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3.3v logic level mosfets

Discussion in 'Electronic Design' started by colin, Feb 28, 2007.

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  1. colin

    colin Guest

    Hi,
    Im needing a 200v/1A nch mosfet wich can be driven from 3.3v logic,
    preferably dual in s08 package but having trouble finding any, is this
    asking too much ? as id rather avoid a driver if i can. ive found a few
    small ones (200vds vgsth <3v) and a few much larger ones but nothing in
    between.

    at the moment ive got a device with pullups to 5v, (it was in my parts bin)
    but this still doesnt quite cut it and it turns on at power up wich is
    unwanted.

    I also have lower voltage fets such as 2n7000p wich work fine driven
    directly.

    Colin =^.^=
     
  2. Mook Johnson

    Mook Johnson Guest

    Do you need mosfets or would a darlington due. With a beta around 5000 it
    would only take 200uA to drive a 1A load.
     
  3. maxfoo

    maxfoo Guest

    What's wrong with the 2n7000,too big? Use a smaller BSS138 in a sot-23 package.
     
  4. colin

    colin Guest

    2n7000 is fine but Its only 60v rated, I need about 200v, ... 150v + some
    inductive overshoot.
    darlington is an idea wich I'd not thought of,
    but its for a low power inverter running from the 5v so low fwd drop is
    preferable.
    the somewhat complex pwm control is done from a PIC. the 2n7000 is for -20v
    inverter.

    I think i might have to go for a driver and some beefier mosfets as I cant
    get enough power from the ones I have even with 5v pullups, its not quite as
    low power as id hoped lol. Il have to bring in 12 for the driver I can run
    the inverter off 12v instead of 5 too.

    However I did have a look at fairchild's site and its very nice to use,
    found one that I might try anyway, thats if I can get hold of it - Fqt4n20.
    although looking for that one in Farnell i came across a bsp297or zvn2120G
    but the later is no longer stocked, They didnt stand out in previous
    searches.

    thanks,
    Colin =^.^=
     
  5. Needs way more than 3.3V drive unless you want to cook it. Vgs(th) is 3V
    max, they spec it at 10Vgs to really turn it on, 5V will turn it on some but
    3.3V and it's really gonna be a bar-b-q with any current.

    I'm thinking maybe the OP should look into IGBTs. I don't know about the
    3.3V drive, but it's way easy to find IGBTs that will withstand 600Vds and
    switch large currents. The down side is that giant cap sitting on the
    gate(base). As long as you can dump the juice in fast enough, they're
    slick. Of course I only played with them once and I used 5V drive. Mr Hill
    is the man to ask about them.
     
  6. colin

    colin Guest

    That vn2224 looks realy cool, maybe I can add an offset to the gate drive of
    1volts,
    that will give me 4.3 volts drive and still be sure its off,
    maybe set the vdd to the max of 3.6 volts and/or go for 1.2 volts, gate
    offset.
    the open drain 5v pull up isnt so nice when programming the device and the
    outputs go tri state.

    the graph shows at 1 amp its saturated at vgs=3v., I hadnt intended to need
    more than 1amp,
    its an inverter so the current is ramped from 0-1amp, average 50% duty
    cycle, so 0.25A avg
    I only need about 1.5W real power output from the device im driving,
    although it seems I need a fair bit more due to reactive currents in the
    load,
    at 4vgs its over 4amps wich should be ample for my needs, typical ofc.

    Theres a quad package too, maybe 2 in parallell will easily do, or I can use
    the other 2 to save a discrete elsewhere.

    As for devices with large input capacitance such as igbt, the current from
    my PIC is somewhat limited, so switching speed will become unaceptable so I
    would need a driver anyway wich then makes the fet selection easy anyway.
    the output from the inverter is 1/2 sine wich is why its PWM is driven from
    the PIC. it should drive the 2nc gate charge of the vn2222 quick enough.

    Id rather drive everything from the 5v but its a bit of a jump from 5v to
    150pk and demands a bigger device but the 12v supply is via a linear
    regulator, the 5v via a switcher.

    Unfortunatly the VN2222 that is stocked by RS and farnell seem to be 60v
    spec to92 devices.
    seems to be some confusion with part numbers here >.<
    neither stock the vn2224 quad.
    is the 200v version an unobtanium part ?

    Think I might just plop for a driver and be done with it. I just tried a
    dual driver I had available but cldnt get it to work, then reading the data
    sheet realised the shutdown input had a threshold of 11v lol.

    thanks
    Colin =^.^=
     
  7. If I understand what I'm reading, Vds will be 1V at 3Vgs and 1A, that's a
    dissipation of 1W. It's gonna get real hot if you draw an amp thru it.
    Am I not reading this right? At Vgs=4V and Id = 4A, I come up with a Vds of
    5V. Isn't that 20W of dissipation?
    I agree that the PIC should have no trouble charging the gate of that
    device.
    All I know is that I couldn't find any FET that I could afford/use at 400V,
    I had to use an IGBT and a gate driver:
    http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010666
    TC4423 by microchip $2 in singles at Digikey. Good to 18V on the output and
    should work fine at 3.3V.
    4423 dual inverting
    4424 dual non-inverting
    4425 dual one-of-each
     
  8. colin

    colin Guest

    yea those dissipations are quite high, its a current ramp though so only the
    tail end power will be that high, the average will work out a lot lower.
    the on resistance would of course severly limit operation to less than 4amp
    as id rather use 5v for the inverter. At 12v i would need less current too
    ofc.
    Well depends how fast you want to switch it, 100khz would be ideal.
    the dspic33 is a bit weedy on the io pin current side.
    rated 4ma but seems to easily sink a peak of 10ma.
    I had a dual SO8 500v mosfet wich i used as a class d stage, fed by an
    inverter.
    Im trying to simplify it drastically by doing away with the inverter and the
    hi and low side driver.
    dual low side drivers should be cheaper than that I hope,
    I dont need 3amp gate drive, thats as much as drain current,
    or discrete will do.

    thanks
    Colin =^.^=
     
  9. Guest

    Does pullup to 5V really help any? won't the voltage just get clamped
    by the output to something close to 3.3V?

    -Lasse
     
  10. Ok, most of the ones I tinker with will supply 20mA.
    Those are for fairly healthy sized MOSFETs and IGBTs. Microchip's AN786 app
    note might interest you. It has some good info on how to "correctly"
    calculate the amount of drive current you will really need to switch in a
    certain amount of time. Those microchip gate drivers above will switch in
    30nS or better. I guess it all depends upon what you need. :)
    http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en011878
    It's about PWM'ng power supplies that have high and low side drive, but
    might interest you anyway.
     
  11. colin

    colin Guest

    the particular PIC outputs can be set to open drain and then have a 5v
    tolerance.

    Colin =^.^=
     
  12. whit3rd

    whit3rd Guest

    If your present mosfet drives OK, and you really care about high
    voltage,
    consider a cascode. Either another MOSFET, or (I'd prefer this)
    an NPN transistor, with base connected to your 3.3V rail, should fit
    the
    bill. Connect the base through a parallel RC to get lower ON
    voltage.

    It's not a perfect switch-to-ground, but it's quite easy to drive.
     
  13. colin

    colin Guest

    yes a cascode arangment that was something I had considered as an
    alternative to a driver.
    and a 2 mosfet cascode needs no extra resistors.

    Colin =^.^=
     
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