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85C Cap ESR vs 105C Cap ESR!


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Hi
I can remind some times ago some one has written that it is better to replace 85'C Caps with 105C caps in old devices. (I think it was audioguru)
Today, I was reading an article about SMPS troubleshooting. Look what a repair man has said:

"(From: Jeroen H. Stessen ([email protected])

Electrolytic capacitors like to be kept cool! If there's anything that these capacitors can't stand, it's heat. It causes
them to dry out.
Electrolytic capacitors exist in (at least) two different temperature ratings: 85 C and 105 C. The latter are
obviously more temperature resistant. Unfortunately they also tend to have a higher ESR than their 85 C
counterparts. So in an application where the heat is due to I^2 * ESR dissipation, the 105 C type may actually be a
*worse* choice! If the heat is due to a nearby hot heatsink then 105 C is indeed a better choice."

Is it really true? ::)

ThanX
Shahriar

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Look what another guy has said about this topic:

ESR is usually something to be minimized in a capacitor. However, where the original design depended (probably
by accident) on a certain ESR, this may not always be the case:
(From: Lee Dunbar ([email protected]).)
Substitutions of low ESR caps into circuits which had lousy caps is not always the good idea that it appears to
be.... Caution is advised, as low ESR caps will not limit surge currents.
The circuits' series impedance drops (compare substituted capacitors ESR when new with the original capacitor's
ESR was when it was a new capacitor), which, in turn, lets the surge magnitude rise, the higher currents destroy
can semiconductors and other components.
I guess what the industry needs is a good capacitor cross reference guide for aluminum electrolytics!

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It really depends on the application.

... the heat is due to I^2 * ESR dissipation...


This is true, but you have to use the RMS value of the ripple current, something that is not always easy to calculate or measure.

Sometimes you want ESR to be high, sometimes you want it to be low. For example... in a SMPS, the input filter likes a higher ESR to... 1) kill the "Q" and 2) change the filters "output impedance" relative to the input impedance of the converter (DC-DC conveters can look like a negative input impedance and there can be interaction which can cause oscillations). Output cap's like low ESR to reduce ripple, but the transfer function has a "zero" caused by the output capacitance and its ESR which can be a pain when trying to stabilize the feed back loop.
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