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Mosfet driving questions

ArchiverArchiver Posts: 46,084
edited 2002-05-10 23:41 in General Discussion
Greetings!
I have a question concerning mosfet driving and I was hoping someone might be
able to help me out. I have an oscillator outputting a 140 khz square wave.
(note this is a square wave that swings from both positive and negative
voltages,(+8 to -8 volts and is not a clock pulse.) and I was wondering if
anyone knew any slick ways of interfacing this to an IRFIZ34G N channel HEXFET
MOSFET (stats below) I tried tying the oscillator directly to the gate but when
I measure the gate the wave seems to distort. The wave actually looks like a
sine wave when I do this... I don't understand why this is happening because i
thought the gates on mosfets had a very high input impedance so they would be
easy to drive... I would further like to note that I am aware N channel mosfets
only conduct when the gate is made positive so the negative portion of my square
wave wouldn't do anything.
Any help would be useful!
Thanks!
Randy Knutson
Minnesota State University, Mankato
IRFIZ34G
N CHANNEL HEXFET
VDSS 60V
RDSS .050 OHM
ID 20A




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Comments

  • ArchiverArchiver Posts: 46,084
    edited 2002-05-10 17:42
    At 09:12 AM 5/10/02 -0700, Randy Knutson wrote:

    >Greetings!
    >I have a question concerning mosfet driving and I was hoping someone might
    >be able to help me out. I have an oscillator outputting a 140 khz square
    >wave. (note this is a square wave that swings from both positive and
    >negative voltages,(+8 to -8 volts and is not a clock pulse.) and I was
    >wondering if anyone knew any slick ways of interfacing this to an IRFIZ34G
    >N channel HEXFET MOSFET (stats below) I tried tying the oscillator
    >directly to the gate but when I measure the gate the wave seems to
    >distort. The wave actually looks like a sine wave when I do this... I
    >don't understand why this is happening because i thought the gates on
    >mosfets had a very high input impedance so they would be easy to drive...

    The gate lead on MOSFETs has a LOT of capacitance - anywhere from 3000 to
    10000 pF, depending upon which FET you are using.

    The easiest way is to use an off the shelf gate driver chip - IR makes one,
    as does TI and many others.

    I'd suggest a simple complementary emitter follower pair but your positive
    pulse is a little low for voltage - the minimum gate voltage for that
    family of FET is 10V and 12 - 15V is better. You]ve only got 8V to begin
    with and you'd lose another 0.7 - 0.9V in the emitter follower.

    dwayne



    Dwayne Reid <dwayner@p...>
    Trinity Electronics Systems Ltd Edmonton, AB, CANADA
    (780) 489-3199 voice (780) 487-6397 fax

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  • ArchiverArchiver Posts: 46,084
    edited 2002-05-10 18:17
    Did you happen to notice what the input capacitance of the gate is? Try
    tying that amount of capacitance to ground from your oscillator and see
    if the resultant wave form looks familiar.


    HTH,

    Leroy

    Randy Knutson wrote:
    >
    > Greetings!
    > I have a question concerning mosfet driving and I was hoping someone might be
    able to help me out. I have an oscillator outputting a 140 khz square wave.
    (note this is a square wave that swings from both positive and negative
    voltages,(+8 to -8 volts and is not a clock pulse.) and I was wondering if
    anyone knew any slick ways of interfacing this to an IRFIZ34G N channel HEXFET
    MOSFET (stats below) I tried tying the oscillator directly to the gate but when
    I measure the gate the wave seems to distort. The wave actually looks like a
    sine wave when I do this... I don't understand why this is happening because i
    thought the gates on mosfets had a very high input impedance so they would be
    easy to drive... I would further like to note that I am aware N channel mosfets
    only conduct when the gate is made positive so the negative portion of my square
    wave wouldn't do anything.
    > Any help would be useful!
    > Thanks!
    > Randy Knutson
    > Minnesota State University, Mankato
    > IRFIZ34G
    > N CHANNEL HEXFET
    > VDSS 60V
    > RDSS .050 OHM
    > ID 20A
    >
    >
    >
    >
    > Do You Yahoo!?
    > Yahoo! Shopping - Mother's Day is May 12th!
    >
    > [noparse][[/noparse]Non-text portions of this message have been removed]
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  • ArchiverArchiver Posts: 46,084
    edited 2002-05-10 23:41
    Try using a lighter mosfet such as the 2n7000 which is designed for logic
    level switching (<2volts=off , >3=on) then if you really need that much
    current you could let the 2n7000 switch 12v to the other one. not sure how
    much output current you have available to overcome the gate capacitance at
    that freq. but the 2n7000's have a fairly low gate capacitance, where
    heavier ones have more.

    KF4HAZ - Lonnie
    From: "Randy Knutson" <ken_ryder@
    >
    > Greetings!
    > I have a question concerning mosfet driving and I was hoping someone might
    be able to help me out. I have an oscillator outputting a 140 khz square
    wave. (note this is a square wave that swings from both positive and
    negative voltages,(+8 to -8 volts and is not a clock pulse.) and I was
    wondering if anyone knew any slick ways of interfacing this to an IRFIZ34G N
    channel HEXFET MOSFET (stats below) I tried tying the oscillator directly to
    the gate but when I measure the gate the wave seems to distort. The wave
    actually looks like a sine wave when I do this... I don't understand why
    this is happening because i thought the gates on mosfets had a very high
    input impedance so they would be easy to drive... I would further like to
    note that I am aware N channel mosfets only conduct when the gate is made
    positive so the negative portion of my square wave wouldn't do anything.
    > Any help would be useful!
    > Thanks!
    > Randy Knutson
    > Minnesota State University, Mankato
    > IRFIZ34G
    > N CHANNEL HEXFET
    > VDSS 60V
    > RDSS .050 OHM
    > ID 20A
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