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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Wed, 12 Jun 2024 12:26:51 -0700
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On 6/12/2024 9:13 AM, legg wrote:
>>>> 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.

I'm not "confusing it" -- rather, deliberately using "100" as a "nice round
number" (note my comment re: 41C vs 40C)  Is it just "coincidence" that
the "safety warnings" are issued at 40 and not 39?  Or, 41?  Is there
something "magical" about "40" -- other than it ends in a '0'?

It's been above 100F (38C) for 16 days, already.  People are still working
their normal outdoor jobs, looking at cars on dealer lots, walking 100 yards
across uncovered parking lots to gain entry to stores, etc.

Are phones NOT expected to work in these environments?  (Gee, I wanted to
telephone emergency services to get help for my buddy who collapsed in
the heat but it was too hot for my phone to make the call)

We expect cars to continue to operate in those temperatures.  What's the
criteria that we use to determine what should and shouldn't be expected
to remain operational?

And, how do these limits differ from the (somewhat arbitrary) figures
that we publish as specifications?

>>> 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.

You don't need to use a bare finger to activate a screen.  And, with most
phones, can use voice dialing.

>> 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.

So, a battery stored 1C beyond that maximum storage temperature
is crap?  Of course not.  For *components*, we have graphs that
make these relationships more explicit.  So, I can trade off
temperature vs. performance, etc.

For *devices*, we rely on a single number to express a limit
(operating or storage).  It's fairly obvious that these
limits are not hard and fast and have fudge factors built in.
How large those are is up to the person designing the product and
writing the published specification.

>> 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.

But that's not true.  Sit outside in 93F temperatures (the published
spec for the iPhones I researched) and you will neither be comfortable
nor safe.  Why doesn't everything operate at 25C which is where we
tend to be most comfortable and feel most safe?  Ans:  because we
live in conditions that extend well above and below those limits.

I can recall being outdoors in -26F and +117F.  I was much more
uncomfortable at -26F than at 117F (I was actually hauling 20T of
stone and, aside from the weight of the stone, more comfortable
than I'd been at 80F in New England)

So, what value writing a published specification limit that
you KNOW will be exceeded, REGULARLY?  Are you looking for a
legal leg to stand on when the user tries to dial "911" and
the phone fails to perform as expected -- because it was 94F
at the time?

>> 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.

But kit designed for the vehicle should be operable IN the
conditions that the vehicle is expected to experience, right?
Or, am I supposed to stand outside the vehicle until the
passenger compartment (which houses the kit in question) drops
to "human limits" (how do I start the vehicle to move it into
the shade?  or, activate the air conditioning?  will the air
conditioning sensors operate in those "above human" limits??)

> What's your actual problem?

If I have a device that claims an operating (or storage) temperature range
of [X,Y] what is the ACTUAL operating range LIKELY *designed* into the device.

Ask yourself what YOUR design process is.  Do you design something and
then put some numbers on it's operating range after the design is
complete?  By measuring it's capabilities?  By SWAG?

Do you write a target specification and then keep tweeking your
design to ensure that it continues to satisfy those constraints?
Do you then verify this, empirically?  Or, do you say, "I've got
12 degrees of margin and the temperature rise in the enclosure
will never exceed 7 degrees so I rationalize that I am safe"?

Why do we derate other attributes of our designs (and components)?
Do we not also derate more general characteristics -- like operating
temperature (clearly this happens; the question is whether or not
there is a conscious effort made and what the criteria are besides
"this is how we EXPECT it to be used")

I specify the TIMEliness of my systems' responses.  There's margin
in those figures.  A customer can rely on my meeting them ALWAYS,
because I have addressed all of the (unlikely) issues that could
cause them to vary.  I *design* to a tighter set of values and
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