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Thread: Analog laser diode drivers?

  1. #21
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    Oh Yes, The Fet capacitance..
    I purposely held off on that ...hehe.

    True, Not much inductance in the diode..just the supporting wiring that im worried about as the lengths can start to influence the switching some.
    not much..also at my location I have to worry able RFI , I am a 1/4mile from a 100kW AM broadcast facility. As well as my own radiation from my own ham amplifiers.

    Good luck finding a suitable TO-220 style..
    Ive been hunting for something that was not surface mount for some time now.
    If we can find a FET with the same characteristics as the 2sk2504 or the
    IRLR3105 in a TO-220 or something we could bolt-down that would make me a happy camper. I dont relish the thought of adding metal to a SMD.
    Ive tried the IRFz20, 630, 720, 730, 820 and 830 and didnt like the performance.

    Lately Ive been working on a better solution to DIY drivers and replacement DPSS analog drivers.

    Yes, The FET saturation is very important. As well as the slew rate of the op-amp.
    Last edited by marconi; 08-05-2007 at 11:26. Reason: like to edit
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  2. #22
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    I'm actually looking for a replacement MOSFET which is capable of delivering higher current in TO-220 package as I'm thinking of building a driver for my DPSS unit as well. Do you think MTP6N60 is a suitable candidate? I have many of this type .

    EDIT: Oh, I have IRFP450 in my possession, but I have a feeling that it's not going to be compatible. Here is a list of all the MOSFETs I have: IRF520, BUK456, BUK457, SSP60N06, MTP3055, BUZ71A, BTA12, BTA10, BTA08, BT139, BT137 and BT136. Any of them going to be compatible? They're old parts, so I'm guessing nope. I usually use them to switch high loads, such as 600W globes, etc.
    Last edited by VaThInK; 08-05-2007 at 12:04.

  3. #23
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    I'll watch out for op-amp slew. My current test was with the LFO output of a MIDI to CV controller. Not a stringent test, but it switches fast, and it's easy to get repeatable digital control for accurate levels. If slowing the slew helps I should have an easy time of it, I got by on levels alone. I think that IRF630 might have added plenty slew of its own though.

    I'm after an IRLR3105PbF now, I'm much taken by the specs, lower on voltage, and it looks a lot more capable of linear mod than the IRF630. I think getting the IRF630 was a gaff, even though I'm not entirely to blame for making it.

    Edit: I don't mind an SMT device, those aren't so small they can't get rid of heat. Soldering with indium might help, it has a lower thermal resistance that standard solder, as well as being safer to rework, at lower temperature.
    Last edited by The_Doctor; 08-05-2007 at 11:49.

  4. #24
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    Quote Originally Posted by VaThInK View Post
    I'm actually looking for a replacement MOSFET which is capable of delivering higher current in TO-220 package as I'm thinking of building a driver for my DPSS unit as well. Do you think MTP60N06 is a suitable candidate? I have many of this type .
    Got a datasheet? Google offers over 100 bloated promises, and NO datasheet at all. That's not a good sign, it usually means a part is obsolete and dodgy outfits are trying to coax money out of people desperate to lay hands on them.

    Edit: I think your IRF520 might be useful. Check for low gate capacitance, low on resistance, low turn on voltage, and a decent stretch of roughly linear line for drain current for the first volt or so above about 2 to 3 V if you're after analog mod with it.
    Last edited by The_Doctor; 08-05-2007 at 11:58.

  5. #25
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    Quote Originally Posted by The_Doctor View Post
    Got a datasheet? Google offers over 100 bloated promises, and NO datasheet at all. That's not a good sign, it usually means a part is obsolete and dodgy outfits are trying to coax money out of people desperate to lay hands on them.
    Sorry, it's actually MTP6N60. My bad. The only data sheets I have are only for MTP6N60 which is equivalent to BUK457 apparently, SSP60N06 (man, I get so confused with these numbers. too many "0"s and "6"s) and IRFP450.

    EDIT: Info on IRF520:
    Ciss: 330pF typical.
    Rds: 0.23Ohm typical.
    Vgs: 2.9V typical.

    I'm not sure about the graph, so here is the data sheet: http://www.futurlec.com/Transistors/IRF520.shtml
    It's for different manufacturer though. Mine is made by IR not SGS.
    Last edited by VaThInK; 08-05-2007 at 12:11.

  6. #26
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    "MTP6N60 Datasheet pdf - OLD PRODUCT: NOT SUITABLE FOR NEW DESIGN"

    First line on Google for that amended part number.

    I guess it's down to looking for the parameters you need. I wish I knew what way to simplify the search according to device category, but I don't, it might be best to look for example applications to see what people are using, then look at a few to see what common category they have.

    What seems odd is that most seem to be for switching. Odd choice for linear analog mod, surely? They seem to have a narrow range of dubious looking linearity at best.

  7. #27
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    Quote Originally Posted by The_Doctor View Post
    "MTP6N60 Datasheet pdf - OLD PRODUCT: NOT SUITABLE FOR NEW DESIGN"

    First line on Google for that amended part number.
    LOL , I thought so. I have them in my possession for about 7 years. So yes, they're OLD .

    Quote Originally Posted by The_Doctor View Post
    I guess it's down to looking for the parameters you need. I wish I knew what way to simplify the search according to device category, but I don't, it might be best to look for example applications to see what people are using, then look at a few to see what common category they have.

    What seems odd is that most seem to be for switching. Odd choice for linear analog mod, surely? They seem to have a narrow range of dubious looking linearity at best.
    Yeah as I said in my previous post, I mainly used them to switch high loads such as globes and such. I guess this world is way too advanced for them . Too bad I have to let them rest again after waking them up from 7 years of sleeping.

  8. #28
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    Yikes! - I was meaning to reply to this thread yesterday - seems like it has exploded since.

    First the sales pitch - my drivers are £20 GBP built, tested & set to a requested current. Bare boards are £5 GBP.

    As for safety the input is clamped to the positive supply so you can't put more than 5.7V in to the drive input. That still won't help you if you manage to short the input to a 12V rail elsewhere but it will save you from missing grounds or long floating input cables.

    The only thing I would say for a top side current source and a modulation transistor starving the laser diode is that the topology lets you have one side of the diode at 0V Ground rather than one side at +supply and the other 2V or so lower than +supply. This greatly simplifies mounting especially with multi-diode setups.

    The best TO-220 MOSFET I have seen in a while is the On-Semi NTP30N06L.
    http://www.onsemi.com/pub/Collateral/NTP30N06L-D.PDF
    Vgs_th of typ 1.7V max 2.0V
    Transconductance of 21 mhos
    Input capacitance of only 810pF
    Reverse transfer capacitance of 80pF
    Quite a brute force mosfet but with the drive requirements of a lightweight.

  9. #29
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    According to the MTP6N60's Transfer Characteristics graph, there seems to be a much better linearity. Gate capacitance is less than that of IRF630, and surprisingly, HALF that of the ones Robin is using in his Die4drive. I thought low gate capacitance was important, but maybe not in that case. I guess anything lower than 1 nF is low. I don't know enough to comment further, I need to try a few different FET's... Linearity (or lack of) is the thing that really bugs me.

  10. #30
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    The Die4drive uses IRLR3105PbF which has an input capacitance of 710pF.
    The 3105s reverse transfer capacitance is only 28pf which means it would be faster switching a load from supply. With the Die4Drive circuit though only the input capacitance is significant because the voltage movement on the drain is so small. This means the 30N06 would perform just as well. The 30N06 is extreme overkill for a DVD writer diode but it would let you drive 3W+ 808nm pump diodes with ease.

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