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General Q&A Regarding Laser Diode Drivers

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    General Q&A Regarding Laser Diode Drivers

    My answers:

    The following questions are for using a large heatsink (e.g. host body or diode heatsink) for an SXD driver running at 4+ Amps:

    1) Which component(s) of the driver needs to be heatsinked the most? What does that component look like on the board? Since many rectangular driver boards have components on both sides, which side needs to contact the heatsink (face down position) ?

    Just the main chip needs to be heat sinked, nothing else.

    2) If the driver has a potentiometer, is it best to keep the side of the board with the potentiometer face-up for future access to the potentiometer?

    Yes, but if you can't, see if you can make a hole through what ever it is mounted on to be able to adjust it. Worst case, if you cannot do that, adjust it before mounting but that is a no-brainer..

    3) What is the best way to attach the heatsink (Arctic Silver or another brand)? Or holding the board down mechanically with a bar of some sort?

    You can hold it down with a bar of some sort that is what I try, but if not, heat sink epoxy is what many use, I've used it and later wanted to reposition the driver and was able to pop the driver off with a screw driver anyway and file down the residual glue from the driver chip and reapply, but I suppose that depends upon how well it was glued and which brand of epoxy

    4) What areas of the board or components should be avoided when applying thermal glue? The legs of the CPU or the tracing on the board or.....?

    I would not allow excess glue to go anywhere if you can help it, but that is not very practical, it will happen. As far as an effect to the board, I don't believe it will cause a problem, never has for me.

    5) Is it best to apply a thick layer or thin layer of thermal epoxy? I assume thicker means greater electrical insulation from the metal heatsink (good) but greater thermal insulation (bad)?

    When using heat sink compound or glue, thin is what you want. As thin as possible for full contact.

    I have had the same concern for some of the drivers, some allow a direct contact because there is a thin layer of paint on the board over a copper plane which is designed to be there to better transfer heat out of the PCB. You can buy the thin heatsink pads which are a half mm thick which can work, but they are worse than heatsink compound or epoxy for transferring heat. That said, the pads may be sufficient.

    6) How do I electrically insulate the components of the board since they will be touching the metal heatsink? I think DTR uses a special thin tape ?

    You can do that with some tapes which designed to transfer heat, but what I've done is find small copper pieces on ebay which are pre-cut or file them down to the size I need and glue it to the top of the chip which needs heatsinking, then the circuit board is raised enough the components don't touch. It irritates me the designers of these drivers have built them where that is necessary, in some cases. I believe all of those drivers should have a flat piece of copper already glued on top of the chip so we don't have to deal with it. Of course, there will be increased cost for that.

    7) Has anyone experienced the driver 'popping off' from its location where it was bonded, after the thermal epoxy cured ?

    YES, file down the excess glue, clean with solvent, try again. It is a good to first file the top of the chip to roughen up the surface before mounting with glue, this will help it stick better.

    8) What about thermal pads, clay, putty? Are they all the same thing? Has anyone experienced a blob of clay debonding and rattling around inside the host?

    Never had that, I don't like using thermal pads as they have more resistance to heat than simple heatsink compound or epoxy glue

    9) Is it better to have a driver heatsink which is separate from the diode heatsink so both are not contributing heat to the same heatsink?

    Yes, better, especially for high power laser diodes which produce more heat from the diode itself, as well as the driver too.

    10) How much of the total heat is generated at the driver vs. the diode?

    The diode produces far more heat than most drivers. Many drivers are very efficient, 95 percent or higher, unless a homemade driver using something like a LM317, then they can get very hot too

    11) What is the best way to attach a driver to the inner surface of the battery tube of the host (since the board is flat and the tube is curved)?

    You can either make, or buy half-moon inserts to match the curvature of your tube.

    12) Generally, which comes first: attaching the leads to the driver or attaching the driver to the heatsink (for convenience and ease of
    assembly) ?

    Attaching the wires to the driver first, then the driver to the heatsink.