Mega ceramics

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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.

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1 MHz isn't fast - tantalums should be fine, as well as some electrolytics.

Leon Heller G1HSM

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leon_heller wrote:
1 MHz isn't fast - tantalums should be fine, as well as some electrolytics.

Leon - using Kemet spice - most tantalums seem to have about 10x the impedance of their ceramic counterpart at 1MHz. Plus - they don't stack nicely due to their case design. I don't think tantalums are going to be the right choice here.

As for electrolytics - can you point me towards any that have low impedance at 1MHz? All those that I've seen have been awful once you start switching quickly.

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45V is pretty high for tantalums. It really narrows the options. Also, they are not very safe in situations where a lot of source current is available. They "fail" by developing pin holes in the dielectric. The holes are repaired by a limited current flow though them, then its all OK, again. My hunch is that 6A is near (maybe even above) the upper end at which this repair mechanism functions well. If the available current is too high, the holes will get worse rather than better, and they ultimately fail shorted (not good).

As far as ceramics are concerned, its all a matter of materials. Only in the last 2-3 years have we gotten anything above 100uf, for example. Basically, the thinner the layer, the higher the capacitance and the lower the breakdown voltage. For a given material, the C*V product tends to be a constant. So, if you need higher voltage, then the layers are thicker and, for a given physical size, you need more of them. Thats just the physics of the thing. Stacked caps are basically a kluge, offered for folks like you who value real-estate very highly. If that solution costs you more than you can afford, then you need to re-evaluate the approach!

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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you can use an inductor in series with a cap to filter.
You can simulate this using LTSpice. You may find some low ESR electros under the 12mm height limit.

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Maybe a combination of ceramic and electrolytic/tant? I see this often on commercial boards.

The ceramic to reduce the switching ripple caused by the ESR and the electrolytic/tant for bulk capacitance to handle bigger transients.

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Worst case scenario: Use another board on top of your current board - it'll look awful, but effectively double your board area

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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You don't need a giant ceramic - use an aluminum electrolytic for bulk capacitance, in parallel with a ceramic. Up to about 10uF ceramics are small and reasonably cheap. I don't like using tantalums having been bitten twice, once by them becoming more expensive than gold nuggets due to some war in West Africa where the tantalum is mined, and later by having significant numbers of them pop their tops. This second was due to a supplier buying an economy batch from a chinese dumpster, I think, but I still won't trust them ever again.