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Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Voltage reversal is really the killer for pulse applications. Even junky caps can tolerate a few high current pulses before the internal bus structure sends itself through the side of the can .
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
... I don't even see indication of this being DC... Yet the datasheet says 20% voltage reversal. Are there polarity markings I am missing or is there knowledge I am missing?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
>> ... I don't even see indication of this being DC... >> Yet the datasheet says 20% voltage reversal. >> Are there polarity markings I am missing or is there knowledge I am missing?
No, Yes.
>> Why is voltage reversal particularly hard on pulse caps?
Among other things: voltage reversal comes from the same circuit values that cause more of the pulse energy to be dissipated in the capacitor materials.
The capacitors don't care to which polarity they are charged. But if the pulse discharge is underdamped, so the voltage oscillates before settling down to zero, this is stressful. A very rough analogy in mechanical design is the fatigue failure of metals, which is aggravated when the cyclic load actually reverses the sign of the stress. Capacitors designed to tolerate high voltage reversal have construction details that increase cost and/or compromise other parametric ratings.
Here's a good treatise. You could start by looking at Table 1 and Chart 2. Section V gets all technical on the problem.
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