For high power smoothing has anyone tried TDK Ceralink caps? https://au.mouser.com/new/epcos/tdk-ceralink-flex-capacitors/
They use some fancy dielectric ceramic which actually increases capacitance with applied DC voltage bias. You actually need to derate capacitance if there isn't a DC bias. Despite high unit costs, they seem cost competitive with X7R/X7T in a similar voltage/automotive class when you account for DC bias capacitance reduction.
Specifically, a poled dielectric: an electret. Some of these (the earlier series?) are poled at the factory, and
must not be heated to the Curie temperature during soldering, meaning they must be hand soldered, and carefully at that. Others, I don't know how it is they work, but they make the poling voltage/temperature such that they pick it up on first use, and these can be reflow soldered normally.
Do RTFAN, they're unusual beasts, and quite attractive in the right application!
For example, look up Google Little Box. IIRC, the winning team used these, combined with a GaN inverter, to do what amounts to shunt PFC on the DC input -- one of the curious specs of the challenge was a particularly low input ripple, necessitating energy storage for a full inverter output line cycle. Their solution used less space, in total, than electrolytic capacitors you'd need to do just this passively!
Edit: I've also been looking at ceramic capacitors at ~20A RMS but for use as resonant capacitors in a CLLC For the 20A current level it seems like one needs to go for multiple parallel capacitors to stay within characterised 20°C thermal rise. As other have mentioned, Kemet and TDK seem to be pretty good about making available ESR and ripple current vs. temperature rise data. Murata also has similar data available but KEMET and TDK caps seem more cost effective from what I've seen. ESR and ripple current rating can vary significantly for same dielectric, voltage and capacitance but different package so be sure to try check a few, particularly those in bigger packages.
For resonant, type 2 dielectric are no good. You need C0G, but availability sucks in low voltages and high values. (Incidentally, they have higher energy density than most anything else, if you're using a few hundred volts -- it might even be worthwhile putting taps on those inductors so you can make use of this!) Next best is film, which, SMT films suck ($$) so consider PP in THT. You can get some quite beefy pulse and snubber type caps (Illinois PPB comes to mind, or a few series by EPCOS/TDK but I don't remember what numbers). Though again maybe not in low voltages.
The last possibility is aluminum polymer, which serves a very similar role to film caps, but at low voltages and respectively higher values -- they have similar energy density. The tan delta (~= 1/Q) may not be enough, though (which is also to say, respect the ripple current ratings). And they don't like much reversal, so you'd have to use anti-series pairs with DC bias.
The final option would then be: you must be doing something wrong, reconsider your network or topology; or just don't bother with resonant at low voltages.
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
Tim