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555 to drive mosfet/igbt

Discussion in 'Electronic Basics' started by kell, Mar 25, 2005.

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  1. kell

    kell Guest

    I was experimenting driving various mosfets and igbts with a NE555 at a
    low freq around one per sec turning a small bulb on and off just to
    check how well the 555 works before I try to build the actual circuit I
    have in mind, which will have an inductor operating at a few KHz in a
    topology similar to a boost converter. (The inductor is intended to
    reach a peak current of several amps.) I used no gate resistor, just
    connected pin 3 directly to the gate.
    Anyway, when I tried the IRF740 and a IGBT designed for car ignitions
    (HGTP14N...), which both come in the TO-220 case, the bulb turned on
    and off cleanly. But when I tried a couple of devices that come in the
    big TO-247 case, the bulb didn't turn off cleanly. With the IRG4PC30F
    IGBT, the bulb dimmed before it turned off. With the STW18NB40 mosfet,
    the bulb didn't even turn off, it just flickered a little.
    I used a sealed lead acid battery and connected the Vcc pin of the 555
    through a 75 ohm resistor to B+. I had a 100uF electrolytic across the
    power pins.
    I was powering the 555 through a resistor because that is the way I
    intend to use it in the final circuit, as a way of protecting it from
    the inductive spiking.
    What is it about these big mosfets and IGBTs, that a 555 can't turn
    them off?
     
  2. jsmith

    jsmith Guest

    Possibly you may not need the electrolytic capacitor since your B+ is a
    battery??
     
  3. Perhaps the 555 does not really switch off. What's the voltage at pin 3
    when load should be off? The 555 can sink about 100 mA so the gate
    should be grounded very well. What else do you have connected to the
    555? (pins 2,5,6,7). Maybe something is ringing or even oszillating
    which prevents the 555 from switching off correctly.

    Regards
     
  4. The 555 should really be able to shut them off. Test the MOSFETs that
    fail to turn off by disconnecting them from the 555, and grounding the
    gate. Does it turn off?

    The gate threshold can be as high as 5V. It may take 10V Vgs to turn it
    on completely.

    Some mosfets may take even more voltage to turn completely on. The
    IRF740 has a slightly lower Vgs(th) than the other.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. me

    me Guest

    Anyway, when I tried the IRF740 and a IGBT designed for car ignitions
    Could be the gate capacitance, try a low value resistor (maybe 200 ohm)
    from gate to ground in the circuit...
     
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