Re: Operating temperature derating

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