Re: Any problems with using X5R/X7R caps as buck converter output capacitors?

Terry Given wrote:
Joerg wrote:
John Devereux wrote:

"Joel Koltner" <zapwireDASHgroups@xxxxxxxxx> writes:

Before I start dropping some decent fraction of a dollar per capacitor (sorry, Joerg, the PCB layout guy claims he doesn't have room for electrolytics), I was wondering if anyone here has had any problems (or successes) using regular ceramic chip caps with X5R or X7R dielectrics as their main output capacitors in a buck converter? I need ESR under 250milliohms, which -- while ceramic caps don't normally spec this -- seems much higher than any guesstimate at ESR I can make by looking at impedance graphs from the likes of AVX (my guesstimates range from about 10-100milliohms depending on the particular cap I look at). I'm looking for some tens of uF here... probably a 22uF cap would work well.


One thing to watch out for is the the ESR being too *low*. Some SMPS
chips are unstable without enough ESR on the output!


Thou shalt not use those. And no LDOs ;-)


The chip ceramics I have tried appeared to have very low ESR - in the
region of 10mOhm or less.


But they do have a current limit.


But yes, I have used them successfully and much prefer them to
electrolytics now, where it is possible to use them. (I test stability
by connecting a 50 ohm output function generator to the input
/ output, and sweeping DC - few hundred kHz. This seems to highlight
any problems). Also check switch-on transient behaviour and current
limit recovery.


That is an interesting trick. I've done it to the input but not to the output. Thanks!


very nice trick!

X5R/X7R caps can also sing - I have seen (and heard) this dynamically varying loads. I built 50,000 50W bucks with X7R output caps - again, 10uF.

bigger caps tend to be more prone to mechanical damage & resonance (Marcon wrote a nice paper on this). And (esp. HV caps) are VERY susceptible to heat damage - soldering irons cause microcracks, which propagate into the cap & boom. green glassy goo all right!

Joerg, when not spec'd what sort of numbers do you use for ripple current?


I like to keep them well under 100mW (I^2 times ESR) and I never use tantalums in designs except maybe for the odd filter or integrator application. This paper explains it in more detail:

http://avx.com/docs/techinfo/mlc-tant.pdf

If a manufacturer isn't forthcoming with ESR/current data I move on and exclude them from the ECO. Mainstream companies such as AVX are usually pretty good with respect to support.

--
Regards, Joerg

http://www.analogconsultants.com/
.

Relevant Pages

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    ... Joerg, the PCB layout guy claims he doesn't have room for electrolytics), I ... caps don't normally spec this -- seems much higher than any guesstimate at ESR ... The chip ceramics I have tried appeared to have very low ESR - in the ...
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  • Re: Any problems with using X5R/X7R caps as buck converter output capacitors?
    ... Joerg wrote: ... I need ESR under 250milliohms, which -- while ceramic caps don't normally spec this -- seems much higher than any guesstimate at ESR I can make by looking at impedance graphs from the likes of AVX. ... electrolytics now, where it is possible to use them. ...
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