I want some really freaking huge ceramic capacitors. I'm working on a 6.6A, 43V DC/DC boost converter that is operating at 1MHz - and I want as little ripple as possible. That speed means that, as far as I know, ceramics are pretty much the only useful capacitor. (or maybe some tantalums are good at these speeds? I think all electrolytics are out). I have limited board real estate available, but plenty of vertical space (about 12mm).
Problem is - most ceramics are about 2.5mm tall. So they are a terrible waste of board real estate.
Luckily - there already is a proper solution for this on the market. It's called a "[url=http://www.kemet.com/kemet/web/h...$file/F3114_MIL_CE_Stacks.pdf]ceramic stacked capacitor[/url]". Problem is - they ain't that good!!! (as far as I can tell).
Looks like their biggest 50V part is 47uF, Kemet part # L1XN305476KB65. It is in a size 3 case, and is 16.5 mm tall and its footprint is 27.3x12.7. That's one freaking huge capacitor - 765.8 mm^3.
Now - looking on Digi-Key - their biggest Kemet 50V ceramic capacitor is C2220X106K5RACTU - a 10uF part in a 2220 package that is 1.1mm tall. So let's do some maths. You would need 4.7 of the 2220s to get the same volume as the stack. So the total volume would be: 4.7 * [1.1 * (25.4 * 0.22) * (25.4 * 0.20)], or 146.8mm^3. That's less than a fifth of the volume of the stack!!!
Both are X7R parts, by the way.
So this leaves me with two questions:
1. What gives? Why do these stacks suck? (and I bet they're ferociously expensive too)
2. Any suggestions for getting massive amounts of ceramic capacitor in a small footprint? I'm thinking I might make my own stacks of the bastards. It'll be cheaper and have higher energy density. It'll also be a pain to solder - but luckily this is a one-off.