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NNTP-Posting-Date: Fri, 31 May 2024 11:41:56 +0000
From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Fri, 31 May 2024 04:41:56 -0700
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On Fri, 31 May 2024 11:20:53 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>john larkin <jl@650pot.com> wrote:
>> On Fri, 31 May 2024 00:12:30 -0000 (UTC), Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> 
>>> john larkin <jl@650pot.com> wrote:
>>>> On Thu, 30 May 2024 14:58:36 -0400, Phil Hobbs
>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>> 
>>>>> On 2024-05-30 09:37, john larkin wrote:
>>>>>> On Thu, 30 May 2024 11:29:18 -0000 (UTC), Phil Hobbs
>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>> 
>>>>>>> piglet <erichpwagner@hotmail.com> wrote:
>>>>>>>> On 29/05/2024 17:39, Phil Hobbs wrote:
>>>>>>>>> On 2024-05-29 11:56, piglet wrote:
>>>>>>>>>> bitrex <user@example.net> wrote:
>>>>>>>>>>> Optocoupler datasheets seem like kind of a mess, I try not to use them
>>>>>>>>>>> too often in situations where there's any kind of power budget because
>>>>>>>>>>> other than "shove some relatively huge current through the LED like 5-10
>>>>>>>>>>> mA" it's hard to know what you can get away with.
>>>>>>>>>>> 
>>>>>>>>>>> A light load on the transistor side will definitely reduce the forward
>>>>>>>>>>> current required (and of course slow the speed to a crawl) but who can
>>>>>>>>>>> say by how much while still ensuring the thing will turn on sufficiently
>>>>>>>>>>> to saturate the output?
>>>>>>>>>>> 
>>>>>>>>>>> The CTR varies widely from process variation, varies with temperature,
>>>>>>>>>>> varies with collector emitter voltage, varies with forward current, and
>>>>>>>>>>> the data sheets are full of caveats like "At I_f < 1 mA, note CTR
>>>>>>>>>>> variation may increase" and "Graphs are representative, not indicative
>>>>>>>>>>> of actual performance." ????
>>>>>>>>>>> 
>>>>>>>>>>> Any suggestions for how to approach methodically/mathematically
>>>>>>>>>>> selecting drive current would be appreciated, thank you! ("Don't bother"
>>>>>>>>>>> a valid option)
>>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>> Why do you want to saturate the photo transistor?
>>>>>>>>>> If you don?t you can get much higher speeds out of even jelly bean cheap
>>>>>>>>>> couplers. Even without a base connection it is possible.
>>>>>>>>>> 
>>>>>>>>> Because unless there's overall feedback, running it unsaturated gives
>>>>>>>>> you a beta-dependent circuit that's further dependent on the LED
>>>>>>>>> efficiency, the transparency of the white snot filling the opto package,
>>>>>>>>> temperature, you name it.
>>>>>>>>> 
>>>>>>>>> Cheers
>>>>>>>>> 
>>>>>>>>> Phil Hobbs
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> Sorry, maybe my language was sloppy. I meant keep phototransistor
>>>>>>>> collector from bottoming and reduce C-B miller effect. Not necessarily
>>>>>>>> by rationing photons. Keeping Vce constant by feeding straight into a
>>>>>>>> transistor base is brutally effective. See the post about halfway down here:
>>>>>>>> 
>>>>>>>> <https://electronics.stackexchange.com/questions/136928/under-what-conditions-does-an-optocoupler-work-fastest>
>>>>>>>> 
>>>>>>>> piglet
>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>> 
>>>>>>> If you have the base pinned out, you can do more stuff, true.  But at the
>>>>>>> end of the day you?re still dealing with a phototransistor.
>>>>>>> 
>>>>>>> BITD TI and HP made optos with actual specs, but these days, not so much.
>>>>>>> 
>>>>>>> Linear mode works great when there?s overall feedback, as in your typical
>>>>>>> offline switcher, which has a TL431 to do the actual regulating.
>>>>>>> 
>>>>>>> Cheers
>>>>>>> 
>>>>>>> Phil Hobbs
>>>>>> 
>>>>>> A c-b schottky clamp would help, sort of a 74LS photocoupler.
>>>>>> 
>>>>>> But the really good logic couplers these days aren't optical.
>>>>>> 
>>>>> 
>>>>> Yup.  Even with a better photoreceiver, most of the usual speedup tricks 
>>>>> don't work with LEDs, on account of their diffusion-dominated carrier 
>>>>> dynamics.
>>>>> 
>>>>> Cheers
>>>>> 
>>>>> Phil Hobbs
>>>> 
>>>> I did test a Cree white LED for speed. It hit my detector response of
>>>> about 7 ns, phosphor included. I was surprised.
>>>> 
>>>> 
>>> Yes, some LEDs are much faster than others. 
>>> 
>>> We sell a LED-based pulsed light source that has <6 ns rise and fall times,
>>> using any  of three part numbers at different wavelengths.
>>> 
>>> With a fancy $20 LED, it gets down to 2 ns. 
>>> 
>>> Speedup caps , reverse bias, and so on do zilch to speed it up. 
>>> 
>>> Cheers 
>>> 
>>> Phil Hobbs 
>> 
>> Why are IR LEDs so much faster? A 10 GBPS SFP transceiver module costs
>> $16 from Amazon (with Prime free shipping!)
>> 
>> 
>
>Those are lasers. The carrier dynamics of a laser running above threshold
>are dominated by radiative recombination, which is much quicker. 
>
>Lasers are also designed to avoid the horrible diffusion delay of most
>LEDs—lower doping, thinner epi, and so on. 
>

I have noticed that the capacitances of LEDs vary all over the place,
like 50:1.