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Thread: flexmod P3.3 catastrophic failure

  1. #61
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    Aug 2010
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    Quote Originally Posted by VDX View Post
    ... I've built my first drivers around current controlling with the LM317/LM338 IC's because all other available 'modern' drivers with DC-DC-converters had overcurrent spikes which would have killed the laserdiodes over time (or pretty fast).
    Attached is a DSO-image of a driver with DC-DC-converter, set to 100mA output current and measuring the voltage drop across a 0.22 Ohm resistor as load when switching the output off - a peak with 6 Volts shows a current peak of 27 Amps !!! (I = U / R)
    With the laserdiode it gives nearly the same pulses, but 'only' 5-10Amps peak when set to 1Amp output
    Attachment 49866
    Viktor
    Hey Viktor, I have a few specific questions about this somewhat alarming post !

    You mention "modern" drivers with DC-DC converters.... (this has been playing on my mind lately) ....are you referring to the supply to the driver being a DC-DC convertor........ or the driver itself incorporating this technique ?

    Unfortunately I cannot see clearly from the "DSO" image the exact pulse width of the spike you have measured.

    I am currently using cheap switch mode supplies followed by (also cheap) DC-DC converters to cut down the voltages and then using a classical type of driver (eg. die4drives) and relying on just lasorbs to protect the MM diodes (mitsus, NUBMO's etc.) from any spikes, like the one you outlined above.

    Do you consider that it is likely that I am getting spikes, so narrow that my old scope (no spec available) won't be able to see them ? ...and that I am slowly battering the diodes to death !

    Do I need a new scope?...... and if so how fast does it have to be, to catch these flashes, before I incinerate a load of expensive laser diodes (albeit over time)?

    Can I insert some kind of reactance/capacitor, to avoid these spikes, and if so what values should I be exploring so as not to interfere with typical diode modulation frequencies used in QS controlled laser shows .

    Any feedback would be gratefully received, if you have the time !

    Cheers

  2. #62
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    Oct 2012
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    ... the base of the spike in the image is somewhat around 100ns wide, so you'll need a scope with 50MHz to see it clear ... but a scope with 20MHz and a good trigger electronics could be enough to get the spikes too.

    Nearly all MOSFET drivers without or with only small coolers use some sort of DC-DC conversion with small coils and around 100kHz converting frequenzies - my guess is, that most of them will generate spikes when switching powers.

    Capacitors will reduce the spikes, but they too reduce the maximal possible bandwith - so most high current drivers are limited to some ten to 100kHz switching resolution.

    I'm actually driving my diodes with 200kHz to 2MHz switching resolution and have some applications with up to 20MHz, so try to design my drivers without any spike filters

    Viktor

  3. #63
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    Nov 2014
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    on one driver i built to run a car ignition coil i ran into a problem where i added inductance with some capacitance at the input caused a lager spike, my guess is that it caused ringing at the input to the driver and i could only see it on a fast scope so i lowered the values of the parts on the input side, i also added a lasorb to the input of a programmable timer, i use it as a trigger transformer for a flash lamp, i did that after i blew a timer out and so far this has worked to protect the timer. you can find oscilloscopes used on ebay and some other sites as well, what i use is a Philips/Fluke combi scope as they called them i have a PM3394 that i got for less then 150 bucks, the reason i like the combiscope is it has both analog and digital functions in one unit, new they where quite expensive but since people like scopes with an LCD display the value has gone down a lot on these units, mine is a 4 channel 200mhz but they did make them in 2 channel and lower frequency, the 3394 was there highest one i believe
    http://www.ebay.com/itm/Fluke-PM3394...0AAOSwl9BWJ-HN

    That's the model i have but using the search name of combiscope gets hits on most of the product line, i used one at work in the past and fell in love with it, i have two other digital ones,one is a spare and i can fix the second one i am going to try and sell it, the company that i got it from did not package it right and got it in kit form if you will, the common fault i find with these is a capacitor on the high voltage line shorts and will load down the supply to the point it may not turn on and if it does turn on you will not have any display, it's "usually" an easy fix
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