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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Optocoupler datasheets
Date: Fri, 31 May 2024 11:20:53 -0000 (UTC)
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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. 

Cheers 

Phil Hobbs 

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics