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MOSFET Driver cannot drive a MOSFET? or something wrong?

Discussion in 'Electronic Design' started by [email protected], May 13, 2008.

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  1. Guest

    Here is my circuits, simple, yet, when I disconnect the MOSFET and
    load from the circuits, the waveform, duty cycle, amplitude,
    everything else is correct. After I connect the mosfet and load (here
    I didn't use a laser diode, in stead I use a 1 ohm power resistor),
    all 're in a messy! MOSFET is hot coz it's not fully on, square wave
    form contains a lot of ringing, overshoot, duty cycle? no duty cycle
    but some weired square waves with spikes.

    It is a very common circuits but I wonder something wrong?


    http://www.dianyuan.com/bbs/u/65/2567951210659430.jpg
     
  2. Bob

    Bob Guest

    My guess is circuit layout or your 1ohm resistor is a wirewound
    type with significant inductance.

    Acording to the IXDD414 datasheet it has a risetime of under
    30nanoseconds and can source or sink 14Amps.
    With those numbers it dosn't take much inductance or
    reflection to spoil a sharp edge.

    If you have wired this up with six inch long hookup wires then
    you are going to see ringing.

    Bob
     
  3. James Arthur

    James Arthur Guest

    Where's your gate resistor? You need one.

    (Methinks your FET screams.)

    Cheers,
    James Arthur
     
  4. Guest

    1-2 ohm but didn't work.


     
  5. Mark

    Mark Guest

    what rate are you switching at?

    what is the gate capacitance of your FET?

    can the driver drive that capacitance at that speed?

    Mark
     
  6. [bottom posted for clarity]
    A gate resistor is not always needed, but it seems more likely there is not
    enough bypass capacitance on the driver circuit. When the driver tries to
    turn on, it needs to get its current from a voltage source that can handle
    a high speed pulse. The driver should be bypassed with a low ESR capacitor
    at least 10x the MOSFET gate capacitance. Also, the gate drive and return
    tracks should be very short.

    The source resistor shown in the schematic should be of low inductance, and
    careful consideration must be given to the ground path for the high current
    output. It is not even clear why the source resistor is present, unless it
    is used for desaturation detection or current mode PWM.

    Please provide the entire circuit, including details on what sort of power
    supply is being used, and a PCB layout or sketch of wiring if it is on a
    protoboard.

    There is good information on high speed circuit design in a Linear
    Technology app note AN47. It is more about amplifiers, but applies to this
    as well. And AN25, "Switching regulators for poets". Also look at
    International Rectifier app note AN-944 on gate charge and drivers for
    MOSFETs.

    Paul
     
  7. Guest

    now it's an open loop circuits. the 0.015 is for further close loop
    circuits use. even I remove 0.015 ohm res the problem is still there.
    I did it on bread board. I think it is caused by the inductance of
    load, power supply wires. I don't know how to solve it. I added some
    big capacitors between vcc and ground, I use separate power supply
    for that mosfet, it didn't improve too much.












     
  8. You will get more help if you don't top post like this.

    Paul

     
  9. James Arthur

    James Arthur Guest

    1-2 ohms? What are you trying to do? Pulse a laser diode, it seems.
    How fast, how much current, etc.?

    That's a massive FET. http://www.irf.com/product-info/datasheets/data/irl3803.pdf
    (IRL3803: 140A, 0.006 ohms, Qg=140nC) Something smaller would be
    easier. Are you sure you need such a monster?

    To turn that beast on super quickly you'll need super layout, tight
    decoupling, intelligent current paths, short traces, and so forth.

    By temporarily using a 10 ohm gate resistor, much will be revealed.

    Cheers,
    James Arthur
     
  10. James Arthur

    James Arthur Guest

    (I just saw this post, after posting my other reply...)

    You can't do that. It won't work. You need a groundplane. And a
    smaller FET with a larger gate resistor wouldn't hurt.
    Build the thing on a solid piece of copper clad. Dead-bug style is
    good. These problems will vanish.

    Cheers,
    James Arthur
     
  11. Just a 0.1 uF or so bypass capacitor directly across the driver power
    terminals should help a lot (like you said, decoupling). Also the
    connection to the gate and back to the ground pin should be reasonably
    short. I have a circuit that uses a Fairchild HUF75645 which is almost as
    beastly as the IRL3803, and I have driven it up to at least 300 kHz with an
    inductive load without problems, using a TI 9 amp driver. UCC27321. And my
    first prototype on a perfboard was OK at 100 kHz.

    The IRL2803 has 5000 pF gate capacitance, but it has 230 nSec rise time, so
    a super fast gate driver is not really necessary. A 9 amp driver can
    provide about 1.8 V/nSec, and realistically about 20 nSec to full on, but
    the device itself will take ten times that to conduct fully.

    There is an app note on the UCC35705 that is used to provide the PWM
    signal: http://focus.ti.com/lit/ug/sluu141/sluu141.pdf

    This is a 4 MHz part, which may be overkill for the application (or at
    least the MOSFET). More information is really needed, such as supply
    voltage and drive frequency.

    Paul
     
  12. James Arthur

    James Arthur Guest

    That's with just 4.5 volts of gate drive (Fig 10a. in the datasheet)
    through a 1.3 ohm gate resistor, but 12v and a hot driver could
    do the job much faster.


    You can do a lot on a smart perfboard. The OP's "breadboard" though
    clearly isn't cutting it, so recommending a groundplane makes up for
    a lot.

    It all comes down to what the OP is doing, which he hasn't told us.

    If he's trying to get 15nS pulses from a laser diode with this
    setup...

    Grins,
    James Arthur
     
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