When does temperature coefficient matter?

[ftransform]

russian resistor tempco must be pretty bad

[JoeyP]

You can spend much more time on capacitors. All kinds of fun parasitic effects to talk about: ESR, ESL, voltage coefficient, dielectric absorption, microphonics, aging and even tempco .

OK, I'll bite The voltage coefficient of large ceramic caps is the one I find most amazing. If you use them anywhere near their rated voltage, they practically disappear. Take a look at this chart for a 4.7uF 50v X5R cap. Notice that at 50v its value has been reduce by more than 90%. It makes a better varactor than a capacitor!

[PA4TIM]

Wow, i knew X5R had a rather large VC not this extreme.  Upto now I never measured this. I was plannimg because I used a X5R 50V 470 nF in a boostconverter casacde for a pulser. I made the same one, using the same coil before. The old one died while exerimenting so I reused the coil.  The new one i made in smd. Instead of MKT 470 nF cps I now used 470nF X5R caps. The first trew out 130V, the new one stuck at 81V. ( changing the feedback solved this but it was strange)

Now seeing your graph it makes sense. I use them very near to their max voltge rating.
My bridges can work with a DC bias. So that gives inspiration for some nice tests.

[Jay_Diddy_B]

Hi,
That graph for the capacitor variation with applied voltage does not look the graph for X5R. It looks like a graph for Y5V or Z5U.

Jay_Diddy_B

[PA4TIM]

http://www.niccomp.com/help/VoltageCoefficientofCapacitors-032012-R1.pdf
More info about VC including that same graph

[Neilm]

The VCR of a cap depends on how the cap has been constructed, as well as the dielectric. I have seen and used caps with X5R dielectric that still had 80% of rated capacitance at full voltage (3kV)

Neil

[JoeyP]

Hi,
That graph for the capacitor variation with applied voltage does not look the graph for X5R. It looks like a graph for Y5V or Z5U.

Jay_Diddy_B

The data sheet is attached below. Take a look for yourself.

Here are some actual measured values for a couple of parts. Lower value parts seem not to be as bad, but still very significant. I looked at different brands as well and found similar results:

Taiyo Yuden 4.7uF 50V X5R 1206 (UMK316 BJ475KL-T)

0v: 4.11uF
10v: 2.56uF
20v: 1.63uF
30v: 1.10uF
40v: 0.78uF
50v: 0.61uF !!!

Kemet 1uF 10V X5R 0603 (C0603C105K8PAC7867)

0v:0.891uF
1v: 0.887uF
2v: 0.854uF
3v: 0.804uF
4v: 0.740uF
5v: 0.675uF
6v: 0.611uF
7v: 0.545uF
8v: 0.495uF
9v: 0.443uF
10v: 0.400uF

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I wonder if this is easy to test using 555 timer (at different voltages) and performing frequency measurements.
i have not played with one in a while but iirc their frequency output varies quite a bit with voltage.

What kind of impact does this voltage dependant capacitance have on decoupling supplies? I figure the 100nF decoupling caps commonly used are not so bad, but how about more extreme cases?
Take the http://www.ti.com/lit/ds/symlink/tps7a3301.pdf for example. They say one should use 47uF ceramic decoupling caps for the output. Maybe this works OK when the unit is outputting 2 volts, but what if I want a -30 volt supply? Should I rethink their use of ceramic capacitors?

I'm afraid the designers did not take this into consideration? They recommend using high value X5R type capacitors. If you look at the spectral noise density graphs over a voltage range (page 9) you can see a increase in 2x over a 5 volt increase in output voltage. Perhaps this is because the ceramic capacitor is crapping out at this point?

[JoeyP]

The January 2013 EDN has an article which discusses ceramics used in capacitors. The author points out another interesting point about this voltage dependence, which is that it is mostly independent of the voltage rating of the cap. It's much more dependent upon the physical size of the cap. He uses an example of two different 4.7uF caps one rated at 16v, one rated at 25v. When biased to 12v, the value of either cap ends up being about the same.

[marshallh]

Here's some graphs on how package size alone will affect effective capacitance...

http://www.maximintegrated.com/app-notes/index.mvp/id/5527

[JoeyP]

Here's some graphs on how package size alone will affect effective capacitance...

http://www.maximintegrated.com/app-notes/index.mvp/id/5527

That looks like the text of the same EDN article. I see that it indicates it also ran in EDN in November 2012. They must be desperate for material to run it again so soon.

[Bored@Work]

That looks like the text of the same EDN article. I see that it indicates it also ran in EDN in November 2012. They must be desperate for material to run it again so soon.

Or EDN got paid a second time for running it.