to inform the community that there is a solution to Capacitance stability AND reliability issues.
We have a limited range available at Mouser or Digikey if you want to check them out.
Our go on our website : http://www.ipdia.com/
Or, just drop me an email if you have any questions, i would be glad to help :
laurent.lengignon@ipdia.com
Best regards
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ralphieboy
1/31/2013 5:35 PM EST
Hey Mark,
You're right, ceramic caps are not nearly constant capacitance w.r.t. operating environments.
Another damning feature of ceramic caps is the problem of LVIR failures. That stands for Low Voltage Insulation Resistance. Cracks or natural fissures in the ceramic material can foster dendritic growths of the plate material to cause a plate to plate connnection of filamentary dimensions. This wrecks the dc blocking ability of the capacitor in high-impedance circuits. In low voltage applications this causes circuit failure, whereas at high voltages, the filament is zapped off when it comes close to reaching through. This arcing only causes a noise disturbance in the circuit that may not occur again for hours or even weeks.
I believe the capacitance you are reporting on might be better termed "small-signal capacitance", i.e., dQ/dV. Some capacitor applications, such as relaxation oscillators, exercize the capacitor through a wide voltage swing. In these applications the small-signal capacitance does not apply directly. The voltage effects on relaxation oscillator frequency are somewhat less severe because the amount of change in stored charge is the integral of the small-signal capacitance over the range of the voltage swing. Obviously, the circuit analysis calculations are becoming more complicated.
When you get this deep into correlating design calculations with laboratory results, you will find that many thin-film resistors also have a significant voltage coefficient of resistance. If the thin film resistor data sheet does not mention voltage coefficient, do not assume it is zero! You will find that the terminal-to-terminal resistance goes down with increased voltage. And again there is a temperature effect on the resistance due to the ambient plus the part's self-heating, which is retarded by the thermal inertia (heat energy storage)of the part.
Thank God for non-linear circuit simulators and complete component models.
I wish you the best....
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Mark Fortunato
2/7/2013 2:00 PM EST
Ralph,
really good insights here. BTW this is NOT small signal dQ/dT. I was looking at changes in capacitances vs DC bias.
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HowieRF
2/8/2013 10:19 AM EST
It is disturbing that the capacitors do NOT have their rated capacitance at their rated voltage.
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Mark Fortunato
2/8/2013 1:23 PM EST
Howie, at first I agreed with you, but on second thought I am not sure. This would just turn the problem around. You would then have a part, or example, rated at, say, 1u at 10V butit would be 5u at 0V. If I am using it as an AC coupling capacitor in a circuit where is will see only mVs of bias, it would be just as far off, but in the other direction, from the case where the part is specified at 0V and operated at the rated voltage.
Then you have to consider that no good engineer would operate a part at its rated voltage, so specifying it there is to specify the part under a condition that it should never be used.
The overall issue is that a single number does not specify the part well enough. In fact, the main point of the article really was not that ceramics can have nasty voltage variations (although it is an import 2nd point); the main point was that it is good practice to look at the actual full specification for a part and not assume that the single number spec spells it al out.
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Rebob
2/18/2013 11:50 AM EST
Thanks for a very informative article Mark, but now I'm wondering how I snuck by in the past, considering the number of times I've selected a capacitor voltage based simply on breakdown/withstand considerations or convenience (parts inventory on hand). Do you think this is largely a result of SMD component shrink, is it most pronounced with ceramic capacitors, and is it accounted for at all in the capacitance tolerance spec?
Regards
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Mark Fortunato
2/18/2013 11:25 PM EST
Rebob,
It is definitely gets worse for higher volumetric efficiency so as we go to smaller and smaller parts this characteristic gets worse. I have not evaluated other types of caps.
I think most people have gotten by becasue large value caps tend to be for bypassing. If the capacitance is inadequate, they simply add more capacitance. I only ran into it because I was using a relatively large valeu cap as part of an RC time constant.
I would not be surprised if there are similar compromises made in other components as they get smaller.
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