Polymer Caps

[RoTTe]

Hi! I need some help. What is the diference between polymer caps and normal aluminum caps? the low ESR? or other factor?. I need recap some PCB's and I have some problems with the polymer caps (the shipping cost). I have thought about use normal caps, double the capacity or use two normal caps in parallel (divide the ESR). Some suggestion? Thanks.


Sorry Bad Einglish :P

[willawake]

Re: Polymer Caps

polymer capacitors use a solid polymer electrolyte unlike normal al e-caps which use liquid electrolyte. The benefit is very low esr and high ripple current.

you may replace polymer capacitors with normal low esr aluminimum capacitors and it is recommended to replace with about 2x the value of the polymer caps. if the circuit needs similar low esr you must replace with very low esr caps like rubycon mcz etc.

many designer specify polymer caps like oscon etc.... in many case this quality of capacitor is not necessary

please more information about your pcb to help you further

[gonzo0815]

Re: Polymer Caps

If possible you should figure out the real ripple current, the caps are exposed to.
The problem is, that some oscons are rated for ripple currents up to about 9A. Even the best elktrolytics woudlnt stand that high currents for long. So i suggest look how many phases the pcb or vrm unit has. if there only 2 phases and outputcurrents are higher than about 40 to 50 A (e.g AMD Athlon xp) the ripple current for the input caps could be as high than a quarter of the outputcurrent. Most elektrolytic are rated between 1 to 4 Amps.
On the outpusection things arn`t that bad, there an replacement with equal esr would be sufficient. Another point is the heat. In small server casses there are in many cases higher temperature, if proper cooling isn`t there. Again, the oscons are better suited for high temps.

[arneson]

Re: Polymer Caps

Is size a factor ? Specifically height?
More capacitance, higher working voltage and temp, in a smaller package is a winning combo.