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NNTP-Posting-Date: Mon, 03 Jun 2024 22:05:58 +0000
From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Mon, 03 Jun 2024 15:05:58 -0700
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On Mon, 3 Jun 2024 21:41:59 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>On Mon, 03 Jun 2024 09:03:10 -0700, john larkin 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?
>> 
>> I don't think that there are any standards that manufacturers use to
>> make datasheet abs max specs.
>> 
>> I like to test parts to destruction and estimate what they can actually
>> do. For example, max voltage ratings can sometimes be safely exceeded,
>> or a surface-mount resistor can dump a lot of power if it's well heat
>> sunk with copper pours.
>
>Some fellow on Youtube a while back managed to get a kilowatt out of a 
>TO-220 packaged device rated at 100W by blasting it with plumber's Arctic 
>Spray.
>
>> Thermal imaging can quantify hot-spot temperatures.
>> 
>> Pushing parts past specs is only a good idea if there is a serious
>> performance upside.
>> 
>> RF parts are especially likely to be under-rated. Like a 2 volt rated
>> schottly detector diode that starts to leak at 7. Or RF amp parts that
>> actually run at 2x rated max voltage in normal use.
>
>Probably because it's common in RF to end up with the part being subjected 
>to voltages double its normal supply rail figure.

When I use RF parts in time domain/pulse applications, I have to make
up my own rules.

It's rare to get even basic DC curves on RF parts. The data sheets
say, literally, adjust the bias until it works. Or AC couple in and
out and don't ask questions about "volts". Which opens up the
possibility of novel biasing schemes.

>
>> 
>> GaN fets are interesting. As are mosfets that avalanche at some
>> unspecified voltage.
>> 
>> A bit of air flow can really change things. I assume that most specs are
>> based on still air.
>> 
>> I've seen a few parts that were unreliable within their abs max specs
>> and needed to be derated from there.
>> 
>> Blow up some parts. It's fun.
>
>Best fun you can have with your clothes on as they say.

Clothes on and safety goggles.