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| Are SuperCaps on 100-amp VCore supplies a thing yet? |
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| frogblender:
I have a product which has a 300-amp VCore supply at .7v (Yes, the asic it is attached to is fully capable of sinking battleships and setting your lab on fire). The asic has very spiky current draw - which I figger easily exceeds the 300A supply, for usecs at a time. Despite the underside of the asic being wall-to-wall with ceramic caps, there is still voltage droop during the current spikes (tens of millivolts, for a couple of microseconds until the switchmode turns on the jets. But then once the current spike is over, the voltage will overshoot by the same amount). So... Are SMT small supercaps with super-low ESR a thing yet? A quick peruse of digikey shows most supercaps have huge ESR, and are for RTC's and such (where ESR matters not, and almost all are giant radial pinthru cans). But some caps claim tens-of-milliohm ESR, which is getting into the ballpark of usefullness for huge-current switchmode supplies. Anyone have any experience? |
| thm_w:
What switchmode IC is being used, could it run at a higher frequency or response be tweaked? What is the capacitance of the ceramic caps used? Have you looked at tantalum-polymer capacitors? https://www.digikey.ca/en/products/filter/tantalum-polymer-capacitors/70 You may compare to motherboard designs, they deal with ~200A+ with vcore of ~1.2V |
| tom66:
The voltage droop is a function of capacitance, but you only need ~1,000uF, not ~1F, that an ultracap may give you. A polymer electrolytic may be the best compromise between capacitance and ESR. Tantalum capacitors are also still competitive in low-ESR applications. Finally, multi-phase SMPS converters get effective switching frequencies well into the 10's of MHz, and eliminate voltage droop by responding very quickly. I would consider all of these before ultracapacitors. |
| T3sl4co1l:
Exactly, the time constant isn't there -- for nanoseconds to microseconds, you need ceramic caps; for microseconds to milliseconds, you need electrolytics (roughly including solid tantalum and aluminum polymer). Supercaps just barely begin to be useful in the milliseconds; they're best in the seconds range. Still further out, various types of cells serve seconds to minutes (the new lithium titanate nanoparticle stuff?), to minutes to hours (LiPo, ion, NiMH, etc.), to hours or days (lead acid, etc.). Interesting to note, you're largely not storing energy here. I mean, you are, a lot of it, but that's not why you're doing it. It's not energy for the sake of energy: energy is only incidental to keeping the supply impedance low as hell. If we had capacitors that have C ~ 0 for most of their voltage range, then C ~ nominal for voltages within say 10% of operating voltage -- we'd be glad as hell! Supply voltage can ramp up quickly from zero, because there's almost nothing on it, then in the last few percent, a lot of charging work is needed, storing only the energy over that range. Then under normal load fluctuations, only a percent of whatever of that is actually used -- as bypass gets better, voltage change gets smaller and energy exchange drops towards zero. So, the fact that supply ripple is small, is proof that it's not about energy storage -- if it were, voltage would be allowed to change more, to make more efficient use of those capacitors. As it turns out, such devices do in fact exist -- poled (electret) ceramics. Whereas an ordinary ceramic has maximum capacitance around zero bias, and less and less at elevated voltage, these have an electric field frozen in, so their maximum capacitance "zero bias" point falls at some offset instead! I don't think this works at low voltages though; the example I'm aware of is designed for industrial application, 400V or thereabouts. (For sure, 0.7V is just about "zero bias" for most parts I've seen, no worries there. :P ) Tim |
| amyk:
--- Quote from: thm_w on February 19, 2021, 09:29:51 pm ---You may compare to motherboard designs, they deal with ~200A+ with vcore of ~1.2V --- End quote --- I wondered how much current modern CPUs draw, since I haven't done stuff in that area for a few years now, and according to https://linustechtips.com/topic/1137619-motherboard-vrm-tier-list-v2-currently-amd-only/ and some manufacturer's specs, there are mobos which are capable of up to 700A at ~1v :o :o :o I'm a little suspicious of those numbers, since that's an effective impedance of around a miliohm, and I know how thick a 200A arc welding cable is... |
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