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| Electrolytic Capacitor Selection: Nod to Dissipation Factor or Ripple Current? |
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| JDW:
Consider these two capacitors which are the same physical dimensions, same Ripple Current rating, yet with different Dissipation Factor DF and Voltage and Life ratings, both made by Nichicon: DF=0.3, Ripple Current=210mA , 10000hr@105°C ULD1E101MED DF=0.19, Ripple Current=210mA, 5000hr@105°C UPV1A101MGD We know the calculation: ESR=tanδ*Xc (Ω), where tanδ is the Dissipation Factor, which is the "tangent of the loss angle." Xc = Capacitive Reactance. We also know ESR usually affects the Ripple Current rating of a capacitor. Yet in the case of the above two capacitors, DF is different but Ripple Current remains the same. So it would seem that in the case of the capacitor with DF=0.3, Xc must be lower to yield the same ESR which ultimately results in the same Ripple Current. Is this correct, or am I misunderstanding something? Here is why I am asking. If we are searching for high quality, aluminum electrolytic, off the shelf capacitors and we come across two capacitors such as the above 2 that would both satisfy a particular design's requirements for physical size, capacitance, voltage rating and life, should we consider selecting the capacitor with the lower DF, assuming the Ripple Current is the same? Now consider the following 2 capacitors: DF=0.24, Ripple Current=40mA, Life=5000hr@105°C UTT1C100MDD1TP DF=0.15, Ripple Current=33mA, Life=5000hr@105°C USV1E100MFD1TP These two are interesting because despite the significantly lower DF=0.15 of the USV1E100MFD1TP, the UTT1C100MDD1TP has a higher Ripple Current rating! So in this case, assuming either capacitor would satisfy the design criterion, and seeing the high volume price of each is nearly the same, would you choose the capacitor with the lower DF or the capacitor with the higher Ripple Current, and why? Thank you. |
| MagicSmoker:
--- Quote from: JDW on August 03, 2019, 07:22:54 am ---...Yet in the case of the above two capacitors, DF is different but Ripple Current remains the same. So it would seem that in the case of the capacitor with DF=0.3, Xc must be lower to yield the same ESR which ultimately results in the same Ripple Current. Is this correct, or am I misunderstanding something? ... --- End quote --- DF goes up over time so this could very well be a case of the manufacturer setting a higher DF for the first capacitor just so it appears to meet spec at the end of its 10kH rated life - in other words, the two capacitors might very well end up at the same DF after 5kH have passed, all else being equal. Another possible difference is the (rarely specified) thermal resistance: the capacitor with the higher DF might also have a lower thermal resistance, and therefore be capable of the same ripple current. Nichicon is good about showing the tradeoffs of each of their capacitor lines with respect to its most common substitutes and they make a very good capacitor in general. If you will be coming anywhere close to the ripple current rating - especially if ambient is 55C or higher - then I'd go with the second one (UPV series) because it is listed as a low-impedance type, but if this a general purpose application then either will be fine. |
| David Hess:
The higher DF capacitor which is otherwise identical can have the same ripple current rating if it operates at a higher temperature which might be the case if it has an electrolyte which does not evaporate as quickly. The lower DF capacitor might be used for lower ripple even at the expense of operating life. Like MagicSmoker says however, the DF rating is not the nominal value when the capacitor is new. It is the maximum DF at the end of its operating life. If you want to know the real values when new, then you will have to measure it. |
| JDW:
Gentlemen, thank you for your detailed replies. I was asking for general use purposes. There are sometimes when i need to select a capacitor for a design I myself created, but there are other times I may be recapping a vintage computer built 30+ years ago -- recapping everything from a switching power supply to logic board to a CRT driver board. When recapping old computers there is no schematic available, nor do I know technical details of the capacitors used beyond what is written -- usually capacitance, voltage and temperature (most often 85°C). That's why I am often unsure when selecting replacement capacitors if I should give priority to the Ripple Current spec or the DF spec. I do try to pick replacement caps that are rated at 5000hr@105°C or 10000hr@105°C whenever possible. Some vintage computers might actually prefer a low ESR replacement, especially decoupling caps on a logic board, but I need to be careful when recapping their switching power supplies. For example, using polymer electrolytics like OSCON with 40m-ohm or lower ESR could in fact cause instability in some old switchers (in theory, but I've not experimented with this), which is why I stick with regular aluminum electrolytics, even the "low ESR" kind because the ESR in the hundreds of milliohms is perhaps not too low for an old switcher. But again, I was just trying to better understand what my capacitor choice should be in light of DF and Ripple Current specifications. And based on your replies it would seem I perhaps should give the nod to lower DF (even if that spec pertains to end-of-life). (Also, most all caps have a DF spec shown, but the ESR spec is often not shown in the data sheet.) Thank you. |
| Conrad Hoffman:
For line applications, go by DF. For switching PS circuits, go by 100 kHz ESR. Look at the rated life too. I've always suspected that when they optimize one parameter, something else has to suffer. Finally, measure. It seems that many cap specs are sort of standardized, and a given series might consistently do a lot better than the DF chart, whereas another series might be near the published values, yet they use the same chart. |
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