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mosfet gate drive impedance

Discussion in 'Electronic Design' started by gearhead, Jan 14, 2008.

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  1. gearhead

    gearhead Guest

    I have a situation where I need to use a pretty stiff pull-up resistor
    on a mosfet gate because I'm going to use a comparator
    to turn it off, and I plan to limit the current the comparator sinks
    to about 5 mA.
    The N-mosfet source will be sitting at 15 volts and the gate drive
    comes from a voltage doubler.
    That's a bit less than 30 volts so the pull-up resistor needs to be in
    the 6k ballpark.
    I'm going to use the NTP90N02:
    http://www.onsemi.com/pub/Collateral/NTB90N02-D.PDF
    Speed doesn't matter -- this is basically an on-off switch, so I have
    only to consider the dc behavior of the mosfet.
    The mosfet has to conduct ten amps.
    My question is, how much resistance can you put in front of the gate
    before it gets too weak to turn the mosfet on hard?
    Mosfet models don't address this, so far as I can tell. I haven't
    ordered the NTP90N02 yet so I can't simply experiment
    on it, but I remember other mosfets having problems turning on when
    the gate resistance gets into the tens of kilohms.
     
  2. Joerg

    Joerg Guest


    Attention: Keep in mind that the abs max for Vgs is usually no more than
    +/-20V, including the FET you have selected.


    It can be very high but two things can cause some grief:

    a. Slow turn-on, on account of a high Cgs and Cdg. In your case Cgs is
    over 2000pF so a 6K would slow that down to 10usec or more. During this
    time the FET transitions through its linear range, briefly dissipating
    lots of power and possibly going kablouie. Then there is Cgd which works
    "against" turning it on, further slowing it down.

    b. Depending on what is connected Cgd might cause a "dent" in the
    turn-on phase and possibly oscillation around that point. With a stiff
    load the latter is usually accompanied by a bang, molten solder
    splattering about and so on.
     
  3. Marra

    Marra Guest

    I design my own MOSFET amplifiers, up to 600 watts RMS.
    I usually use a 330R in series with the gate.
    The drive to the 330R is very low impedance.
     
  4. John Fields

    John Fields Guest

    ---
    Since there's no DC path through the gate, the only limit to the
    value of the pullup will be how long it takes the gate to charge up
    through it (and, therefore how much power the MOSFET will be
    dissipating) until the MOSFET turns on hard.
    ---
    ---
    They should all turn on unless they have ridiculously high gate
    leakage currents, It's just a question of how long it takes for them
    to turn on.

    Question...

    Since you're doing high-side driving, why can't you use a P channel
    MOSFET, like this: (View in Courier)

    ..+15V>---+-----+----+----+
    .. | | | |
    .. [R1] | [3K] |
    .. | | | |
    .. +----|+\ | S
    .. | | >--+--G IRF4905
    ..IN>-----|----|-/ D
    .. | | |
    .. [R2] | [LOAD]
    .. | | |
    ..GND>----+-----+---------+

    where, assuming IN is positive true, R1 and R2 are selected to
    cause the voltage at the + input of the comparator to be about
    halfway between the swing of IN?
     
  5. John Fields

    John Fields Guest

     
  6. Joerg

    Joerg Guest

    But according to Walmart there is Watts PMPO :)
     
  7. JeffM

    JeffM Guest

    You're joking. Right?
    Back in (what?) the early '70s, EIA decided that
    instead of using the logical term "Watts, continuous"
    they would call it "Watts RMS" for audio gear.

    That is is similar to *root mean squared* makes this confusing
    and is a sign of bad judgement on their part.
     
  8. It appears to have been the Institute of High Fidelity - a group of
    manufacturer's
    http://www.hifi-writer.com/he/misc/rmspower.htm
    This, and other articles, seem to agree that it is, at best, confusing.
     
  9. JeffM

    JeffM Guest

  10. Guest

    It is indeed possible to calculate RMS power. However, the result is
    a mathematical curiosity and is not useful. If you calculate RMS
    power, the result will be 1.225 X the value you get by multiplying
    (RMS voltage) X (RMS Current). See the article in JAES (Journal of
    the Audio Engineering Society) "RMS Power: Fact or Fancy" by Eargle
    and Locanth for the details. The latest IHF Standard for Amplifiers
    was released in the late 60's. I does not make reference to "RMS
    power". In its table of minimum amplifier specifications, it refers
    to "Continuous output in watts per channel". Partially in response to
    audio equipment manufacturers' abuse of power ratings ("RMS Power",
    "Peak Power", "Music Power", etc), the FTC published its "Electronic
    Code of Federal Regulations". Part 432 of this regulation specifies,
    among other things, that power output shall be disclosed as "rated
    minimum sine wave continuous average power output". In fact, the only
    use of the term "RMS" in this regulation is with respect to the power
    line voltage and current. "RMS Power" makes absolutely no sense.
    Regards,
    Jon
     
  11. Winfield

    Winfield Guest

    That's a 35-year-old article, which makes it a bit hard
    to get. But it's no doubt interesting. Could you put
    up a scan?
     
  12. Guest

    The articel can be obtained by visting the AES website: www.AES.org
    Regards,
    Jon
     
  13. Joop

    Joop Guest

    Ah yes, Peak Marketing Power Output ;-)
     
  14. gearhead

    gearhead Guest


    Hey John, I'll explain what I'm doing here and why I think I need a
    high-side drive.


    In the interest of economy the sketch below leaves out most of the
    circuit I'm building, but it shows the part involving `the mosfet I
    asked about in the original post. It's a kind of
    synchronous recitifier that turns on and off at relatively lengthy
    intervals when the generator revs up or turns off. With the generator
    turned off or idled, the mosfet acts as a blocking
    rectifier to keep the battery from discharging into the generator
    windings.



    Vboost
    |
    ,-----------------, ,-+-,
    | | | |
    | | Rp |
    | ,--+-----|---|---,
    | | | | |/| |
    ) V S|_ | /+|-'
    ) - _||--+--< |
    ) | D| \-|-,
    ) | | |\| |
    ) '--+---------|---'
    ) generator | |
    ) windings | |
    ) ___+ |
    ) _ |
    ) ___ |
    | _ |
    | ___ |
    | _ - |
    | | |
    | | |
    '-----------------+---------'
    |
    -----
    ---
    -

    The rationale: using a mosfet avoids expensive, bulky heatsinking a
    conventional rectifier would require. The mosfet is also cheaper than
    a rectifier for the same current. Less than $2 in small quantities
    for the NTP90N02 from Digikey. It needs a charge pump for the drive,
    but having the charge pump there also allows me to use a cheap
    jellybean comparator on the high side.
    Without the charge pump, to run 10 amps wihout a heatsink, using a p-
    channel like an STP80PF55 would require two in parallel at $2.50 each,
    and a more expensive rail-to-rail comparator like the LT1716, which
    only has one comparator in it and costs $2.50 (in small quantities).
    But I need four comparators altogether in this circuit, three of them
    on the high side, and with an LM339 I get it all for about 50 cents.
    So I come out ahead even though I have to spend an extra buck or two
    on a charge pump. Two STP80PF55's and three LT1716's comes to more
    than ten bucks.
     
  15. Bob Monsen

    Bob Monsen Guest

    Your mosfet has an internal schottky diode from source to drain, which, in
    your picture, will conduct when the generator is on, regardless of whether
    you have gate drive. It also enables you to get rid of the external diode.

    However, the reason the mosfet is able to deal with 10A is that it has a
    very low Rds(on). A diode won't have that, and so 10A will cause it to
    dissipate more heat than you might expect.

    Also, I'm not sure what the rest of the circuit does, so I'm not sure what
    the mosfet drive is going to do. Are you going to charge the battery with
    it? If so, you probably want the Source to be next to the battery. However,
    that makes the substrate diode conduct when the generator is off, which you
    say you don't want.

    I wonder if you can go with a simpler circuit? Here is one, assuming I
    understand your requirements:

    generator--->|----(battery +)---->|---load

    When the generator is on, it'll charge the battery and power the load. When
    the generator is off, the battery will drain through the load. You may want
    to control the charging current somehow, since dumping a huge current into a
    lead acid battery is a mistake (it'll overheat and vent). For the cost of
    another a power resistor, you can do that easily:

    Gen o------>|----o----------------------------------.
    | |
    | |
    | ___ |
    '---|___|----o-------->|-----------o
    1 Ohm 5W | |
    --- .-.
    - Battery | |
    --- | | LOAD
    - '-'
    | |
    GND o--------------------------o---------------------'
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

    Here is a schottky diode that is about a buck in single quantity from
    digikey:

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=497-2749-5-ND

    Here is a suitable resistor:

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=1.0W-5-ND

    It is only 5W, but you'll need to evaludate that, and may need a beefier
    one.

    Total circuit cost is about $2.50.

    Regards,
    Bob Monsen

    PS: Vista mail doesn't like this message, so it wouldn't put the appropriate
    '> ' in front of the replied text. Some formatting thing is screwing up its
    parser. Please forgive the top posting.


    Hey John, I'll explain what I'm doing here and why I think I need a
    high-side drive.


    In the interest of economy the sketch below leaves out most of the
    circuit I'm building, but it shows the part involving `the mosfet I
    asked about in the original post. It's a kind of
    synchronous recitifier that turns on and off at relatively lengthy
    intervals when the generator revs up or turns off. With the generator
    turned off or idled, the mosfet acts as a blocking
    rectifier to keep the battery from discharging into the generator
    windings.



    Vboost
    |
    ,-----------------, ,-+-,
    | | | |
    | | Rp |
    | ,--+-----|---|---,
    | | | | |/| |
    ) V S|_ | /+|-'
    ) - _||--+--< |
    ) | D| \-|-,
    ) | | |\| |
    ) '--+---------|---'
    ) generator | |
    ) windings | |
    ) ___+ |
    ) _ |
    ) ___ |
    | _ |
    | ___ |
    | _ - |
    | | |
    | | |
    '-----------------+---------'
    |
    -----
    ---
    -

    The rationale: using a mosfet avoids expensive, bulky heatsinking a
    conventional rectifier would require. The mosfet is also cheaper than
    a rectifier for the same current. Less than $2 in small quantities
    for the NTP90N02 from Digikey. It needs a charge pump for the drive,
    but having the charge pump there also allows me to use a cheap
    jellybean comparator on the high side.
    Without the charge pump, to run 10 amps wihout a heatsink, using a p-
    channel like an STP80PF55 would require two in parallel at $2.50 each,
    and a more expensive rail-to-rail comparator like the LT1716, which
    only has one comparator in it and costs $2.50 (in small quantities).
    But I need four comparators altogether in this circuit, three of them
    on the high side, and with an LM339 I get it all for about 50 cents.
    So I come out ahead even though I have to spend an extra buck or two
    on a charge pump. Two STP80PF55's and three LT1716's comes to more
    than ten bucks.
     
  16. gearhead

    gearhead Guest

    ** There is no an external diode, that's a diagrammatic representation
    of the mosfet's internal body diode. Yes, the mosfet conducts anytime
    the generator comes on. That's what I want. It's not there to block
    charging current. It's there to block the battery from _dis_charging
    into the generator.

    (snip)
    **Right, that's why the mosfet is oriented the way it is. So that it
    won't conduct when the generator is off.
    **I appreciate the effort you went to, Bob. The diagram I posted is
    only a detail of a larger circuit, which includes voltage regulation.
    But what I posted ought to be enough to understand what the mosfet is
    doing. It's a blocking recitifier. At first blush, you might think
    the mosfet is backwards, but it's not. The body diode acts as a
    blocking diode (when the generator is turned off). Then, when the
    generator turns on and the voltage at the source rises above the drain
    voltage, the comparator detects it and relaxes its hold on the mosfet
    gate. The voltage doubler then turns on the mosfet through the pullup
    resistor, allowing it to conduct with much less loss than a mere diode
    would. If you take a closer look at the circuit, you'll see how it
    works.
     
  17. There are ORing-controller ICs that drive FETs
    of your choice for the power-supply ORing job.
    For example, the Intersil isl6144, TI tps2410
    and 11, and LTC LT4351 for "+" supply lines,
    and IRF IR5001s for "-" supply lines. All
    of these feature built-in charge pumps, and
    have comparators, MOSFET drivers, and other good
    stuff. The LTC4412 uses a p-channel MOSFET.

    Whether any of these ICs interest you or not,
    studying their datasheets and app notes could
    turn up aspects you hadn't considered.
     
  18. Oh, and Maxim makes a half-dozen parts too.
     
  19. gearhead

    gearhead Guest

    Just what the doctor ordered.
    Thanks.
     
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