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| Should bulk capacitors generally be electrolytic? |
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| cbc02009:
hello all, I'm design a pcb for my capstone project, and I've realized I don't really know much about the differences between ceramic and electrolytic capacitors for this particular application. Given the same voltage rating and capacitance, is there a reason to use one over the other for bulk capacitors? How about tantalum? Thanks! |
| T3sl4co1l:
Ceramic: low ESR Electrolytic: high ESR (relative to size) Ceramics have the downside of tending to ring with nearby inductances. Putting in some ESR (in the right places) helps dampen that ringing. The other downside of electrolytics is, the ESR varies wildly with temp and age. They can be fine, but used carelessly, they can make a poor choice that much worse. (In that case, the wiser choice would've been a stable-ESR type like tantalum or polymer, or a low-ESR ceramic or polymer with added external resistance.) It's all a matter of "how much?" Some circuits are fine with a small electrolytic, or just one or a few 0.1's; others need a lot of both. The determining factor is, how much voltage ripple can your circuit tolerate, and how much AC current does it draw? (It's not DC current that matters, it's the change. Say your circuit is switching rapidly between 0 and (lots)A, it's basically worst-case 100% AC*. :P Anything else, probably easier.) *Well, 50%, or something near there, depending on how you define it. Tim |
| tautech:
Tim I've always been of the understanding tantalums can serve as both bulk and local decoupling providing the necessary precautions for using them are observed. I'd welcome your advice. |
| cbc02009:
--- Quote from: T3sl4co1l on May 20, 2018, 01:36:18 am ---Ceramic: low ESR Electrolytic: high ESR (relative to size) Ceramics have the downside of tending to ring with nearby inductances. Putting in some ESR (in the right places) helps dampen that ringing. The other downside of electrolytics is, the ESR varies wildly with temp and age. They can be fine, but used carelessly, they can make a poor choice that much worse. (In that case, the wiser choice would've been a stable-ESR type like tantalum or polymer, or a low-ESR ceramic or polymer with added external resistance.) It's all a matter of "how much?" Some circuits are fine with a small electrolytic, or just one or a few 0.1's; others need a lot of both. The determining factor is, how much voltage ripple can your circuit tolerate, and how much AC current does it draw? (It's not DC current that matters, it's the change. Say your circuit is switching rapidly between 0 and (lots)A, it's basically worst-case 100% AC*. :P Anything else, probably easier.) *Well, 50%, or something near there, depending on how you define it. Tim --- End quote --- Thanks for the explanation! The circuit is a Solar MPPT controller using an atmel SAML21. Do you think it's possible that the current change from say a cloud covering the panel is enough to want a bulk capacitor on the panel input? The only other place I have them is on the outputs of the switching regulator (for the 3.3V and 5V rails). |
| T3sl4co1l:
--- Quote from: cbc02009 on May 20, 2018, 02:30:10 am ---Thanks for the explanation! The circuit is a Solar MPPT controller using an atmel SAML21. Do you think it's possible that the current change from say a cloud covering the panel is enough to want a bulk capacitor on the panel input? The only other place I have them is on the outputs of the switching regulator (for the 3.3V and 5V rails). --- End quote --- Ah good, we have a start. Now the question is: how fast? So first of all, I take it there are at least three things in this system -- you're leaving a LOT out, I suspect! -- the solar panels (DC), the MPPT converter itself (switching, with harmonics into the 10s MHz??), and some digital logic (at least an MCU, clock of 10s MHz, harmonics in the 100s?). Which one has the highest rate of change? How much power? How much impedance or energy storage is required for each? Presumably, this is charging a large energy reservoir, like a battery, or feeding the grid. So, slow changes, like clouds, simply change the charge rate. Rate of change (to anything the circuit cares about) is essentially zero for that. So what does that leave? :) Tim |
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