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Old 16th Jan 2012, 1:53 pm   #42
dinkydi
Retired Dormant Member
 
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
Default Re: capacitors in series - pitfalls?

Hi Craig, Hope I haven't misinterpreted your post, but you appear to disagree with my assertion that the analysis you provide assumes that capacitor voltages should be kept equal.

These tiny quotes from your document are revealing: "the ideal value of V/2" page 2 after equation 6, and "the ideal mid point voltage V/2" page 2 after equation 7. (Other references exist.)

Another assumption concerns the advisability of even using balancing resistors - no technical justification is given. As you point out, the problem of the methodology is the difficulty of estimating leakage-current mismatch. However, other problems also exist.

The source of equation 9 (with its uncombined constants!) is not given, and we are not told of its applicability or limitations. I remain to be convinced that the equation is even of the right form. This does not inspire confidence to rely on this analysis in your circuit design.

According to the "Revision to TD001" there was originally a mistake in the analysis (it is now "corrected") - the definition of "difference in leakage currents" in equations 8 & 9 wasn't identical. To "fix" this a factor of 2 was added to equation 9. However, the factor should really be a variable. This simplification compromises the analysis, I believe.

The analysis is not a "worst-case" analysis, and neither is the "Capacitors in banks" analysis. This means that designs based on these analyses may fail, though hopefully infrequently.

The theoretical problem of capacitors in series seems so simple (well at first, anyway) you would think it must have all been sorted out long ago, however, no one seems to have uncovered a technical analysis that withstands scrutiny and is without controversial assumptions. If you want to know the size of balancing resistors for electrolytic capacitors in series, there is no accepted method of calculation, unfortunately.

Peter
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