On Sun, 09 Dec 2012 10:18:28 -0800, the renowned Jeff Liebermann
wrote:
On Sun, 09 Dec 2012 05:43:40 -0500, Spehro Pefhany
wrote:
On Sat, 08 Dec 2012 22:35:19 -0800, the renowned Jeff Liebermann
wrote:
Yes. The conspiracy theory is that components are selected to survive
no longer than the warranty period.
I think it's more a design spec than a conspiracy.
My first clue of a conspiracy was about 15 years ago, when I was
fixing a Sony CRT monitor (in the days when Sony made complete
monitors). It suffered from the usual electrolytic capacitor
failures. On any other brand, I could replace one or two caps, and it
would work. However, on this Sony, I discovered that a rather large
number of caps failed an ESR test, making repair uneconomical.
Inspecting the components, I noticed that there was a wide range of
rather strange voltage ratings on the electrolytics. In the past, I
would see dozens of electrolytics in a small number of standard
values. In this Sony, I saw a rather wide assortment of values and
voltages. I didn't do any lifetime calculations, but my impression
was that the voltage ratings were calculated for some target lifetime,
and no more. The result was near simultenous failure of all the caps.
Looking at a typical specs of the internal parts of a linear wall
wart- at full rated current, the winding temperature rise is 70°C.
At a 30°C average ambient, if we assume the temperature in the wall wart
interior rises only 15°C (say half rated current), and 10°C internal
cap heating from the ripple current, we'll have the cap at 55°C. With
a 105°C/2000 hour rating, we should get
Life = 2000 hours * 2^((105-55)/K) where K is 10-15
Assuming K is 15, we get a life of 20,000 hours. Plugged in all the
time (24/7) that's about 2-1/3 years.
Run it at full rated current in a high ambient and you'll be at a
small fraction of that.
That sounds a bit on the low side. Let's see how it compares with one
of the online capacitor lifetime calculators:
http://www.illinoiscapacitor.com/tech-center/life-calculators.aspx
For the wall wart, I used:
L1 = 2000 hrs Load Life Rating
Vr - 16VDC Maximum voltage rating of capacitor
Vo - 12VDC Operating voltage of application
Tm - 105C Maximum temp rating of capacitor
TA - 30C Ambient Temp.
which results in 483,000 hrs or 55.1 years.
Or 9.7 years using 55 C for the ambient.
Sheesh, no input box for ripple current. Plugging the same numbers
into the above equation:
x = (Tm - Ta + Tr) / 10 = (105 - 30 + 10) / 10 = 8.5
L2 = L1 (Vr/Vo)*2^x
L2 = 2000hrs*(16/12)*2^8.5 = 2,666*362
L2 = 965,000 hrs = 110 years
Here's yet another formula:
http://www.low-esr.com/endurance.asp
L2 = L1 * 2^(T1-T2/10) = 2000 * 2^((105-30/10)) =
L2 = 362,000 hrs = 41.3 years
I assumed 55 C temperature, so you'd get 7 years with that formula.
The transformer core and copper loses and capacitor ripple current
both heat the cap (plus a bit from the diodes and thermal fuse), and
the adapter will heat the air around it to some extent. If the ambient
is 30°C (ambient for the adapter), the cap will be warmer when the
adapter is plugged in and much warmer when it is doing something.
Some of the crappier ones even use 85°C caps. Some of them use crappy
lams so they run quite warm to the touch even without load. I'll bet
some Chinese caps are 85C marked as 105C.
The ones I spec'd I had reasonable confidence they will last 5 years
under normal use (so far, so good), but I don't think it would be
harder to consistently kill them faster than that.
Assuming that I'm not making any of my usual math errors, there seems
to be a discrepancy between the various methods of estimating
capacitor life.
Sure, they are just estimates.
There are caps that will last longer, but they're too expensive to use
in an AC adapter.
There are extended life capacitors, with up to 10,000 hrs rated life
for commodity components. I don't have a clue on pricing. Example:
http://www.niccomp.com/help/presentations/AlumE-CapExtendedOperation0809-revA.pdf
http://www.low-esr.com/endurance.asp
From the prices I've seen, it would cost ~10x as much to use the
cheapest such 10K-hour caps (maybe doubling the cost of the adapter).
They're also physically larger, pushing up the size of the case,
pushing up the size of the packaging, and allowing fewer units to fit
in a TEU, so increasing sea shipping costs.
If you can get, say, 7+ years typical life, and 99% will last more
than 5, meet all technical specs and safety requirements, as well as
meet the price point marketing wants, what would you choose?
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com