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Low Rds(on) tempco

Discussion in 'Electronic Design' started by Tim Williams, Jan 24, 2010.

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  1. Tim Williams

    Tim Williams Guest

    A little while ago, someone was asking if anyone makes MOSFETs with a low
    Rds(on) tempco. Fairchild is claiming an usually low ratio of ~1.63 =
    Rds(on)(175C) / Rds(on)(25C) for some low voltage FETs:
    http://www.fairchildsemi.com/ds/FD/FDP8860.pdf

    But they're also claiming 254W dissipation in a TO-220. Tempco of just the
    leads, in air, at rated current, is more than Fig.3 I'm pretty sure.

    Tim
     
  2. Hammy

    Hammy Guest


    Even Lower from TI their new CICLON line.

    http://focus.ti.com/lit/ds/symlink/csd25301w1015.pdf

    Full line up is here.

    http://focus.ti.com/general/docs/ge...iclonannouncement&HQS=NotApplicable+OT+mosfet

    They sent me an email about them. They are nice for LV only 25VDS max;
    easy drive requirements as well.

    It's mainly the lower voltage ones that have the best tempco's. When
    you get up in higher voltage they are all usually about 1.8 to 2 x
    25C. I'm sure I used to now why this is but forgot.

    Anything over 50VDS I just go at 2x 25C rdson to be safe.
     
  3. Tim Williams

    Tim Williams Guest

    That is quite flat.

    'Spose it helps not having leads, too. Average metal has an awful tempco.
    I spotted this phenomenon while exploring a low volt, high amp synchronous
    supply. I'm looking at things a bit harder to drive though (~200nC gate
    charge). :)
    I would guess channel length. Something about short channel effects, or the
    way they have to build them when Vds(max) > Vgs(max)?

    I wonder, does anyone know what the typical feature size is for power
    MOSFETs? Still micrometers as usual? Anything gained from teensy sizes?

    Reminds me of an idle thought I had the other day... just imagine, if you
    took all the transistors on one of those big pigfucker FPGAs and put them
    all in parallel. You'd only get maybe 1.5Vds(max), but think of the
    amperage and switching speed. ;-)

    Tim
     
  4. legg

    legg Guest

    Fig 11 shows 1ADC @30V limit, with Tc @25degC, so obviously something
    is not quite up to snuff, where data sheet accuracy is concerned here.

    RL
     
  5. Hammy

    Hammy Guest

    Yea that Fairchild device is quite the brute. Have you seen the
    IRL3713S same ratings only half the gate charge at the same price
    it's a DPAK.

    http://canada.newark.com/international-rectifier/irl3713spbf/mosfet/dp/63J7773

    NXP has some cheap LFPAKS with similar ratings as well; with a quarter
    the gate charge

    http://canada.newark.com/nxp/psmn4r0-30yl/transistor/dp/04R7692

    The biggest FET I have gate charge wise is the FQA24N50, TO-3P,
    500VDS,24A,0.20ohm RDSON, 90nC. Newark had them on sale for $1.14 each
    so I grabbed 25 of them.

    http://canada.newark.com/fairchild-semiconductor/fqa24n50/mosfet/dp/58K1512

    Sub 0.25 ohm RDSON fets with 500 or 600VDS blocking capability usually
    cost an arm and a leg.

    Infineons IPB60R099CP is a prime example $14.

    http://canada.newark.com/infineon/ipb60r099cp/mosfet-n-to-263/dp/33P7131
     
  6. Tim Williams

    Tim Williams Guest

    I want to avoid SMT since it's nonobvious to heatsink (I don't have PTH
    facilities at home), and at 100A, 3mohm is 30W of PCB-melting toastiness.
    The duh answer, use four in parallel, but then gate charge is back where I
    started. So I figure heatsinking is best.

    I have been more and more tempted to buy SMT though. So far, the only thing
    I've built with SMT didn't use a PCB:
    http://webpages.charter.net/dawill/tmoranwms/Elec_JT3.jpg

    With solder paste, gobs of flux and a heat gun or toaster oven, I'd probably
    enjoy it a lot, but I don't quite feel like investing in yet another parts
    line, made of things I can hardly see and certainly can't tell apart (99%
    unmarked capacitors. seriously. wtf.).
    Mmm, nice price.
    I have some IRG4PC50UD's that clock in at some 270nC. They were expensive,
    too (~$8 each).

    Fairchild's Field Stop IGBTs are like popcorn nowadays.
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMv5VNPK0aSx5MnkNzNNxVPi14Kshq0fnKk=
    A c-note of them could switch an entire village.

    IGBTs rule above 300V. They're not even bad down around 100V:
    http://www.mouser.com/ProductDetail...GAEpiMZZMv5VNPK0aSx5InJD4DRwCpIT9JScd%2bUcNY=

    I put a bunch of those together, way cheaper than any IGBT or FET module
    you'll find for a motor drive:
    http://myweb.msoe.edu/williamstm/Images/EV_Parts0006.jpg

    Tim
     
  7. Hammy

    Hammy Guest

    There are too many good parts out there not to use SMD particularly
    FET's in the sub 60VDS DPAK. You don't really need any special
    equipment for most packages. I use a $10 solder iron from Canadian
    tire and toner transfer method for PCB and I do fine. Just keep track
    of the parts in labelled coin envelopes.

    Agreed on the caps some marking scheme would be nice ;)

    Yea for kW SMPS you would be looking at IGBT's they do have some good
    high speed capable FET's but they are slim pickings and they are
    pricey.

    http://canada.newark.com/stmicroelectronics/sty60nm50/mosfet/dp/89K1683

    Usually you don't want to be banging around double digit amps at
    rectified line voltages at hundreds of kHz anyways.

    I picked those FQA24N50 mainly for the price for a buck a pop I could
    always parallel them and still be ahead $. They show 1.3uS worst case
    getting eaten up on transitions using a 25ohm signal generator for
    gate drive. I'll have to test a couple of them with a beefy driver.
     
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