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Need help with MOSFET interfacing

Discussion in 'Electronic Basics' started by Keith, Jan 5, 2004.

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  1. Keith

    Keith Guest

    Hi folks. Kindof newbie with electronics at the component level, def
    not an engineer.

    I have a 5-volt pulse, from an Atmel 2313, running about 5KHz at
    around a 60 percent duty cycle, that I am using to drive a heavy duty
    auto ignition coil, currently (no pun intended) at about 28 Volts DC.
    I have good sucess obtaining the quality of Plasma discharge cloud
    that I need, albeit short sucess, using a Darlington NTE262 PNP
    transistor. Short sucess, because it can't handle the amperage for
    any real length of time. So I want to move to MOSFETs, but directly
    driving the MSOFET from the 5-volt signal doesn't seem to drive it
    into full conduction. Do I need to add a intermediate stage to drive
    the MOSFET at a higher voltage than 5 volts? A simple transistor or
    opto isolator perhaps? Thanks for any input.

    Keith
     
  2. happyhobit

    happyhobit Guest

    Hi Keith,

    Do a search for "logic level mosfet". I got 725 matches.

    Jay
     
  3. Keith

    Keith Guest

    Hey! That's what I need. :) Thanks for the tip Jay. I also
    found a super device from Phillips which is a logic level mosfet with
    protection and feedback outputs in a 5-pin unit. Nobody seems to have
    them though, and Digikey says they'd be $35 if they did. :(

    Keith
     

  4. These documents may be of some use:

    http://focus.ti.com/lit/ml/slup169/slup169.pdf

    http://www.irf.com/technical-info/appnotes/an-937.pdf

    I've never played with automotive ignition coils but my understanding of
    them is they are essentially flyback transformers. The primary current
    ramps up to several amps (often limited with an external resistor for
    running off of a 12V automotive battery) and then the primary switch turns
    off. The stored energy then produces a very large voltage since it has
    nowhere to go. This appears as both high voltage on the primary and high
    voltage on the secondary. Eventually the voltage gets so high it normally
    is supposed to arc over on the secondary, thus discharging the stored
    energy.

    My understanding of electronic automotive ignition systems is they usually
    use either a high voltage bipolar junction transistor, a relatively high
    voltage MOSFET, or sometimes an IGBT.

    For IGBTs the IRGB14C40L is specifically designed for this kind of use:

    http://www.irf.com/product-info/datasheets/data/irgs14c40l.pdf

    For MOSFETs devices like the IRF740 (this is a relatively obsolete part, use
    the IRF740A instead) are sometimes used:

    http://www.irf.com/product-info/datasheets/data/irf740a.pdf

    This is a 400V device. If you use a lower voltage rated part the device
    will likely avalanche at a low voltage and the net result is your maximum
    attainable output voltage will be substantially less than possible. Some
    MOSFETs are sufficiently avalanche rugged they do not absolutely require
    external voltage clamping circuitry.
     

  5. Oh yeah. Some practical notes for this device. Digikey has them in stock
    for $3.75 for a single unit or $2.63 each for ten. I believe in normal use
    this device normally expects to see a gate resistor of around 1k. This is a
    logic level device, so it is quite likely all you need to interface this
    device with your microcontroller is a 1k resistor.

    I recommend using this device. A relatively small heatsink is probably
    required depending upon your circuit's usage requirements.
     
  6. happyhobit

    happyhobit Guest

    Hi Fritz,

    Automotive ignition coils are 6 volt for starting. The ballast resister
    drops the 12 volts from the battery to 6 volts for running. The resister is
    jumped out for starting because the motor starter pulls the battery voltage
    down so much

    Jay.
     
  7. Keith

    Keith Guest

    Thanks folks. Good info all around.

    Keith
     
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