[Milsurplus] [ARC5] Cool New DC-DC Converter

Sun Mar 18 13:35:10 EDT 2018

Hi

Like I said, it *really* surprised us when he popped out the papers and the data. 
None of us were aware of an issue. The driving mechanism seems to be the way 
the insulator “grows” in a polar capacitor. If you don’t put “enough” voltage on it, 
the process gets confused. That’s about as well as I can remember it, several 
decades later ….

The really bad news, I’d run into the same thing on tantalum’s *much* (like 6 months
out of school)  earlier. The world of SMD parts was new and wonderful. We
were setting up a screening / life test process to qualify some parts. I ran through 
what we intended to do ( more or less, max voltage / max temp for a month, 
while monitoring leakage) with the vendor. He smiled and said “that will be fine with us”. 
Later at lunch he pointed out that lack of bias (and re-forming after that situation)  
was the real issue for leakage (and possible failure). 

Bob

> On Mar 18, 2018, at 1:08 PM, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:
> 
> Hmmm.  Don't recall seeing the lifetime vs voltage appear as a bathtub curve. Life vs Time, yes, but not vs voltage.  Do you know why? Seems odd to me.  The only thing I can think of that would explain it has to do with operating at a much lower voltage for a long time and then expecting the cap to be OK right away once returned to operating at a much higher voltage.  All 'lytics pass current (we call it leakage) as part of their natural ongoing process of oxide decomposition and reformation.  Operate a 450V cap at 50V for a long period of time and it will reach a new equilibrium, eventually becoming a 50V cap.  To use that cap at much higher than 50V safely would require slowly reforming (re-thickening) of the now very thin oxide layer until an new (high voltage) equilibrium is reached.  
> 
> We see a version of this situation whenever we power up an old piece of equipment whose lytics have been dormant for years.  If they haven't dried out they will properly reform if given the chance. That is why we are careful to severely limit the current into those caps while we allow the oxides to slowly reform.  Slow growth creates a better quality of oxide and insures that the negative affects of self-heating are keep at a minimum.
> 
> Dennis AE6C
> 
> On Sun, Mar 18, 2018 at 9:44 AM, Bob kb8tq <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> wrote:
> Hi
> 
> The whole “less is better” approach is what we had been using across the board on parts.
> 50% max was our magic number. Went into the design review and started going through 
> everything. Made it to parts and one guy’s eyes lit up. He reached into a folder and pulled
> out a coupe of studies. (Odd that he just *happened* to have them along with him …). 
> They pretty well showed that on some caps, you get a bathtub curve for reliability vs voltage.
> Go to low and the reliability starts to degrade again …… surprised the heck out of all of
> us (to say the least). 
> 
> Bob
> 
>> On Mar 18, 2018, at 12:31 PM, Dennis Monticelli <dennis.monticelli at gmail.com <mailto:dennis.monticelli at gmail.com>> wrote:
>> 
>> The failure of an electrolytic cap vs how close to its rating it is run is a soft function.  It's not like the sudden failure of a solid dielectric cap.  It takes a long time.
>> 
>> To get a decent lifetime a 'lytic should always be run under its rating, just how much depends upon how much reliability you are after.  HP was conservative; their design guidelines were 60%.  Most other makers were less conservative in order to keep cost and size down.  My vintage ham gear seems to apply roughly 80% and sometimes more.  They also don't respect ripple heating enough, mounted high wattage bleeders close by and increasing moved toward ESR hostile cap input filters with solid state rectification.
>> 
>> The other factor is dry-out due to age and heating.  The higher the ESR and the higher the ripple, the more self heating.  The more heating, the more drying. The effect is a very slow runaway condition that eventually manifests in cap failure.
>> 
>> I you buy a modern 105C-rated low-ESR cap for replacement and don't try to squeeze the last volt out of it, it's going to reward you with long life.  They are built to survive in abusive PWM circuits.  Our circuits are a cakewalk by comparison.
>> 
>> Dennis AE6C
>> 
>> 
>> 
>> On Sun, Mar 18, 2018 at 9:12 AM, Bob kb8tq <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> wrote:
>> Hi
>> 
>> The real point is that a “250V” cap may or may not have a max rating of exactly 250.0V.
>> It may indeed have an actual max voltage on that sample a bit higher than 250V. These
>> days it’s very much a “who knows” sort of thing.
>> 
>> Oddly enough the same rule that says “don’t use the last 10     %” also says “don’t use less
>> than 60%” on an electrolytic cap. Learned that one in a design review …. on product headed
>> for space no less …. gulp ….
>> 
>> Bob
>> 
>> > On Mar 18, 2018, at 12:06 PM, Scott Robinson <spr at earthlink.net <mailto:spr at earthlink.net>> wrote:
>> >
>> > Well, a switching power supply designer who is even older than I am told me that if you avoid the using the last 10% or so of an electrolytic cap's voltage rating that it will last longer.
>> >
>> > FWIW,
>> >
>> > Scott Robinson
>> >
>> > On 3/18/18 7:33 AM, Bob kb8tq wrote:
>> >> Hi
>> >>
>> >> In some cases, indeed a 250V cap is quite happy for a very long time at 300V. It’s not
>> >> all “great balls of fire” sort of stuff ….
>> >> Bob
>> 
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