Fast but small optocouplers?

Phototransistors are horrible, as I find I've said before. A photodiode optocoupler can be a good deal faster, but you need an amplifier on the isolated side. Even a homemade phototransistor might work--you could use a quick transistor with a Baker clamp, for instance.

Cheers

Phil Hobbs

You could consider my almost-famous totem-pole optocoupler thing

(this one modified for speed and low power consumption)

Transformer plus DC restore wouldn't be bad, if you can keep the max duty cycle a bit below 100%.

It's going to be tough in the area you're budgeting. What's the load?

"almost-famous"? Bwahahahahaha! Almost-infamous is more like it.

Except the floating bases on the phototransistors cause dramatic Miller feedback and kills the speed... unless you use outrageous peak currents.

(Tested and verified.)

The Baker clamp is nice but all the tiny ones I found had only four pins, so no access to the base :-(

So post an idea, instead of your usual whining. What is this, s.e.d. or s.e.w.?

And I put a cap there so that there would be suitably outrageous peak currents but low average current.

Contribute something substantive instead of clucking.

I'm talking about using a photodiode optocoupler, not a phototransistor one. With phototransistors, your're doomed.

Cheers

Phil Hobbs

Well, I don't really want to build my own optocoupler :-)

I can, and thought about that because I've used this on medical gear. But even really little transformers are big when they come with isolation ratings. Plus I'd need more stuff to restore a clean DC, just a rectifier wouldn't be accurate enough. We are really space-constrained on this one.

It's essentially hi-Z inputs (

Googleing found this:

SWITCHING CHARACTERISTICS PARAMETER TEST CONDITION SYMBOL MIN. TYP. MAX. UNIT Rise time VCC = 10 V, IC = 10 mA, RL = 100 ? tr 3.5 ?s VCC = 10 V, IF = 16 mA, RL = 180 ? tr 1 ?s Fall time VCC = 10 V, IC = 10 mA, RL = 100 ? tf 14.5 ?s VCC = 10 V, IF = 16 mA, RL = 180 ? tf 20.5 ?s Turn-on time VCC = 10 V, IC = 10 mA, RL = 100 ? ton 4.5 ?s VCC = 10 V, IF = 16 mA, RL = 180 ? ton 1.5 ?s Turn-off time VCC = 10 V, IC = 10 mA, RL = 100 ? toff 29 ?s VCC = 10 V, IF = 16 mA, RL = 180 ? toff 53.5 ?s

Never mind !!!

Can you cite where a part will survive your "outrageous peak"?

I did, I said your concept has problems. It does, but you will never admit. Show us, with a suitable opto part number, that it meets Joerg's edge speed requirement... and survives.

And stop with the "clucking"... you're exposing yourself as a mental midget with limited vocabulary ;-) ...Jim Thompson

With photodiode ones I am doomed as well, then I need too many parts on the receiving end to turn the PWM into clean DC. No available space.

Unless you believe in Larkin's totem pole phototransistor folly ;-)

...Jim Thompson

:-)

RL of 180ohms and IF 16mA is pretty unrealistic in real life, that sounds like written by marketing EEs. Plus it's still too slow.

I had in mind using ordinary couplers here. Given no base resistors allowed, this circuit at least switches one opto on hard against the other one running at low current. Might make 1 us edges.

Nasty problem. You'll need a higher-order filter to get that sort of step response, and 100KHz PWM likely isn't fast enough. I glad it's not my problem; I've got enough impossible things going on as-is.

One way to do fast PWM is to have the two optos (or transformers) drive a cmos flipflop, so the data is edge-sensitive, not level sensitive.

Don't give in to the coders!

Put parts on both sides of the board!

Demand more board area!

Even a baker clamp as simple as it maybe can also be a challenge to implement if you're trying for a clean switch.

The last one I did use high speed S diodes with low cap and a

-0.5 supply for the emitter to get it down close to 0 volts and the use of RC lead and lags to remove parasitic on the corners.

Jamie

On a sunny day (Sat, 01 Sep 2012 10:45:05 -0700) it happened Joerg wrote in :

Use small transformer, send impulse only, use oneshot with fixed pulse and lowpass to make DC again at rx end.

No need for a one-shot, a comparator will do (NRZ scheme). But unfortunately transformers with isolation ratings are large.

Good idea. But unfortunately the usual small ones are still well above

Well, you can goose a PWM if needed. Like the kick-down switch in some automatic transmission cars.

But they also need to have a life :-)

We already do, and it's all full. Like our drill sergeant said "The day has 24 hours, and if that ain't enough you've still got the night!"

They have to be retrofitted into a given space, no luck there.

On a sunny day (Sat, 01 Sep 2012 11:24:49 -0700) it happened Joerg wrote in :

The sort of transformer I would use would be just 1 turn insulated wire to ???spec in a ferrite bead. Whats you insulation voltage spcec? I mean in that case it is specced for the wire insulation.