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Discussion in 'Electronic Basics' started by Randy Day, Feb 24, 2007.

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  1. Randy Day

    Randy Day Guest

    I've built some TTL motor/relay controllers
    out of NPN transistors in a Darlington
    configuration, and I'm considering using
    mosfets instead.
    - )|
    ^ )| relay
    _R_ |/
    TTL in -|___|--|
    (created by AACircuit v1.28 beta 10/06/04

    My experience with mosfets is zero. Is an
    n-channel mosfet pretty much a drop-in
    replacement for an NPN device, aside from
    gate/base resistor values? If so, how do I
    calculate an appropriate value for R on a
    mosfet? I'm looking for full-off/full-on
    switching, not voltage regulation.

    The Mouser website lists the FDN5630 and
    FDC655BN, which seem to have the current
    capacity (and low price) I want. Are there
    any other things I need to consider when
    designing for mosfets, aside from ESD?

  2. John  Larkin

    John Larkin Guest

    You don't need a gate resistor at all. Just run your logic level to
    the fet's gate. There are some second-order reasons to use a gate
    resistor (kill rf oscillations during the slew, slow down the edges,
    protect the driver if the fet dies) so 1K, close to the fet, wouldn't
    Make sure your logic high is enough to turn the fet on hard, not
    merely "gate threshold" voltage; that's on the datasheets.

    Be aware that an open-gate fet could be on or off, so make sure your
    driver circuit keeps the gate low when it should. Disconnected cables
    can get interesting.

    Most fets like this are fairly esd hard, so don't go overboard on
    handling. I take no esd precautions when working in my lab, and have
    never blown a mosfet. My manufacturing people do all the strap/mat
    stuff, and they don't blow fets either!

  3. In this application, I'd say yes.
    You will likely not need R, there is no current flowing. What you will need
    is active drive, high and low. FETs are voltage controlled, the only
    current to flow into the gate (base) when you turn it on is what is required
    to charge up the tiny gate capacitor, after that the current flow ends. To
    turn them off, the gate capacitance has to be discharged, just letting it
    float won't turn it off.
    Allot of MOSFETs require high voltages to turn them on (usually you have to
    pull the gate higher than the highest voltage being switched. For example,
    if you are using 12V to power the relay, you would have to pull the gate
    higer than 12V. This can really suck when you want to use a microcontroller
    to do the switching. Enter the "logic level" FET. They only need 4 or 5
    volts to turn them on hard, even when switching higher voltages. Much more

    I've had good luck with NTE-2985 parts. 5V of drive and the on resistance
    is .05 ohms max. 60V breakdown voltage and 30A max continuous current. I'm
    sure there are many many others out there, but I accidentally happened upon
    these whilst looking for something else.
  4. John  Larkin

    John Larkin Guest

    Not so. The curves of drain current versus drain voltage, at fixed
    gate voltage, are essentially flat. What is true is that fets with
    higher max drain breakdown specs do *tend* to need more gate drive
    voltage, because the oxides are thicker. I've never seen a fet spec
    that went above +10 gate voltage, even to switch 600 volts, and there
    are lots of mosfets around that will switch 50 volts with 3.3 volt
    gate drive.

  5. Oops, I stand corrected. :) After you make the point about 600V, it does
    seem ridiculous to expect that much gate voltage to turn one on. As for the
    3.3V that switch 50V, how long have those been around, or are they just more
    of the "logic-level" collection? While you're here, is this all that
    "enhancement mode" means; logic level/low gate drive?
  6. Guest

    Don't think that's correct Anthony.
    Typically the gate threshold for a 30A MOSFET is 2-4 volts. For
    example take a look at a BUZ11, maybe you're defining
    this as a logic level fet too. ;)
  7. Vgs(th) for the NTE part is 2V max. .05 Ohms Rds(on) is at 5V gate drive.
    At 4V drive, the on resistance is .07 Ohms max. Sounds like logic-level to
    me. ;-)
  8. It takes more voltage to turn a mosfet on than it takes to
    turn on a darlington, but the gate looks like a small
    capacitor, rather than two diodes in series to ground, so
    instead of putting a current limiting resistor between the
    logic output and the gate, put it between the +5 supply and
    the logic output, so that the logic high voltage will be the
    full logic supply. 2.2k to 10k should help get the fet
    turned on more than just the TTL output would do.

    And you will have to find a mosfet that is called a logic
    level type. This means that its on state resistance is
    specified with a logic level gate drove. Most other mosfets
    have the on resistance specified with 10 volts of gate drive.
  9. John Fields

    John Fields Guest

  10. If I was actually going to try and switch 30A with it, I guess I'd use a
    gate-driver of some type. I have some in my shack that are made by
    Microchip (IIRC) that I used for tinkering with some IGBTs. They have very
    fast switching times and will supply much larger currents (>3A) during the
    transition. The NTE transistors that I mentioned seem to work fine being
    driven by a PIC at 5V. I was only switching a couple of amps though without
    a heatsink.
  11. Randy Day

    Randy Day Guest

    John Larkin wrote:


    Wow! Lots of really excellent advice from everyone!
    I think I'll put the 1k in for driver protection.
    For peace of mind, if nothing else. ;)
    The FDC655 lists 33 mOhms Rds(on) at 4.5v, which
    seems more than adequate. I think I'll go with it.
  12. John  Larkin

    John Larkin Guest

    If you draw the "saturation" curve of a mosfet, Id versus Vd, there's
    some drain voltage where it stops being resistive and goes into
    constant-current mode. That voltage tends to be about the same as the
    applied gate voltage. To get it to saturate below X volts, you need at
    least X volts of gate drive. You can see that effect in the curves on
    the datasheets. Maybe that's what you were referring to. But you can
    switch a bazillion volts of load with just a 5 or 10 volt gate swing,
    if the final drain current isn't too high.

    Logic-level fets are fairly new, last 10 years maybe. Classic
    n-channel mosfets fets were barely turned on at 3-4 volts, and not
    fully enhanced til 6-10 volts; p-channels were worse. The new parts
    use IC technology (cellular layout, polysilicon gates, thin oxide) to
    get low thresholds, at the expense of breakdown voltage.

    Enhancement mode means the drain-source channel is inherently
    nonconducting, "normally off", and applying a gate voltage makes it
    conduct. Depletion mode devices are already conducting, "normally on"
    and require a negative voltage to turn them off. Most depletion
    devices are jfets or gaasfets, although a few people (like Supertex)
    make depletion mosfets, which can be very handy in certain apps.

  13. John  Larkin

    John Larkin Guest

    The drain current at "gate threshold" is only 1 mA. The Rdson at 10
    amps is spec'd with +10 on the gate. So this is of course not a
    logic-level fet, because it wouldn't be prudent to run a beast like
    this with just 3.3 or 5 volts on the gate.

  14. As you can see, I'm not much on analog that's for sure. :) I was thinking
    that I had read somewhere that conventional FETs worked the way I described,
    and it sounds like you might be saying that they sort of do if you need them
    "really turned on". ;-) And I was under the impression that "ordinary"
    P-channels would have to have the gate pulled below ground for the same
    Logic level FETs sure are convenient for guys like me that like to use PICs.
    I also like the high-side drivers that are available, pretty slick features.
    Ok, so it really doesn't have anything to do with what makes the FET turn on
    better at lower gate voltages?
  15. You are referring to the BUZ11 right?
  16. Lionel

    Lionel Guest

    On Sat, 24 Feb 2007 13:03:23 -0600, "Anthony Fremont"

    I'm a complete amateur WRT MOSFETs, so I'm finding this discussion
    interesting. I hope you'll bear with me if this question sounds dumb:
    If you're driving the gate from 5V CMOS logis (eg; a micro), can you
    get around this by putting in a series diode after the logic output, &
    a large (1M+) pullup resistor from the MOSFET gate to your 12V (or
    whatever) relay supply?
  17. John  Larkin

    John Larkin Guest

    Nope. A diode doesn't give voltage gain. You can shift the gate swing
    up by one diode drop, so it swings 0.6 low to 5.6 high. but that
    doesn't help a lot.

  18. John  Larkin

    John Larkin Guest

    Yup. The typical curves show it conducting pretty hard, 8 or 9 amps,
    with +5 on the gate, but that's not guaranteed. If your load is just a
    couple of amps, it's probably safe to go with +5 drive. But I wouldn't
    bet a production run on typical performance curves.

    At 3.3, it might conduct just a few mA, if that.

  19. Guest

    Come on, the BUZ11 isn't a beast, It's a beauty. It's quite versatile
    and cheap too.(84cents at digikey). One of the least expensive N-
    Mosfet's available.
    Here's a PWM using a 555 and a BUZ11.
  20. Just wanted to be sure. The NTE part I mentioned turns on hard at 4V drive.
    They aren't the cheapest things, but so far I like them ok for a jelly-bean
    part. Do you have a favorite cheapie logic-level part in this current
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