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High current/heat DC PCB traces

Discussion in 'General Electronics Chat' started by Oznog, Dec 27, 2005.

  1. Oznog

    Oznog Active Member

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    I'm looking to build a MOSFET replacement for a diode-based battery isolator for a high current car DC system. For now, let's please just assume "I have my reasons". I have a high side driver that can drive an NMOS.

    I'm really liking this Power SO-10 pkg. Very, very good rDS-on and the dissipation through the tab can be pretty high.
    http://www.st.com/stonline/products/literature/ds/6654/stv160nf03l.pdf

    So the system can be around 160 amps here and I was figuring 2 in parallel would be best, keeps the voltage drop and heat down. That's 12W per device if they're cool, if rDS increases due to temp rise we could be up to 19W at max junction temp. The case will not have a fan and will not be vented since it's "under the hood" and could get splashed.

    OK, I now have 3 issues:
    1. Even if I use 3 oz board, can the traces be made large enough to handle the current?
    2. How can the traces be made to dissipate the devices' heat?
    3. I will probably be lugged 1/0 cable here too. Mechanically, standard board can't take this kind of stress. Nor do I want to have to use 2 wrenches to ge it on. Using a stud attached with 2 permanently affixed nuts is a bit hoakey even if mounting stress weren't the issue since the contact area with the lug is reduced when the lug grabs the nut.

    I've seen "aluminum core PCBs" but I don't think this is going to be possible to do at home and it would be expensive to mfg. Should I cut out copper sheet to lay over the high heat/current areas? I'm thinking the lugs themselves could be used to conduct away most of the heat, does that sound right?

    How much does 3oz board insulation between sides? I could make the bottom of the board below the components all copper, grounded, and stick a flat heatsink on it. But that means a layer of fiberglass between the component and sink which is hardly ideal.
     
  2. zachtheterrible

    zachtheterrible Active Member

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    To beef up the PCB traces for more current you could solder a bare wire along the trace. Just an idea :lol:
     
  3. Oznog

    Oznog Active Member

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    But as far as I can tell "a wire" isn't going to make a difference. For example, 4 ga wire with high temp insulation can only go to 140 amps. Now without insulation the wire will dissipate much better but look how thick 4 ga is, it can't really be soldered like that.

    I looked inside a power inverter I have, I saw there's a solid copper busbar, maybe 1/8" thick, turned on its edge with tabs on that edge to solder into the board. That's nice because it exposes 2 faces for dissipation. I don't have a source for a component like that though.

    I suppose I could cut one the same way I might cut a strip that would have soldered flat on the trace. It'd need to be thicker and a bit more complex to cut accurately. I guess one thing I'm looking at is the flat strip would be easier to bolt to, though as I mentioned I'm not sure what the best way to do that one is.
     
  4. dave miyares

    Dave New Member

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  5. justDIY

    justDIY Active Member

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    this is sort of an off the wall comment, but I'll throw this out there

    while sifting through my Digikey catalog, I saw some mosfets (from fairchild I believe) that were in 1/4" threaded stud package (think welding diode / automotive diode). The drain and source were both 1/4" thread, and the gate was a little pin sticking out of the end of source side.

    something heavy duty like that, you could bolt it onto some copper (or aluminum) rod/bar and carry enormous current without regard for heat.

    I'll try to find 'em again and get you a link

    maybe look into how modern motherboards are made - their onboard buck converters turn 12v into Vcore (1.2-1.6v) and develop some serious amperage (think a 100watt dual core cpu at 1.2 volts = 83 amps)
     
  6. Glyph

    Glyph New Member

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    Tedious solution...

    Really "out of the box" idea here so don't try it unless you're daring :)

    How much time do you have to build this? a week? a month? a few months?

    If the last one then you might try a really tedious but versatile solution: You electroform the copper pad structures onto the PCB.

    Essentially you "grow" the pads right on the PCB atom by atom.

    The idea is you get your PCB and etch the traces as usual but also include large "pads" of copper that will serve as the heavy current traces. Then you cover up all of the traces you want to keep "normal" with waterproof tape leaving the high current traces alone. You then fix a wax mold (with plenty of solution access holes) onto the PCB to make the traces grow into the mold in the proper shape, this is to prevent them from growing toward each other and shorting out. Then you connect it to the cathode of the high current low voltage (10-50 amps at 0.3v) power supply and dip the PCB/wax mold into the copper plating solution and let the copper ions plate onto the exposed sections of PCB.

    Takes some time to get a good deposit and you need to carefully control the solution chemistry to get high conductivity of the final product.

    At the end you take it out, break off the wax and remove the tape and after some sanding and buffing you'll have a PCB with a huge THICK copper pads that could take a few hundred amps of current (or more, i never tested the upper limit).

    The amount of copper you'll add on can be a millimeter or a couple of centimeters thick depending on how long you leave it in the solution (the longer the thicker)

    The other traces that were covered in tape are normal so you can solder your control components.

    The thick copper is strong and drillable so you can drill holes for the bolts or clamps you'll connect the cables with.

    Very tedious process to learn and do (your first ten tries WILL look like crap, but you'll get better), but in the end you can make things that can handle gargantuan currents.

    Also a fair bit expensive if you're just starting off.

    I first used this process for the unrelated task of making copper plaques for ornamental purposes. Then i applied it to making an extra-thick copper heatsink/conductor pad for a switch mode power supply i was building.

    I tested the 5 millimeter thick pad (which covered about 6 square inches) with a current of 100 amps, didn't even get warm. (i didn't test further for fear i might cause something else in the circuit to explode...)

    it was thick enough that i could securely drill and screw on a rather large diode and mosfet combination that had screw connections for its leads.

    the copper is fully solderable if you sand it down smooth enough. (i didn't bother since i wanted to able to screw and unscrew replacement mosfets everytime it blew)

    for more information look up "copper electroplating" on the internet. the process is not used for electroforming commercially (metal casting is faster, simpler and WAY cheaper) but copper plating is used extensively by the electronics industry for through-hole connecting.


    ....btw the same skills for electroplating copper also work for chrome plating your own car parts, restoring them to showroom finish.... (HINT HINT)

    ********


    If you don't have the time to do something this complicated then i would suggest just buying some copper stock sheets from ebay or onlinemetals.com. Cutting them to shape, sanding them down, and soldering them over the traces you have on your PCB. Faster, simpler, and cheaper.

    or do what JustDIY said and get some different components that use screw connections. Buy a copper plates that's 1/4" thick, drill holes and screw the components in. Your control electronics only need little 10 miliamp wires for their connections.
     

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