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NNTP-Posting-Date: Thu, 30 May 2024 21:52:39 +0000
From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Thu, 30 May 2024 14:52:39 -0700
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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.