Electronic – Capacitors: 200 mOhm ESR @ 100 Hz, attainable

Thanks for the help everyone, I ended up manually making the spreadsheet, with capacitors ranging from 7.5 to 82g and 16mm to 40mm diameter. Basically the higher voltage the better, within a capacitor family. Some capacitor families go up to 550V, but the most energy dense families only go up to 450V. The next largest factor is the connection style, in that snap-in capacitors are much lighter for a given capacity than either screw terminal or radial leaded capacitors. It also seems that smaller diameter capacitors had a higher energy density, but length had a fairly small effect. Manufacturer basically doesn't matter as their high end capacitors all have the same capacitance for a given case size and voltage.

Because of this, I ended up choosing the smallest diameter and longest snap-in capacitor available, the 22mm diameter, 50mm long, 400V 330uF United Chemi-Con ESMQ401VSN331MP50S, and am planning on putting 3 of them in parallel. They don't list the mass of their capacitors, but based on TDK's mass for the same size it is 2.66 J/g. The highest energy density I could find was 2.99 J/g, but that and several other capacitors at the highest energy density were not stocked anywhere.

For filtering SMPS ringing noise, you need a snubber. There are many guides on designing such. Ridley engineering is one and it's freely available: http://www.ridleyengineering.com/component/content/article/15-dc/127-71-designing-snubbers-for-nonisolated-converters.html?showall=1&limitstart=

Here's another: http://cfile9.uf.tistory.com/attach/2706B1465448758E2E7E4E

If you need more thorough reading, consider buying Rudy Severn's book on it. There are various different ways of designing these, simple RC snubber is the most popular one, followed by a RCD snubber. For high power/efficiency designs you can also design a lossless snubber where the energy is stored in a capacitor and fed back into the system.

On subject of using different size capacitors in parallel, this has a side-effect of creating an impedance spike as the inductances and capacitances of different components resonate. It may or may not matter but you're not damping that particular frequency much if at all.

edit Old Unitrode seminars have good stuff. Not to mention the way they draw basic SMPS circuits would give one our regulars a fit. http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slup100