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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Operating temperature derating
Date: Wed, 5 Jun 2024 16:57:51 -0000 (UTC)
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On Mon, 03 Jun 2024 15:05:58 -0700, john larkin wrote:

> 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.

Yeah, RF biasing - well, everything to do with RF in fact is a blast, 
isn't it? No wonder the guys at the cutting edge of it have always coined 
the BIG BUCKS.

>>> 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.

Indeed. It's amazing how many decibels you can get out of even a 10uF 
electrolytic.