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Using Logic ICs to make short pulses

  1. Jun 13, 2007 #1
    Hello All,

    I'm designing a driver board to control some MOSFETs. I have some mic4427 MOSFET drivers to drive the gates into avalanche mode but the function generators I am using to trigger the chips only put out 50% duty cycle square waves. I'm working in a wide frequency range from 1 kHz to about 500 kHz and I simply want to use the shortest possible pulses to turn on and off the MOSFET driver chips.

    I am wondering if there is a way I could use some logic chips to produce extremely short pulses ~1uS to trigger the MOSFET drivers? I would simply feed my function generator output into the chip(s) and they would, in turn, make the shortened pulses to drive mic4427s. If not, does anyone know of a simple way I could accomplish this? I'm trying to think of the most bare bones way to do this with the least components.

    Jason O
  2. jcsd
  3. Jun 13, 2007 #2


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    Staff: Mentor

    Can you just capacitively couple the square wave, in order to get pulses on the edges? You can use the positive pulse to turn on the FET. You may need to diode clamp the negative-going pulse, if it exceeds any reverse breakdown voltage specs of what you are driving.

    Put an explicit discharge R after the C, and tune the RC time constant for your pulse width.
  4. Jun 13, 2007 #3


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    It's a simple solution, berkeman, but the "pulses" are not going to be anywhere near a microsecond long -- ideally, they'd be deltas if the function generator has a nice fast slew rate -- and they're going to be rather poorly shaped.

    If it were up to me, I'd use a monostable multivibrator (a "one shot"). A tiny IC won't take up much more board space than a large-valued capacitor, and it'll be a lot more configurable and precise.

    - Warren
  5. Jun 13, 2007 #4
    Hi Warren & Berkeman,

    I really like both of your approaches and would like to test out each one to see what works best.


    I would not be driving the MOSFET gate directly from the function generator + short pulse setup, the MOSFET driver chip will drive it directly. I just need a signal to the FET driver that is fast enough and with a short enough width to do it. According to the datasheet for my FET drivers, the input needs 1.5V to V+ to turn on and is monitored by a comparator which will smack the gate of the MOSFET on in under 43ns once the threshold is reached. In my case, V+ is a 9V battery. Could you tell me how to wire the RC setup to test out your idea?


    I really like the multivibrator idea because it is closest to what I was imagining before. But what kind of IC specifically should I use?

    Jason O
    Last edited: Jun 13, 2007
  6. Jun 13, 2007 #5


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    The LM555 or equivalent is the simplest and probably best choice. You can look up its monostable configuration in its datasheet.

    - Warren
  7. Jun 13, 2007 #6


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    I agree with chroot; use a monostable multivibrator chip.
    1us is hardly 'extremely short' relative to the speeds of
    basically *any* common digital logic chip you can
    easily buy today will generate pulses of 100ns to 5ns and
    correspondingly operate at frequencies from 10MHz to
    200 MHz, and it's hardly uncommon to find 500MHz to
    2GHz rated parts if one needs them.

    Even the decades old 555 timer/multivibrator will generate
    about 1uS pulses in astable mode, though it isn't the
    best choice due to retriggering issues and that being
    up there in the range of its speed capability.

    Anyway, try this multivibrator (or a similar one with
    schmitt trigger inputs and edge triggering)..

    Use the TTL 74LS prefix equivalent part version
    (check the speed first) if you'd rather not work with the
    relatively more electrostatic discharge damage prone
    CMOS parts.


    330pF and 5.6k will be just about a 1uS pulse time
    for that part.

    Beware -- check the output drive current specifications
    of the MOSFET drivers and the gate capacitance of the
    MOSFETS; it's entirely possible that large higher power
    MOSFETS will be difficult to turn on and off in 1uS
    simply because the gate capacitance can become
    rather large and that (as well as other inductance and
    slew rate limitation issues) limits the maximum frequency
    one can toggle them on and off at given a reasonable
    amount of gate drive current.

    For low frequency repetitions of short pulse
    periods, however, at least thermal dissipation issues
    due to gate drive power shouldn't be a limitation since
    you're letting them stay off for 500 times the desired
    pulse period.

    Also beware that the logic levels and drive requirements
    of the MOSFET gate drivers are compatible with the
    logic levels of your multivibrator chip.
  8. Jun 13, 2007 #7


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    PS -- oops I thought you had said 500Hz to 1kHz rep.

    Actually at up to 500kHz pulse rep. rate you better
    make sure that your driver and MOSFET gate current
    and power dissipation / thermal specifications aren't
    being exceeded by driving the MOSFETS on and off at
    such a frequency. It's not unthinkable that the larger
    capacitance MOSFETS may have issues pulsing at quite
    that speed for extended periods of time.
  9. Jun 13, 2007 #8
    Hi Xez,

    Thanks for all the great info. The MOSFETs I am using are IRF840s and you're right, they have a lot of gate capacitance; that is why I need to use the FET drivers so I can get them to turn on fast at high frequencies. I can directly drive them up to 1MHz off of my function generator but I have a Coax line between the MOSFET drivers and the board containing the fets and there starts to be a lot of reflections and other garbage on the line once I get up around 600 kHz and up. But for the range I'm using they are switching on very cleanly. I'll check out the links you posted, thanks for your help :-).
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