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How Important Is Soft Start?

cncyana

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Is there a pre-made add-on soft start module which can be added to our lasers?
 



Giannis_TDM

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No, and to answer this question once and for all: Soft start is nothing but a cosmetic feature, as long as the current is actually constant without overshoots the diode doesn't give a shit. If you read the datasheets on most diodes you will see that they are made to be able to switch at crazy high frequencies (100mhz!) Aka 0A to say 1A in a few nanoseconds. Also, might I point out, all of the chips used with drivers of mine and by other driver makers always have an internal soft-start in the 0.5ms -10ms range, That is there to help the chip not to overshoot, anything other than that is purely cosmetic.
 

cncyana

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ok, thank you for that explanation. This is good news for me.

I assume the 250millisecond soft start feature of the ACS4500BU driver is nice but not necessary to increase laser diode lifespan.

So, regardless of whether we are using a 50mW laser or a 7W laser:
1) Turning a laser on and off manually using the power button does not decrease or adversely affect laser diode life span, regardless of the number of cycles.
2) Each manual on-off cycle is basically equivalent to a single pulse at 100MHz in terms of electrical stress on the diode.
3) 0.5ms soft start is long enough, time wise, to prevent overshoot because 0.5ms is a longer duration than a single pulse of a few nanoseconds at 100MHz.

A few questions:
1) Is a Lasorb necessary or is it another cosmetic feature in a laser pointer?
2) Besides the temperature of the diode and current overshoot, what other things commonly affect laser diode lifespan?


Thank you.
 
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paul1598419

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Since we normally push these diodes far past what they were designed to do I don't think it is all that simple. Having a soft start can help if it is slow enough as these diodes were never designed to be switched off and back on at these currents at 100 MHz. It isn't just a cosmetic feature.
 

Giannis_TDM

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Since we normally push these diodes far past what they were designed to do I don't think it is all that simple. Having a soft start can help if it is slow enough as these diodes were never designed to be switched off and back on at these currents at 100 MHz. It isn't just a cosmetic feature.
As long as the current is actually regulated right soft start is purely cosmetic not taking into account my previous point, being that soft start is mainly there for the IC to catch up without overshooting, that 100mhz spec refers to the diode's typical forward current but should only minimally change even if overcurrent. Let me remind you telecom diodes switch way faster at way higher currents to accommodate for losses in the fibres. If you still believe that a soft start beyond the timeframe I mentioned will benefit the diode somehow please enlighten me with more than just I think that they were not designed hence they can't.
 

Giannis_TDM

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ok, thank you for that explanation. This is good news for me.

I assume the 250millisecond soft start feature of the ACS4500BU driver is nice but not necessary to increase laser diode lifespan.

So, regardless of whether we are using a 50mW laser or a 7W laser:
1) Turning a laser on and off manually using the power button does not decrease or adversely affect laser diode life span, regardless of the number of cycles.
2) Each manual on-off cycle is basically equivalent to a single pulse at 100MHz in terms of electrical stress on the diode.
3) 0.5ms soft start is long enough, time wise, to prevent overshoot because 0.5ms is a longer duration than a single pulse of a few nanoseconds at 100MHz.

A few questions:
1) Is a Lasorb necessary or is it another cosmetic feature in a laser pointer?
2) Besides the temperature of the diode and current overshoot, what other things commonly affect laser diode lifespan?


Thank you.
1) No because the power button switches the power going to the driver, the driver will always perform the 1-15ms SS when switched
2) No because of what I said above, the driver always soft starts when switched on
3) The diodes themselves don't need a soft start, Again though only if the current regulation is good, the soft-start is purely to help the driver not to overshoot.


1) A lasorb is an esd protection device, always good to have.
2) Current setting, If you run say a 1w diode at 700mw (say 700mA instead of 1A) it will last longer due to the typical degradation taking way longer since you are running it below its designed nominal output and hence stressing out the PN junction less.
 

cncyana

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One more question: What about circuit 'noise'?
I assume that as long as no overshoot is involved in that 'noise', circuit 'noise' in the driver doesn't adversely affect laser diode life either?

Why do driver designers tend to mention 'low noise' as a positive design aspect in their drivers?
 

paul1598419

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The signal to noise ratio is a well known aspect of many circuits. Low noise is always a criterion that all circuit designers work for.
 




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