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metallized polypropylene capacitors for DC supply

metallized polypropylene capacitors for DC supply

metallized polypropylene capacitors for DC supply

(OP)
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.

RE: metallized polypropylene capacitors for DC supply

This sounds like a peculiar application, or maybe an ordinary application with a confused designer? Why use a polysomething* capacitor for what sounds like a reservoir application? An electrolytic would be the usual choice, with a low ESR film capacitor in parallel if required, especially if size is at a premium. Why is this unsuitable?

*Polysomething: Polycarbonate? Polystyrene? Polyester? Polypropylene?

Do you need very low ESR and ESL? Very high ripple current rating? Very high ripple frequency? Elevated temperature? There is a lot of detail which is missing - it would be easier to help if we knew what was going on!

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: metallized polypropylene capacitors for DC supply

Apologies for the polysomething comment: just noticed it in the subject line blush.

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: metallized polypropylene capacitors for DC supply

(OP)
Well I am not a confused designer!!! Maybe need to give more detail. The AC-DC rectification will take place in a junction box that is filled with oil and box is mounted on a underwater ROV,that goes 15000feet down in the ocean. Electrolytic capacitors are best for this AC-DC application but when dealing with air cavities under pressure, electrolytic caps FAIL!! That's why I am looking for metallized polypropylene capacitors, which are wrapped or epoxied in a moisture resistant epoxy resin

RE: metallized polypropylene capacitors for DC supply

I suggest making an air filled custom case for the electrolitic cap and/or using higher ferquency e.g.
400 Hz, reducing cap-s and transformers etc.

Plesae read FAQ240-1032
WEB: <http://geocities.com/nbucska/>

RE: metallized polypropylene capacitors for DC supply

Air filled?? This is for use in a liquid filled 5000PSI environment.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: metallized polypropylene capacitors for DC supply

An air filled container will probably collapse - that seems to be what the capacitors do. It will probably be tough enough to bring 50 Hz down a 15000 feet cable. 400 Hz next to impossible. If you have a possibility to use three phases, you will have a lot less ripple if you use a six pulse rectifier. Needs very little capacitance for smoothing.

Gunnar Englund
www.gke.org

RE: metallized polypropylene capacitors for DC supply

Good points skogs.  (Did you say you were a cable salesman?)lololo

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: metallized polypropylene capacitors for DC supply

yes,Keith, air filled. A 1000uF 63V ( i looked up this one)
is 10x25 mm.  A solid iron cylinder of this size would be
less than 50 gram.

It must be air filled to remove the pressure from the C.
I didn't calcilate it but I am willing to bet that less
than 4 mm. thick wall will be enough.




Plesae read FAQ240-1032
WEB: <http://geocities.com/nbucska/>

RE: metallized polypropylene capacitors for DC supply

I think the rest of the electronics won't be able to
survive this pressure either e.g. transistors and IC-s.

Why not use DC ?

Plesae read FAQ240-1032
WEB: <http://geocities.com/nbucska/>

RE: metallized polypropylene capacitors for DC supply

There are comparatively thick walls and very little, if any, hollow room in those other components. They will probably surwive.

DC sounds good. But there is probably a voltage drop issue involved.

What about supercaps? They are compact, no voids. Need to use several in series and a potential equalizer, though.

Gunnar Englund
www.gke.org

RE: metallized polypropylene capacitors for DC supply

Wirs insulation wise, you are much better bringing down a higher voltage DC and using a switcher to get the voltage you want.  There are small epoxy switcher modules that operate at 1MHz that should have suitable internal components that can take pressure.  This would be a good quistion to ask a manufacturer like VICOR.

RE: metallized polypropylene capacitors for DC supply

There is some similarity here to the electronics contained in the Crash Survivable Memory Module in a black box flight recorder which under the ED-112 specification must survive a month at 20,000ft (60 MPa) water pressure.  

I agree with nbucksa that the rest of the electronics will need some help to survive this pressure.  One technique used in CSMU's is to contain the electronics in a titanium container filled with silicon gel.  I don't know of any small metal pol capacitors though and so the suggestion of a 3 phase supply has a lot of merit as the filtering requirements are a lot less.  400Hz would simplify things even more but you will need to treat the 15,000 ft cable as a transmission line to control the losses which would otherwise be large.  There are a few challenges here.

RE: metallized polypropylene capacitors for DC supply

I worked for a semiconductor company that supplied ceramic DIL ICs with a metal lid.

For use under pressure (downhole), the lid had a hole in it and the cavity was filled with silicone oil of some kind.

RE: metallized polypropylene capacitors for DC supply

(OP)
Many IC components have no issue in surviving very great depths. It's only the electromechanical relays, electrolytic capacitrs, crystal oscillators, anything with an air void in it.

RE: metallized polypropylene capacitors for DC supply

Icar have a really good range of polypropylene caps, mainly for the power electronics market. I'm not sure if they make anything at the low voltage rating you're after, and the volume of the HV stuff is likely to be significantly greater. Probably worth a look, or an e-mail to their technical people. Evox-Rifa are or were a partner with Icar: I have bought Icar caps through Evox-Rifa before.

Are you tied to 28V AC? If this is for a production part have you considered a hybrid as a power converter? They can be packaged to suit just about any environment.

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: metallized polypropylene capacitors for DC supply

1.) If the input is high VTG DC, you don't need large cap.

2.) At 400 Hz the 15,000 ft is .006 lambda - no reason to
handle it as tr. line.

3.) BRIAN: if the Ti container carries the pressure the Si gel is only for heat xfer.

4.) What about the communication signals, connectors, cables? Fiber optics or Cu ?

5) I submit that each component must be evaluated for the
pressure and the design is far from trivial...

Plesae read FAQ240-1032
WEB: <http://geocities.com/nbucska/>

RE: metallized polypropylene capacitors for DC supply

nbucksa,

Have to agree with all you say although wrt the CSMU the other specs it must pass include 2 hr @ 1,100 degC + 9 hr @ 260 degC + absolutely huge shock and vib so the silicon in that case is not for heat transfer but impact protection and heat blocking.  Virtually no internal heat is generated anyway.

RE: metallized polypropylene capacitors for DC supply

Whoa ! this is a horse of different color !
at 1100 C Au,Al,Cu are molten and there is no polypropylene



 

Plesae read FAQ240-1032
WEB: <http://geocities.com/nbucska/>

RE: metallized polypropylene capacitors for DC supply

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.

RE: metallized polypropylene capacitors for DC supply

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

RE: metallized polypropylene capacitors for DC supply

(OP)
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)

RE: metallized polypropylene capacitors for DC supply

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....

RE: metallized polypropylene capacitors for DC supply

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.

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