Optocoupler datasheets

El 29/05/2024 a las 17:15, bitrex escribió:

CTR reduces with age, so you must leave some margin.

Right, that's a good idea.

Situations where they're just left on for DC most of the time and trying to get minimal forward current by optimizing a resistor value seem hopeless.

Could wrap a DC feedback loop around it which would only make sense in the situations where you have a lot more power available on the secondary side for some reason, hard to think of a use case.

Logic-output couplers seem somewhat better specified in the sense they explicit state the relatively high minimum forward currents or all bets are off.

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

Not really answering the question, but there's more than one way to skin a cat...

SMT optocopulers don't cost much more than discrete transistors, so they can be useful sometimes for level shifting, current flipping, reference shifting, and other situations that don't strictly require isolation.

It would be cool if there were some cheap implementation of an abstract element you might call something like a "current flipper", with a predictable CTR. Current goes through one side and comes out flipped and/or scaled on the other.

The current mirror is sort of like this but it tends to work best placed referred directly to ground or the supply, not "in between" stuff. Also it usually tends to need too many parts to do anything useful, to make it regularly worth using with discretes.

You can also turn the LED off a bit faster and reduce off-time interference/noise by pulling the anode low, activiely. Not sure how much better it is though.

boB

I thought IREDs and LEDs are very fast, far faster than the phototransistor but someone has done as you suggest, see:

piglet

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:

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

Interesting approach, John.

Piglet, the first comment says where I first saw this which was from Avago/HP and their IGBT driver chips.

boB

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

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