Tantalum cap failures

Hello Mark

There may be some merit in what you suggest, but this would require substantial redesign and, even assuming we could fit this in the available space, would have profound implications for project time scales and costs. (This is a military product and as such has to undergo extensive Qualification testing that can take many months to complete succesfully, with associated costs that can run into six figures).

The optimum solution would be for us to find a replacement component that we can fit in place of the SFEs, in which case we can put forward an argument for Qualification by analogy with the existing design. Only if all else failed and we could justify the additional costs could we pursue the redesign route.

Ted

That's pretty much the conclusion I'm coming to.

At the time we specified them, (around five years ago now), the SFE range was the best we could find - hence we ended up using them. Unfortunately, the manufacturer didn't say: "Oh, by the way, our caps will develop spurious internal shorts during normal use", so it's not until we were using them in anger that these problems came to light.

As I've said elsewhere in the thread, we can't use electrolytics in our designs for aircraft use, so in instances where we need relatively large amounts of capacitance, this has invariably meant using tantalums in our applications.

In the light of what's been said on here however, I shall check this - it may well be that the technology has moved on sufficiently for us to be able to use them.

Ted

If the typical operating temperature is not too extreme, highly derated high-temperature aluminum caps may meet your life specification.

Best regards, Spehro Pefhany

The article seemed to indicate a 10 year life under full load, and very long life in storage with low humidity:

That was in 2001 - things should have improved since then. Since the solid polymer caps have lower ESR, less capacitance may be needed to meet your requirements. This means you could derate heavily and improve the lifetime.

How do other companies meet a 25 year life? Even the best computer grade electrolytics have much shorter life under load. And as you have shown, tantalums are not suited for this application.

Electrolytics may have the shortest life in a system, so a solution to your problem could help us all.

Mike Monett

Considered wet-slug tantalums, like the MIL CSR-series? They don't have the short-circuit failure mechanism that the dry-slug parts do.

Or Maxwell supercaps?

John

That sounds truly awful.

Sounds like you need to dump arcotronic for starters !

Graham

Hello again Mike

The short answer as to how other companies meet a 25 year life requirement is that I don't know that they do.

The 10 year shelf-life/25 year operational requirement is not universal across BAE, but depends on where the product is to be used - Typhoon requirements are particularly onerous in all areas.

I only get to see what's going on in external suppliers' products when I'm asked to sit in on a design review, (usually as a result of problems with the product), and I would certainly ask questions if I saw electrolytics being used on a product for Typhoon.

However, things are changing on the military electronics front and some of the old restrictions are being reviewed, in the light of relatively recent changes on the component front - the acceptance of the use of plastic devices being one example of this, brought about by the rapidly diminishing availability of military ceramic packaged parts.

Ted

Thanks John

I'll have a look at these, together with the various other sugestions.

Ted

Hi Ted,

I didn't know you were working on the Typhoon - avionics has got to be one of the toughest design problems, due to rapid and extreme changes in temperature, power supply spikes and overvoltage, severe vibration, arcing problems at high altitude, and so on.

My hat's off to you. Let us know if you find a suitable replacement for the tantalums!

Mike Monett

It depends what the equipment is going to be used on.

In some instances, electrolytics are used, but if I were to try to introduce them to a design for Typhoon, I would have to be able to substantiate any claim that they were suitable for use. The general rule is that we would not use electrolytics on Typhoon, for reasons primarily of shelf life and operating life requirements.

However, that position may well have changed by the time we bottom this particular problem.

Ted

I forget the name of the process involved but I distinctly recall a US aerospace engineer saying that commercial reliability figures were so good that they were being accepted for new build including military.

There's a good reason for this. MIL spec parts are unique and high price without any attendant benefit for the most part. Indeed by their nature, they are often 'old technology' and therefore inferior to modern commercial parts.

I suggest you give BAe a kick up the arse ! They deserve one for so many reasons it has to be said.

Graham

backwards is never a good idea either :)

I once saw our production manager annihilate the DC bus caps in a 10kW drive - they exploded, and sprayed boiling electrolyte everywhere. It took hours to clean up the production test bench! He was testing a 230V drive on a 400V test bench, and using a motor-driven variac to wind up the AC line voltage. Problem was, the variac overshot dramatically (inertia?), and boy the Hitachi AIC caps did NOT like it. Safety goggles saved his eyes, but his face got burned.

Cheers Terry

My impression from this thread is that avionics don't use electrolitics anywhere, is that so? What caps do you use on the power supplies?

.........

Did the bench test failures occur in units that had been previously 'autotested' with the uncorrected test program?

RL

Our test regime was changed around six months ago, so, since then, units have been subjected only to the revised testing, with no noticeable improvement. Nor have we seen any significant change since Arcotronic introduced their addtional screening.

Ted

Ted,

Earlier, you posted this comment:

Perhaps a 10-year cap life is not realistic in this application. Every aircraft must go through periodic inspections to check for corrosion, fatigue cracks, hidden damage, etc. Many items are life or wear limited, such as tires, engines, actuators, and so on. These must be replaced periodically.

Why not treat the avionics the same as the rest of the a/c and simply replace the caps during a scheduled inspection? This would allow you to use whatever performs best in your application, for example hi-rel solid electrolytics. You could also switch to a different cap as the product reliability improves over the years, and save money and time.

Perhaps the avionics already has scheduled inspections. If so, pop in some new caps and the problem is solved:)

As a pilot, I'd much rather prefer the slow degredation of solid electrolytics over the unpredictable and catastrophic failure of tantalums. Consider when you are in a storm approaching the airport with the weather at minimums. That is the worst time for a system failure, and the resulting confusion could cause an accident.

So you could argue flight safety as a good reason for the change.

Mike Monett

Is the power supply a flyback or forward type btw ? Flyback in particular causes high dI/dt.

Graham

Active parts don't store energy.

John

In message , snipped-for-privacy@baesystems.com writes

Most new designs for power supplies increase the frequency and therefore require lower value caps for smoothing. The high value ceramics that can then replace electrolytics rely on very thin dilectric layers with greater quality of material physically bigger caps could/would be less stressed. Interesting to consider active capacitors ie small caps multiplied by gain blocks, could this yield a more reliable product.?