View Single Post
Old 11th Jan 2012, 11:17 pm   #31
dinkydi
Retired Dormant Member
 
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
Default Re: capacitors in series - pitfalls?

Hi GMB, My point is that leakage is the direct result of voltage stress on a capacitor, not only that but leakage is the exact measure of the stress. A capacitor with a lot of volts across it but very low leakage is a happy fellow. One with a lower voltage but higher leakage is not faring so well. However, the stress on a capacitor can only be inferred in a rough general way by comparing its voltage with the rating on the cap body and yet this is the common approach.

I am indeed saying that the big issue is the leakage itself and it could well be the case that the leakages are substantially different (when the caps are apart) as you point out. You are also correct in pointing out that the voltages may distribute themselves in an unbalanced fashion, not in accordance with the voltage ratings of the caps. You suggest that this may be bad news, but what exactly is this bad news? I argue that it is actually good news because the actual leakage is being minimised - this is the fundamental issue, not the voltage rating written on the body of the cap. This minimisation does not occur if you add resistors.

It is indeed bad news if you believe that the voltages should be balanced, but my argument is that this is an artificial constraint that we are placing on the design - this constraint is not a necessary condition for best design. We have all been taught and accepted the wisdom that voltage ratings on components are sacrosanct, it takes a lot of consideration, perhaps, to accept that this is not necessarily true and this approach in this case leads to a better, more robust, simpler design.

The series connection of capacitors minimises the leakage of the "leakier" cap. When balancing resistors are added that cap leakage current increases, so the design with resistors is by default an inferior design.

We don't need to worry about cap voltage ratings because leakage current is the measure of how well the cap is faring under voltage stress and the design without resistors minimises this. Of course we must ensure that the total voltage ratings of the caps are adequate otherwise neither cap may be operating within specification (I don't recommend this!), even if it is operating in the best manner in the circumstances.

Peter
dinkydi is offline