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How Important Is Soft Start?

paul1598419

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Looking at this anecdotally, the BDR209s can be heat sunk and brought up slowly to levels higher than 1 watt. It can be done many times without failures, but trying to drive them at currents approaching 1 amp kills them quickly. This alone would suggest that having a slow start current rise might indeed help in keeping them from failure along with the decreases in drive current we've seen over the past several years.
 



Cyparagon

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Have you definitively ruled out a shit startup waveform? Lead inductances? The DIY drivers I see floating around here these days are a bit sketchy sometimes.

"The plural of anecdote is not data." At best, it is a starting point with something to investigate.

I know that high power Cmount 660 diodes need soft start.
Gonna need a source for that. Is it in the datasheet, or is it something you heard from a guy who has a friend that knows a guy that had one?
 

Coonie

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Gonna need a source for that. Is it in the datasheet, or is it something you heard from a guy who has a friend that knows a guy that had one?
I'll look for it. It was about concerning those 5w FAC cmounts. Remember those? Lazeerer made a build with one. Evil shadow I think it was called. He mentioned the softstart and how he wanted to make a driver with it for this.
 

paul1598419

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Since I use lazeerer's drivers almost exclusively, yes I can rule out crap drivers and I have always soldered the driver to the diode leads, so no stray inductance. To do a real study of these over-driven 405nm diodes one would need a sample set that was large enough to make the data statistically significant. Who would take on that challenge just to learn exactly why these diodes tend to fail? No one would. Use a soft start driver or don't. Drive your 405nm diodes at 500 mA or some value higher than that. I have given my experience after losing several of these diodes. Accept that or don't. I'm done.
 
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paul1598419

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Yes, all the studies on rise time for laser diodes is about getting them to switch faster, often in the 1% range of on time and at threshold currents. No one ever does one to see if they can get drastically over driver diodes to last longer. Because in industry these diodes aren't over driven at all and rise time is spent trying to get them to switch faster not stay on at higher currents.
 

RedCowboy

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Sorry paul, opinions don't equate to facts, you need actual data, science is not political and you can't baffle physics with bullshit.
 

paul1598419

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My point is still valid that my conclusions cannot be said to be false only because no one has done a study on this. That is nonsense in and of itself. You can only conclude that there is no proof. To state that I had been "clowned" out of the conclusion that these do work is just your opinion and stated in a way to make it look like you somehow got over on me, but that is not the case. There is legitimate evidence that bringing the current up more slowly can affect the ability of these diodes to remain functional and after trying different approaches to mitigate these failures that is the conclusion I have made. You can disagree with it, but not having done a study yourself you can't claim my results are false either.
 

RedCowboy

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Just because you haven't been proven innocent doesn't mean you are guilty and just because your opinion hasn't been proven not to be a fact doesn't mean it is.......YOU claimed this was so........you should say it seems like xxxxxx or in my opinion xxxxxx but to say it as a statement of fact is your mistake, you are wrong and you have yet again been exposed as wrong......... ohh and Hillary lost, deal with that too. :ROFLMAO:
 

Cyparagon

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You can disagree with it, but not having done a study yourself you can't claim my results are false either.
This is SO simple; I don't know why this is so hard for you to understand, Paul. Maybe an analogy would help.

There is a large jar of jelly beans before us. We can all agree the number must be either even or odd. There is no third option. You make the claim "there are an odd number of jelly beans" to which we naturally reply, "prove it."

If your best attempt at defending your claim is "no one is going to count all of those, and you haven't proven the number is even, so I'm right," you have all your work still ahead of you.
 

LSRFAQ

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Ever tore into a DVD burner driver? Trust me, the DC bias comes up sloooowy... It may not be 15 seconds, it may be on the order of milliseconds, but it IS there.
The soft start is built into the current mirror startup circuits in the chip itself.

Transient analysis is a huge part of Electrical Engineering training. It may not be stated on the data sheet, but it is assumed you learned to get control of your circuit's startup state in school.
If not, you'll learn about it quickly in your first design review at your first employer.

I know I was white faced when I watched an audio design engineer start penciling in roll-off caps on one of my first commercial designs where I didn't think I needed them. He said "Well, if you had not already bypassed the power correctly, I would have fired you on the spot". He spent a long, long, time with the calculator penciling them in. From the stares of my new co-workers, I received the message. Later he told me the other reason he continued with my employment was that I at least prototyped unit one, and he had stopped by to listen to it without my noting... I am self taught... I do have a degree, but not in what I do.

I had worked really hard to ensure I had a clean start, but I assumed one input reconstruction filter with roll-off between the audio DAC and the amplifier chain was enough. Zorched in the first thirty seconds in design meeting one...😊 Some place there is a DOD spec on audio roll off I was not aware of, and this was to be a low cost, mass produced, product for DOD. Needed or not, the spec calls for HF bypass to limit turn-on thump. Not good to tell the other guys with guns, where you are on start-up.

I have a large pile of dead, single mode, red diodes on my desk where what is widely regarded as the best low cost analog driver in the world had a bad batch. Culprit after staring at the board for hours, one slew rate limiting cap, was too small in value.... Second problem, I assumed that because the past ten drivers were good, the next four ordered would be fine. Moral of the story, incoming test is important. Usually I would have tested each driver with a dummy load and a scope, then with the sacrificial bench diode. Instead of taking care, I put a one Ohm resistor on them, set the Imax with full input and a current limited bench supply, and hurriedly hooked them up. Diodes blew before I ever even saw any laser light.

The diodes stay there to remind me, that watching a whole paycheck vaporize is not to be tolerated. All of them have a mirror faucet blown off.

My point is...

You may not see it on the data sheet but it is there by implication.

Listen carefully starting at 4:05.. You need to make sure the control circuitry is stable before applying power to the diode.












Steve
 
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Buffo

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Interesting to see this old thread come back to life after 6 months! :)

Steve, I agree 100% with your comments about previously-bullet-proof drivers suddenly experiencing a round of failures. In my case it was due to the manufacturer substituting the wrong current-sensing resistor on the pick-and-place line so they could save a few pennies. They swapped in a wire-wound resistor which added a tiny bit of inductance to the output circuit., and of course they didn't tell the supplier about the change. After I killed several diodes with one of these drivers I broke out the 'scope to see what the hell was going on, and sure enough I found these huge current overshoots on the rising edge.

Of course, convincing the supplier that there was a problem with their heretofore "perfect" driver took some real effort. At first they actually insinuated that I didn't know how to use an oscilloscope properly, but after I cut open one of the resistors and sent them a picture of the wire-wound element inside they got the message. Then they got in touch with the manufacturer who admitted to the substitution error. So yeah, things can change without your knowledge and you won't know until something fails.

To the larger topic of whether a soft-start is absolutely needed or not, I would say this: In a perfect world where the driver always functions exactly to spec and all other components never deviate from their tolerance values, then yes, you don't need a soft start. HOWEVER, in the real world there will always be variations that you can't account for, so having a soft-start is like buying insurance. You don't need it until you need it, but at that point it's too late.

Adam
 

WizardG

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LASORB Google it.

Most of our diodes have gotten cheap enough, and the drivers we use good enough, that we don't usually think about ESD and spike protection. But these work and they're relatively cheap.
 

Buffo

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Ahhh, no - not quite. :) Lasorb is not the same as a soft start.

Lasorb is designed to protect against static shocks. It does not trigger based on current, or on voltage for that matter. Rather, it triggers based on how fast the applied voltage is changing. But the rise time of the transient needs to be *very* fast (Ghz) before Lasorb will trigger. You can still kill a diode with over-current spikes even with Lasorb correctly installed, provided the rising edge of the spike is slow enough, which it absolutely will be for most transients that are caused by a poor driver. (Those spikes will be in the range of KHz to Mhz.) Note that this also means you can kill a diode with Lasorb installed by simply turning up the current manually until the diode blows.

For the record, two of the diodes that I killed with the faulty driver described above had Lasorbs installed.

Don't get me wrong: the Lasorb is a fantastic product. It's cheap, and it's very good at doing what it does, which is to protect diodes from static shock damage. It's less important with higher power multi-mode diodes, but if you're working with single-mode diodes you would be foolish not to include the Lasorb in your design. But a Lasorb by itself will not save a laser diode from a poor driver that has spikes or overshoots in the output.

Interesting fact about Lasorb: it *must* be installed within 1 inch of the diode to be effective. (The closer the better.) How did Pangolin learn this? By extensively testing diodes with a static gun. (!) And one of the things they learned was that even if you shorted the anode and cathode of a laser diode together using braided copper strapping (which is ideal for static dissipation), if the length of the braid exceeded 10 cm you could still kill the diode every time with just a single shot from the ESD gun. Think about that for a moment: the anode and cathode are electrically shorted together and the diode still dies if you hit it with static!

Bill discovered that even with just a 5 cm length of braid shorting the diode you have a 50/50 chance of killing the diode with a single ESD shot. So to be sure Lasorb can act quick enough the spec calls for a maximum round-trip path from Lasorb through the diode and back to be less than 5 cm, which is 2 inches, thus Lasorb must be installed 1 inch or closer to the diode.

Adam
 

WizardG

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Thanks Adam. I learned something new today! I wasn't thinking of lasorb as a replacement for soft start but I had been thinking they might be helpful with 'sloppy' drivers.
 

Ramsey_innovations

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Soft start is primarily used to prevent inrush current. The inrush current ( or instant spike @ startup) can effect the inductor on some dc to dc regulators(or "drivers" in the case of laser diodes) causing a momentary output that's outside the specified range of the regulator. Even regulators with a smoothing capacitor on the output can still exhibit inrush current that could kill a diode depending on how the circuit is designed and lots of other influences like the temps of components on the regulator. High pulse frequency drivers are designed specifically to be able to mitigate inrush current while maintaining the ability to switch on/off rapidly. This is partially by implementing larger capacitors on the output as well as the design of the circuit layout and component selection itself.
This link isn't related to driving laser diodes, but it does provide some applicable info.
https://circuitdigest.com/tutorial/what-is-inrush-current-and-how-to-limit-it

Personally i would recommend soft start for this application. Somewhere around 250ms to 500ms is usually good enough.

Be safe and happy lasing!
 
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