Re: Operating temperature derating

On Thu, 6 Jun 2024 14:01:04 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

On 6/6/2024 10:45 AM, legg wrote:

On Mon, 3 Jun 2024 07:52:58 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:
 

Presumably, one should feel comfortable using a device at the
published operating temperature extremes "forever".
>
But, what sort of derating likely went into that specification
in the first place?  Sad another way, how much *beyond* those
limits might want suspect you could operate the device?

 
There are two basic limits to operating environment temperatures.
 
Junction temperature limits are usually proscribed below book
Tjmax for an agreed mtbf. Sometimes the environmental limits
can be extended under reduced performance guidelines, without
exceeding agreed Tj limits.
 
For a whole product, there's usually other component limiting factors
that occur first - electrolytic and film capacitors, insulation system
ratings and surface touch restrictions.
 
Performance is measured with these specified limits in mind.
 
or not.

>
Yes, but is it (measured)?  Or, actively designed with as a target?
>
Or, do you just use rules of thumb knowing that the typical *operating*
environment is likely to be X and chose components specified for Y > X
(but not necessarily Y >> X)?
>
Note this iPhone claims a max operating of 93F and storage of 113F.
It is now 108F outside -- does that mean I can't use it outdoors?
And, when the temperature climbs to 115, I'll have to set it in an
ice bath?  (I'm being facetious, of course).
>
I suspect there isn't a place in the lower 48 that doesn't
see ambient temperatures above 93F at least part of the year.
>
And, Apple isn't a garage shop with few design/test resources at
its disposal.
>
So, clearly there is margin in these specifications.  Whether it
is intended -- and to what extent -- is my point.
>
When designing for industrial/commercial applications, we were always
extremely careful to *ensure* our products would operate in the temperature
ranges (and other environmental factors) that we specified.  A fisherman
out on the North Atlantic would be miffed if his kit stopped working
because it was too cold or too hot (or, too much salt spray).  A craftsman
would be annoyed if the tip of his screwdriver sheared off from "excessive"
torque.
>
Similarly, a factory floor can't shut down because it's an unusually
warm day in the shop...
>
Consumer kit *seems* to have a far more cavalier attitude towards
these things.  And, one that doesn't really hold up to close inspection
(e.g., the iPhone).
>
>

The guidelines are derived from empirical data and accelerating
factors are determined. The arrhenius equation comes to mind.

Predictors determine the design guidelines, but, yes, varying
degrees of physical testing is a standard element in design and
product verification.

Commercial considerations of free enterprise have always pushed
human behaviour towards banditry, so what can I tell you?

RL