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Thermal Design for LFPAK FETs

Discussion in 'Electronic Design' started by Steve K, Dec 6, 2012.

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  1. Steve K

    Steve K Guest

    Some of the NXP (and Renesas) MOSFETs in the LFPAK (SOT669) package look quite attractive (low source inductance in particular appeals to me) if I knew how to heatsink them to a significant portion of their rated dissipation (typically ~50W). NXP's LFPAK thermal design document discusses under-1-watt applications only. Has anyone any clever ideas on how to get the heat out ?
     
  2. Phil Allison

    Phil Allison Guest

    "Steve K"

    Some of the NXP (and Renesas) MOSFETs in the LFPAK (SOT669) package look
    quite attractive (low source inductance in particular appeals to me) if I
    knew how to heatsink them to a significant portion of their rated
    dissipation (typically ~50W). NXP's LFPAK thermal design document discusses
    under-1-watt applications only. Has anyone any clever ideas on how to get
    the heat out ?


    ** The LFPAK is about the same size as the chip used in many TO3P/ TO264
    power fets and bjts.

    So, you could clamp or solder the LFPAK to a copper header and then bolt or
    clamp that onto a large aluminium heatsink.

    IOW create your own large flat pak.

    But why bother ?



    ..... Phil
     
  3. Tim Williams

    Tim Williams Guest

    Some manufacturers offer copper core PCBs (the core of which can be
    soldered to). Don't ask about the price. :)

    Tim

    --
    Deep Friar: a very philosophical monk.
    Website: http://seventransistorlabs.com

    Some of the NXP (and Renesas) MOSFETs in the LFPAK (SOT669) package look
    quite attractive (low source inductance in particular appeals to me) if I
    knew how to heatsink them to a significant portion of their rated
    dissipation (typically ~50W). NXP's LFPAK thermal design document
    discusses under-1-watt applications only. Has anyone any clever ideas on
    how to get the heat out ?
     
  4. miso

    miso Guest

    Not using bonding wires certainly gets the inductance down. But any
    discussion of how to mount the part requires knowing the ambient
    temperature. Since you are switching the FET, what is the duty cycle?
    Does the "on" period exceed a millisecond? [Generally using PWM in
    temperature analysis doesn't work if the on time is too long. For
    instance, 1 day on and 10 days off isn't 10% duty cycle in the eyes of a
    semiconductor.]

    A lot of cheesy consumer grade products spec the maximum ambient at 40
    degrees C because they figure if it is used in the presence of a person,
    said person will get out of Dodge when the ambient exceeds 40. This is
    really common for devices with displays.

    BTW, there are plenty of places in the world where the ambient exceeds
    40 degrees C!
     
  5. Heatsink?
    <http://www.fischerelektronik.de/en/latest-news/press-releases/releases/smt-heatsinks-for-lfpak/>

    Cheers
     
  6. Guest

  7. Steve K

    Steve K Guest

    Thanks for all the comments...a couple suggested soldering to a copper tab and bolting that to a heat sink. That's pretty much what I came up with (though I haven't tried it yet), other than a dubious idea of pumping oil over the PCB.

    Why bother ? Potentially tens of MHz operation (~50% duty cycle) at a few percent of the component cost of RF power FETs (if the heatsinking kluge can be done cheaply). I was looking mostly at the lowest Qg versions of the NXP PSMN series. I had looked at the Fischer heatsinks and they only seem good for a few watts.

    Steve
     
  8. Guest

    isn't that something like where directfets shines? they have the
    "right" side up so you can put a heatsink on top
    Maybe mount them on back side sandwiched between pcb and heatsink?

    another trick I've seen is to using the kelvin connection on a sense
    fet as reference for the gate drive


    -Lasse
     
  9. miso

    miso Guest

    Par for the course when doing datasheets for devices that expect pads of
    copper to be heat sinks is to just solder the part to a big piece of PCB
    and characterize the theta JA. You dremel wide breaks in the copper to
    change the size of the heat sink.

    To get the junction temperature on a chip is easy since you can
    characterize a parasitic diode. [Force a small constant current into the
    pin. Make sure the part is not generating any heat itself, that is,
    don't operate it. Then sweep temperature and measure the diode voltage.]
    For a discrete FET, this could be harder or maybe impossible since there
    is no free diode to play with when you make the chip generate heat.

    There may be a way to put two parts on the PCB with one generating heat
    and the other to measure junction temperature, but I'd have to mediate
    on if that is kosher.
     
  10. Aluminum-core PCB?
     
  11. Nico Coesel

    Nico Coesel Guest

    I have used these kind of heatsinks on a product:
    http://uk.farnell.com/fischer-elektronik/fk-244-08-d-pak/heat-sink-for-d-pak-31-5-c-w/dp/4314001
     
  12. Guest

    $$$$$$!!!!!!!
     
  13. Guest

    I'd think that if you just want to characterize it you could run it in
    reverse
    using the body diode as both thermometer and to generate heat

    toggle between a high current to generate heat and a small current to
    measure temperature

    -Lasse

    Version 4
    SHEET 1 1172 680
    WIRE 320 -64 208 -64
    WIRE 704 -64 432 -64
    WIRE 896 -64 704 -64
    WIRE 128 -48 -96 -48
    WIRE 704 -48 704 -64
    WIRE 432 -32 432 -64
    WIRE 896 -32 896 -64
    WIRE 704 48 704 32
    WIRE 208 80 208 -64
    WIRE -96 112 -96 -48
    WIRE 128 160 128 -48
    WIRE 160 160 128 160
    WIRE 208 192 208 176
    WIRE 208 192 -96 192
    WIRE 208 288 208 272
    WIRE 432 288 432 48
    WIRE 432 288 208 288
    WIRE 448 288 432 288
    WIRE 896 288 896 48
    WIRE 1008 288 896 288
    WIRE 208 320 208 288
    WIRE 208 320 112 320
    WIRE 208 352 208 320
    WIRE 112 368 112 320
    WIRE 160 368 112 368
    WIRE 896 368 896 288
    WIRE 896 368 848 368
    WIRE 896 384 896 368
    WIRE 848 400 848 368
    WIRE 208 480 208 448
    FLAG 704 48 0
    FLAG 208 480 0
    FLAG 896 480 0
    FLAG 448 288 Vdiode
    IOPIN 448 288 Out
    FLAG 1008 288 Vdiode1
    IOPIN 1008 288 Out
    FLAG 320 16 0
    SYMBOL voltage -96 96 R0
    WINDOW 3 -370 51 Left 2
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR Value PULSE(0 10 0 1u 1u 1.5m 2m 5)
    SYMATTR InstName V1
    SYMBOL voltage 704 -64 R0
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V3
    SYMATTR Value 5
    SYMBOL nmos 160 80 R0
     
  14. miso

    miso Guest

    I'd think that if you just want to characterize it you could run it in
    I'm a little unsure about the toggling. I've done this with very slow
    bench meters to measure the diode, but you have added time into the
    equation.

    I'm not ruling this out, but you would probably want to test the scheme
    on a device that does have a spare diode. That way you would have a
    benchmark on the scheme's accuracy.
     
  15. legg

    legg Guest

    If you mount them on a largish copper area, you can couple this to the
    package wall or other radiators either directly or through thermally
    conductive pads/insulators. The large copper area sor of defeats the
    intention of thhe smaller package though, doesn't it?

    There's not much use heatsinking them inside a container, unless
    there's somewhere for the heat to go, but everything will reduce
    thermal impedance.

    The Aavid 5731/5733/5734/7109 types, at 4-16 degC/W are similar to the
    Fischerelektronik parts.

    A 1 in square thermal pad can easily beat the lower end of the above
    numbers, even at a considerable thickness.

    RL
     
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