Gas station without pumps

2013 June 26

Kemet tech reports on capacitors

Filed under: Circuits course — gasstationwithoutpumps @ 08:46
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I was looking through the tech reports from Kemet about capacitors, to see whether there was anything else useful, besides the report that explain why capacitors don’t have their rated values that I mentioned in my previous post about DC bias.  I found one very beginning tutorial that may be useful for the Applied Circuits course: What is a Capacitor.  It talks about the standard RLC model for a capacitor, with series resistance and inductance and parallel resistance for leakage current.

It also does a pretty good job of talking about the different dielectrics used. I was not aware that barium titanate (class 2 ceramic capacitors) had dielectric constants of 3000 to 8000—I can see why it is so popular since tiny areas and thick layers are sufficient to get high capacitance.  They don’t mention a few of the classics from my youth (like mica capacitors), but you’d have to really be into old surplus to find those nowadays.  The low dielectric constant insulators are still in heavy use for high-frequency signal applications, because there are a number of problems with the ceramics and electrolytics in those applications.

I read CARTS – Spice Models with Temperature-Bias-Frequency Concerns also, because I was interested in how they modeled the DC bias concerns, but they didn’t really address that in the modeling, despite the title.  The temperature and bias dependence are handled by a proprietary tool that sets the parameters of the various RLC models for a given fixed temperature and DC bias, which does not at all capture the dynamic nature of DC bias and allow it to be simulated.It may be a useful tool for people designing fixed-voltage power supplies and using bypass capacitors, but it doesn’t really model bias effects in variable-output supplies or circuits with varying bias.

The report Why 47 uF capacitor drops to 37 uF- 30 uF- or lower is still a better presentation of what goes wrong with ceramic and electrolytic capacitors.

I looked at Avnet Power Forum – Capacitor Selection for DC-DC Converters, which is a series of slides from a lecture, not a full tech report.  It has some interesting material on DC-DC converters, which would be useful for a class on power supplies (or for anyone actually using a DC/DC converter), but is not really relevant for the Applied Circuits course.

CARTS – Can ESR Be Too Low describes the instability problems that can arise when using a capacitor that has low series resistance with a power supply controller designed for a capacitor with high series resistance.  The discussion was interesting to me, but relies on the reader knowing a little control theory, and so is mostly unsuitable for the Applied Circuits course.

There are lots more papers on the Kemet page, but most of them seem from their titles to be too specialized for the Applied Circuits course.

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