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Re: Spice models of laser diodes?
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I know, it's just an interesting point.

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I rather suspect you're barking up the wrong tree with a SPICE model of
a diode laser...it isn't the terminal voltage you care about, it's the
light output, and that depends on a whole lot of optical and thermal
things that SPICE is never, ever going to get right.  They have widely
differing timescales, for one thing, which SPICE is horrible at, and for
another thing, small amounts of optical feedback have a _huge_ effect on
DL performance, including feeding back to the terminal voltage.

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Cheers

Phil Hobbs



--
Dr Philip C D Hobbs
Principal
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Re: Spice models of laser diodes?
5ZRI-3XnZ2dnUVZ snipped-for-privacy@supernews.com:

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During the transitions, perhaps, but in steady state operation there seems to
be a remarkable electrical similarity between a dead diode and a live one.
What matters is the way the diode responds to hard electrical changes on the
input, as that's what makes the ringing and damaging overshoots. This is true
with NO consideration of optical nature, and just modelling that alone,
realistically for real laser diodes, is a lot more than we currently have.
And likely not that big an ask, it's just not been done much, it seems.

Re: Spice models of laser diodes?
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Ringing? I never had that. Don't drive them through a built-in inductor :-)

LDs are usually current driven. The prudent way is to impose a constant
DC current, very well stabilized and equipped with belts, suspenders,
cushions, airbags. Then the fast signals are fed in via a current
"robbing" shunt circuit to ground. That pretty much makes sure you can't
fry it.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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Not intentional.. I'm using an LM317. Yes, I know it's not the approved way.
It WORKS, far better than expected, and most people who explore weird tricks
with an LM317 are mightily impressed with it, from the guy who built a class
A headphone amp, or the radio ham who built a transmitter round one...

My physical circuit build, AND the later spice modelling, both indicate that
an LM317 for a cheap way to get up to an amp-amd-a-half of DC coupled
proportional laser drive at up to 500 KHz is very likely to work well. Maybe
the inductance is in the LM317 model, I don't know where else it can be in my
simple circuit models. What's crucial is that it is the SAME overshoot I saw
in the real circuit so spice is already telling me good things, and I've
already improved the driver on the strength of that spice model.

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Definitely true. Shunting is my favourite method, and I might return to it
with this current indea. But half the fun is trying to see how well I can
push this LM317 idea. I've seen circuits that are 'better' that don't seem
either much better or worse than mine, which also happens to be very polite
at startup, no spikes at that moment at all..

Re: Spice models of laser diodes?
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Ohhhhhh.....  You're using a voltage driver, and trying to trick it into
being a current driver.  Bad, bad, bad, bad news.

Due to the rolloff in the loop gain, the output of a voltage regulator
appears inductive, which will reliably give you a big noise peak if the
output cap is too big, and some ringing if it's too small.  As Joerg
said, you're way better off using current drive.  It isn't difficult,
just an op amp and a Darlington.  Use the Darlington's collector as the
output, and sense the current in its emitter.   Adding an outboard
current limit is easy then too.

Cheers,

Phil Hobbs

Re: Spice models of laser diodes?
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I tried to use an LM317 as a constant current driver for a regular old LED (a
high-power white LED -- needed a few watts; this was >5 years ago when white
LEDs were still new and spendy) and it just sat there and oscillated on me.

You do see people suggesting using LM317s (or similar) as constant current
sources all the time, though... when can it work?  It sounds like you're
saying it's almost always a bad idea?

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Compared to an LM317 though, isn't the main difference that the LM317
essentially has Power->drive transistor->LED->sense resistor whereas Joerg's
approach is Power->LED->drive transistor->sense resistor?  ...so you're
essentially isolating the current sensing from the load itself, to some
degree?

---Joel



Re: Spice models of laser diodes?

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Never seen that, though I know it HAS been seen, the guy who built a
transmitter round an LM317 did so to explore it. I use an LED lamp fopr my
main room light when I'm at the compiter, it uses an LM317 with a voltage
reference and a pot to dupe it to work as a dimmer. It's not very efficient
but it doesn't flicker either like most power converters do, and it's much
cheaper. Solid as a rock.


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I tend to trust manufacturers data sheets. It beats assuming they're useless.
:)

Re: Spice models of laser diodes?
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The LM317 has an inductive output characteristic, which resonates with
the bypass cap.  The reason for the inductive characteristic is that
there's a feedback loop inside.  The output of the 317 is an emitter,
which is naturally low impedance--it's a good voltage source, and the
feedback just makes it a better one.  Outside the feedback bandwidth,
it's still a low impedance because it's an emitter.

Collectors make good current sources naturally--a bit of feedback makes
them stiffer and more accurate, but outside the loop bandwidth they're
still current sources.  Using an emitter as a current source gives you
problems, because the feedback is fighting the natural tendency of the
device--the output crosses over from a high impedance at low frequency
where feedback dominates, to a low impedance at high frequency where the
feedback is unimportant.

It's a bit like a LDO, where a collector is forced to act like a voltage
source, by wrapping feedback around it, leading to similar sorts of
stability problems.

The usual 317 fixed current source circuit uses a resistor in series
with the output lead, and the feedback lead connected to the other end
of the resistor.  That works great for fixed current applications, where
there's nothing to excite the resonance, but if you're using the 317 as
a modulator, you're bound to have trouble with the resonant peak.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal
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Re: Spice models of laser diodes?
That's a great explanation (and I appreciate the connection to LDOs) Phil;
thanks!

I suppose that purposely adding resistance in series between the output of the
LM317 (emitter of its drive transistor) and whatever it is you're trying to
(current) regulate ought to help, but that's considered a rather ugly hack
when it's no harder to do it the "right" way? :-)

---Joel



Re: Spice models of laser diodes?

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If you always do everything the 'right' way, how are you going to learn
anything for yourself?

Re: Spice models of laser diodes?
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I don't really see a connection there.  Certainly there's plenty to be learned
from applying ICs such as the LM317 in unorthodox ways, but there's also
plenty to be learned from studying the "right" way to solve a specific
problem.  Indeed, often the "new" right way comes about from someone
experimenting with the old ways and having a flash of insight.

I'm all for people experimenting and figuring out interesting new applications
for circuits originally designed for different applications, but I'm also all
for examining the works of the likes of Phil and Joerg and Jim since they have
a demonstrated track record of building rock-solid designs.

In a hobbyist environment, one should certainly try out whatever wild ideas
pop into their heads.  In a work environment, while engineerins shouldn't
generally be discouraged from trying out wild ideas :-), they need to be
prioritized based on the perceived risk, time/resources needed to experiment,
other project priorites, etc.  I've seen firsthand someone's clever idea
costing lots of money because the idea turned out to have unforeseen
limitations -- any time you're proposing something wildly different from the
traditional approaches, be sure to budget for plenty of testing time.  On the
other hand, you have John's company, where many of his products are completely
unique based on his ability (and his employees') to successfully incorporate a
few wild new ideas into viable products.

---Joel



Re: Spice models of laser diodes?

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That's exactly how I improve code when I write it, I look at what aorks best
and always revisit to see if there;s a further improvement because there
often is. But there is also context. In several posts here I've accounted for
the context of my decision to try this LM317 wheeze. There's no reason not to
try it, and a few good ones to try it.

Re: Spice models of laser diodes?

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Once I'd never have considered it because it has other troubles to overcome
too, I'd used it solely as a fixed constant current drive in every laser I'd
built in the previous 7 years..

One problem is that the series resistor must be on the high side of the load,
most designs using sense resistors put it on the low side. Another is that
1.5V is dropped across that resistor, other designs use far less, so are more
efficient. And another, as you say, is that to damp the feedback for an ideal
response to changes in a supply or load, they fixed conditions that may or
may not be troublesome to us tinkerers who do weird things with it.

The large voltage drop on the sense resistor is actually a boon, it's far
easier to dupe the LM317 accurately when you have that 1.25V to use. I have
modified the idea to drop 1V across the voltage control network leaving just
0.25V on the resistor, saving one watt per amp, pretty cool if you're doing
this with an LM338. >:) It sucks for fast mod though, stability and solidity
is dire, but it's great for a dimmable low-volt light though, a string of
LED's and that trick, and the efficiency approaches that of a decent switched
power converter.

The high-side sensing seems at first awkward, but it's not that bad, just
feed it to the normal input of a differential amp and do the modulation on
the inverting input. A second stage is needed to set input protections and an
agreeable polarity for response to signals but it's still just one 8 pin
chip.

That internal componentry for response to line.load changes was the thing I
thought would make the concept useless, but that's why reaching 500 KHz with
respectable square waves is so nice a surprise.

Viva the LM317. If I can have 100 of those to do entertaining stuff with it
beats having to agonise over Mouser's lists and paying big tax on many small
boxes. I really aim to do things with a minimal range of parts types, it's a
kind of thing I have.. :)

Re: Spice models of laser diodes?
iIbBSu3XnZ2dnUVZ snipped-for-privacy@supernews.com:

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I thought if constant current config for an LM317 is good enough for the
standard data sheet then it's good enough for me. :) (And if you look at
the LaserFAQ closely you'll also see that it's good enough for Winfield
Hill, though he wasn't trying to modulate it..) Like I said, part of the fun
is in making the LM317 do weird and wonderful things. It's fun to know that
wherever you have some, you can do some amazing things normally done with
other parts, usually exotic, more costly, with MUCH more complex board
layouts, etc..

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That fits what I see. I've managed to tame it to something respectful. I
think an 'overshoot' that results in a minimum-to-maximum deviation of
about 3 mA along the 'flat' top of a 500 KHz square wave at 160 mA isn't bad.
People who know a lot more than I do have been content with worse.

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That's what I intend to try too, though I'll try a MOSFET rather than a
darlington. I take it the darlington is to avoid the gate capacitance of a
MOSFET? I can see that it will work because its total Vf will be less than
the laser diode's own. (A quirk my own circuit is exploiting, in a different
way).

Re: Spice models of laser diodes?
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I really like Darlingtons for that job, because they have a lot of
transconductance.  That makes the current source stiffer, especially at
low V_CE where MOSFETs start to crap out.  Their capacitance is much
lower, which is helpful with massively nonlinear loads like laser
diodes.  Also, the V_BE of a Darlington is much better controlled and
less drifty than the V_GS of your average MOSFET.  Their betas are
usually around 10,000, which means that they're as accurate as the sense
resistor anyway.  Quiet, stable, and predictable--just the ticket for
diode laser drivers, I think.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
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Re: Spice models of laser diodes?

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I've not used them often enough. Always liked the idea though, and I agree
that predictable base current and voltage helps a lot. But unless I'm missing
something, they make great switched but lousy proportional controllers, no?

Re: Spice models of laser diodes?
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You can build a current drive out of a LM317, I've even used it as an AM
modulator. But 500kHz, nope, it ain't going to go quite that far.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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Want to bet? >:) I grant that it won't go further, I tried the model at 1 MHz
and it degraded to crude sine waves, but 500 KHz is ok. As far as I can tell,
the main weakness seems to be a small range of acceptable capacitance on my
controlling op-amp's feedback loop so parasitic capacitances alone may, or
may NOT, make it work well. And it's very dependent on the diode too which is
why I want an LD model. But when I built my first real circuit for this
design a few months back the LD showed characteristics similar to the 1N4148
model which are much more favourable to damping high frequency ringing than
if it had been a 'hard' diode like a 1N4005 that barely responds with Vf
changes to current changes and forces. I think LD's do vary Vf a lot with
current so they damp the ringing well enough.

But I'll know more when I've ate the pudding.

Re: Spice models of laser diodes?
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It can work but as Phil had mentioned the LM317's stability becomes iffy
beyond a certain frequency when operated as a current source. It's like
taking a small car past 120mph.


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Remember, laser diodes die quietly. There is no tchk ... *phut* but
there is that painful hit in the bank account.

--
Regards, Joerg

http://www.analogconsultants.com /

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Re: Spice models of laser diodes?

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Ithink I posted enough about that to make it clear I know about that pain.
I'll try it on cheap diodes and take it from there.

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