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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Wed, 12 Jun 2024 12:13:50 -0400
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On Sat, 8 Jun 2024 13:35:16 -0700, Don Y <blockedofcourse@foo.invalid>
wrote:

>On 6/8/2024 12:03 PM, KevinJ93 wrote:
>>>> 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.
>
>It's that, here, now (103F) -- and another 5-10 degrees expected before
>temps start back down.  And, I imagine a few million people are experiencing
>that same sort of temperature.  If 20% have iPhones, that's a shitload of
>devices operating at or above their design maxima.
>
>Where did the "40" come from?  Why not 41C (was that not "round enough"?)
>Or, "100F"?  I.e., was it arrived at by deliberate thought or just picked
>out of the air as "good enough"?

40C is the ambient temperature where public health safety warnings are
issued in most jurisdictions. Schools will cancel track meets etc. 
Some ammend that to wet bulb temperatures, which can be reached in as 
low as 36C environment.Don't confuse it with the Fahrenheit scale.

>
>> 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.
>
>Makes sense.  But, has limits to its applicability.  I.e., if the CPU
>couldn't support the load of running the cellular radio, then you've
>prevented damage but still rendered the phone inoperative.
>
>> I was working on the display/touch hardware; LCD displays stop working at about 
>> 75 deg C (they just turn black)

You'd probably find that surface touch temperature limits are exceeded
before that - screens having direct access to the external 
environment.

>
>Yes.  The technology has lots of environmental limits.  And, too cold
>and it gets sluggish (not a good thing for an AC device).

LCD screens also become unreadable at ~ -20C. Again - don't confuse it
with the Fahrenheit scale.
>
>> 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.
>
>And the battery's failure mode can be spectacular.  So, do they
>rely on the printed specifications to bail them out of any liability
>lawsuits?

He's talking about aging, loss of capacity and charging voltage 
tolernce restrictions. Lithium battery 'cook-off' temperatures 
are much higher.
>
>Note that you don't tend to see different "grades" of consumer kit
>as you would encounter in commercial/industrial markets -- where
>the consumer can buy an option/upgrade/upsell to address a market
>that he feels more typically reflects his usage.
>
>How many consumers actually are aware of these parameters for
>the kit they've bought (often at very dear prices)?

Industrial grade components and equipment are designed for 
higher operating ambients - but equipment designed for personal 
(hand-held or pocket) use assume the limits for human physical 
comfort and safety.
>
>When we were last looking at vehicles, we noticed many of the
>"front-facing technology" would throw errors, before you even
>made it onto the road for a test drive.  "Oh, the electronics
>are overheating from being out in the sun..."  "WTF?  So, can
>I only drive at night?  And, how many kilobucks for this bit
>of kit??"

Vehicular operating environmental limits differ from consumer 
equipment standards.

What's your actual problem?

RL