metallized polypropylene capacitors for DC supply 1

[fuseshut]

Hi I am looking for a metal pol capacitors for DC supply, I am rectifying 28VAC with a bridge rectifier (more efficient than single diode) and will attempt to "smooth" the rectified voltage with a capacitor bank. I have looked at some metal poly capacitors. I would like 60-70WVDC at 1000uF, but in a small form factor. for example, something smaller than

http://www.icwltd.co.uk/smf.html#

(click on technical details).
Any help on vendors would be greatly appreciated.

 

[BrianR]

They use CRES and Ti for the equipment cases. Imagine this test: Impact of 500lb weight with a 1/4" hardened steel penetrator dropped 10ft onto the most vulnerable face of the CSMU followed by a series of 3,400g 1/2 sine shocks followed by either the heat test already mentioned or alternatively one month at 20,000ft water pressure. Then read the memory with less than 1ppm error. Thats what black box flight recorders are certified to and still they get smashed in a crash.

 

[nbucska]

Brian:
all of these are interesting but mostly irrelevant --
they just show what careful design can do...
Fuse:
FYI: <

http://www.sensotec.com>

has .5" to 4" displacement sensors
for undersea application up to 3500 PSI.

Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[fuseshut]

NBU: THanks. Yeah reading through all the posts there are design parameters/consideration. The telemetry backbone is fiber optic and has no issues with ST connectors or CWDMS in oil. I have designed light controllers that operate 500W underwater incandescent lighting -no issues there. The older systems have aluminum pressure vessels where all this DC power stuffed housed before, but that's a lot of weight and for greater depths, say 6000m, it's advantageous to place all electronics into a PBOF jbox (pressure balance-oiled filled)

 

[billbehen]

TRW pioneered the polypropylene cap for the Power Electronics industry, introducing them at POWERCON 5 or so. They are just about ideal electrically, with none of the esr vs. temperature problems of electrolytics. The only drawback was/is size: They could never get the polypropylene film thin enough to make a part below 100VDC, so the parts were not so successful as they had hoped. However, for your application, 100 Volt parts might be just right! At 28VAC, bridge rectified, you're looking at a 40 volt rail, and some overhead never hurts in a power design. Capacities are small, though, due to the volume just being the thickness of the dielectric plus that of the metallization (a current/heating constraint) times the area, which translates directly to capacitance. But the question would be whether you really need 1000uF. The largest part TRW made was 30uF, which would support maybe 1/4 amp if you could tolerate some AC ripple on the output; or were regulating down to a lower DC voltage. Multiple caps would of course multiply this current by N, the number of caps. As you suggest, these parts are solid body epoxy, and are only at hazard from a soldering iron as they are not encased. Finding them is not easy, as TRW sold off the line a while back. I think Cornell Dubilier is where to look! Mouser is the right distributor. So far as esl (series inductance) is concerned, these things resonate up in the hundreds of KHz, vs. 2KHz for electrolytics, so it's no problem! Sorry to be so long winded....

 

[BlownUp]

This application sounds like a tough issue and unfortunately tough issues often require big dollar solutions. Poly caps are fantastic for many reasons (high frequency, super low ESR, self-healing) but are bad because of their large size per uF. It sounds like this is a significant consideration. I have never seen a miniature poly cap and for the reasons billbehen stated above I suspect you will never find a small poly cap.

I can see aluminum electrolytics having the void issues you mentioned but what about a tantalum? I am not 100% sure but their construction might be void free. However they have other issues and failures in them are not generally pretty. I am assuming that you have already considered these and decided they don't fit the application.

Given that it might be worthwhile to consider some other more off the wall type solutions.

You could look at an assembly with ceramic caps. Surface mount cermaics with 1uF capacitance in 1206 packages are very common. Using double sided boards with vias under pads and internal planes it should be possible to get 1000uF down to about a 4x6x2 package. Still not small but less than what I think you will find with poly caps. Some creative board layout or >1uF caps might get this considerably smaller.

You might also consider putting some aluminum electrolytic capacitors under oil and pulling a deep vacuum to remove voids. Not sure if this would work but it might. You would need to be sure that the oil you are using is compatible with the oil the cap manufacturer uses. Also, aluminum electrolytics do vent as they age and I am not sure if they produce gas internally. That might be a problem. Information nn their aging should be readily availble from a cap manufacturer or someone more familiar with their aging process.

Just some thoughts. Sounds like a tough issue. Good luck.