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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Thu, 6 Jun 2024 14:01:04 -0700
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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).