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MOSFET smd - six pins?!

Discussion in 'Electronic Components' started by DaveC, Mar 13, 2007.

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  1. DaveC

    DaveC Guest

    RTQ035N03 in TSMT-6 smd 6-pin package:

    <http://www.rohm.com.cn/products/databook/tr/pdf/rtq035n03.pdf>

    One pin each for source and gate. Four for drain.

    Is the number of drain connections simply for convenience of
    mounting/orientation? One wouldn't need four drains with only one source...

    Or am I missing something (which is my default presumption)?

    Thanks,
     
  2. Palindrome

    Palindrome Guest

    Thermal dissipation.
     
  3. Tim Wescott

    Tim Wescott Guest

    To unpack:

    The leads carry heat as well as electricity, and the lead frame is
    designed to maximize the thermal transfer -- some or all are being made
    out of copper instead of kovar to maximize heat transfer. To really
    deal well with the heat you often need to solder the chip down to a
    copper pour on the board -- the data sheet will tell you how, or will
    point you to an app note.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. DaveC

    DaveC Guest

    The leads carry heat as well as electricity, and the lead frame is
    Oh, you mean like this:
    "Total power dissipation: 1.25w*
    *Mounted on a ceramic board"

    I can find no other references to how to use this chip.

    Hmm... how to mount on a ceramic board. Is it difficult to achieve that 1.25w
    in real-world terms. How many designers use ceramic boards in their designs?

    Thanks,
     
  5. Genome

    Genome Guest

    To, not, clarify. Most of the power dissipated in your mosfet is dissipated
    in the drain.

    The chip is put into the package with the drain bonded to the main leadframe
    and the main leadframe has most of the leads connected to it so you can get
    the heat out.

    How they think you do that is up to them and the marketing department, who
    will win.

    How you do it is up to you.

    I've got problems with spelling dissapated dissipated but, fortunately,
    that's not your problem. It's rather similar to your subsequent
    question(s)......

    DNA
     
  6. Robert Baer

    Robert Baer Guest

    If you were crazy to $pend the money, then you would be psycho-cramic
    crack-pot.
     
  7. Dave Platt

    Dave Platt Guest

    Oh, you mean like this:
    You use ceramic boards in your design when you really need high power
    densities! For designs where you can run your parts at a lower
    dissipation per square, you can get away with less exotic substrates
    like FR4.

    Consider the size of a 2-watt through-lead resistor (which is probably
    what you'd want to use when actually dissipating 1.25 watts), and the
    space needed to mount it up a bit above the board to allow for
    adequate air circulation. Consider how warm it's going to get.

    Now, consider all of that heat coming out of a little SMD package,
    with no free-air under-chip circulation, being carried away mostly by
    radiation and by conduction into the PC board traces. That chip's
    going to get hot!

    I've seen some of the less-expensive PC-board material be singed
    pretty badly from the heat of a TO-220 7805 linear regulator
    which was screwed down (with its heatsink) directly to the board.
    Even with a couple of square inches of heatsink sticking up into the
    air, its several watts of heat dissipation burned the board. The
    heat-per-square density of a 1.25-watt SMD is likely to be even
    higher, and thus mandates the use of a high-temperature-tolerant board
    substrate.

    As I understand it, it's not necessarily any harder to use a ceramic
    board than a glass/epoxy board. You just tell your fab house which
    substrate to use, and use a somewhat larger wheelbarrow to ship them
    the money :)
     
  8. (Dave Platt) wrote in
    Off topic to FET's but how much, roughly, for small quantities of maybe 2"
    x 3", single sided? And can thickness be had at 1/8"? This could be a cost-
    effective method for mounting electronics and optics on a single board for
    laser designs where it is critical that thermal stability is high, and
    expansion is low. I've seen it used in the Coherent C315M laser, and it
    looks like a good way to do it. It seems to allow high quality for making
    small numbers of devices by hand, and an easy way to set and reset
    alignments at will, so even if it costs a bit, it might save far more than
    it costs.
     
  9. Good, cos 'dissipated' is the way to go... Can we enjoy enough dissipation?
    I think not. Cut me some SLACK. >:)
     
  10. I haven't seen this thing, so I don't know if my suggestion has any merit,
    but just possibly... TEC. The small ones often found in modest quantites on
    eBay. Small high pump density devices metallised both sides are often
    custom made, so surplus turns up at times. If you could remove the tinning,
    and somehow protect the body of the device in peelable rubber, you could
    etch tracks and such onto the top side for mounting stuff. That gets you a
    cheap ceramic heatsink AND a pump. :) And an easy way to mount the parts,
    if you have a temp-controlled iron.
     
  11. Robert Baer

    Robert Baer Guest

    I find that Megtron 6 holds up rather well at high temperatures and
    it is a heck of a lot less expensive than ceramic and certainly not brittle!
     
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