[ARC5] Cool New DC-DC Converter
Tom Lee
tomlee at ee.stanford.edu
Sun Mar 18 16:19:28 EDT 2018
They seem to go *very* dead short. They're the closest things to
room-temp superconductors we'll likely encounter.
HP gear is hardly unique. Pretty much any vintage gear that has
tantalums is awaiting an unscheduled power supply crowbar test. I have
lost track of how many 400-series Tek scopes I've brought back to life
by recapping. When a friend brings one to me to fix, that's pretty much
the first thing I check now. I'd say that something like two out of
three have shorted tants. On rare occasion, I'll find a tant that
auto-heals after being off for some time, but most go short and stay
that way.
Tom
--
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)
On 3/18/2018 1:05 PM, Scott Robinson wrote:
> Wehn Tantalums fail, they internally whisker and go dead short, even
> catch fire sometimes when used as bypass caps.
> If I'm fixing HP grad test gear from teh 1970-80s, first thing I look
> for is shorted tantalum caps.
>
> /scott robinson
>
> On 3/18/18 10:35 AM, Bob kb8tq wrote:
>> 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 <mailto: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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>>
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