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From: KevinJ93 <kevin_es@whitedigs.com>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Sat, 8 Jun 2024 12:03:49 -0700
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On 6/8/24 4:17 AM, Don Y wrote:
> On 6/7/2024 9:25 PM, legg wrote:
>>> [Given that using ANYTHING beyond its operating limits leaves you
>>> without a leg to stand on, it would be nice to have some idea as to
>>> what a reasonable expectation for those limits might be, despite
>>> the "fluff" on the spec sheet.  E.g., I would be really pressing my
>>> luck to use something at 80C in that most components would likely
>>> not be specified at those extremes.  But, 55C for a 50C-specified
>>> device?  60C?]
>>
>> Given that you expect to get what's promised when you pay for it,
>> outside of banditry, I only expect performance and a reasonable
>> service life.
> 
> But performance is only formally defined within the "operating conditions".
> And, many bits of consumer kit don't even specify these!
> 
>> I often kick mysellf in the head when I realize that shortcomings
>> in product performance were actually predicted in the written
>> specification - that the performance that I was expecting was not
>> only outside published spec, but might not be physically
>> possible, using current materials and techniques.
> 
> Would you expect a cell phone to operate in the same nominal environments
> that a human being would encounter in their normal living?  If it FAILED
> to operate "above 93F" (which is likely most of the lower 48, at some
> portion of the day, lately), you'd likely be looking for another device
> as you would always have to be in an air conditioned environment to make
> that guarantee.


I used to be in the iPhone design team. At the time we used a 40 deg C 
as the maximum ambient temperature.

As part of the development we would run a "thermal virus" software to 
cause the CPU to dissipate an approximation for the maximum possible.

Under those conditions the internal temperature could get to the 70 deg 
C region. If excessive temperatures were reached the CPU would be 
throttled to avoid damage.

I was working on the display/touch hardware; LCD displays stop working 
at about 75 deg C (they just turn black).  Since the display was within 
a couple mm of the CPU there was not margin.

Under less stressed conditions the internal temperature was much lower.

I suspect that the battery is probably the most sensitive item for 
storage temperature while not operating, especially if fully charged.

kw

> [And, if *storage* above 113 was contraindicated, how many could leave
> it in an unattended/garaged car?  You KNOW that you can't store *ice*
> above 0C and, thus, don't!]
> 
> So, what value writing those limits on the formal product specification?
> Obviously (as we haven't seen class-action lawsuits from folks claiming
> their thousand dollar phones don't work in the Summer months) the devices
> work beyond their stated operational limits.
> 
> Is the limit published solely to give the manufacturer a legal "out"
> if large numbers of units suddenly DO start failing?  While that may avoid
> a lawsuit, today, how likely would it be to endear future purchasers to
> a product if such a constraint became well-known?
> 
> Said another way, why not just *say* "25C" and hope customers are just
> as oblivious of the actual declaration?
> 
> *Or*, say 42.1C after a careful analysis of the design and its actual
> operating temperature limits?
> 
> [I just don't see value in these numbers as they stand, currently]
>