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Hi side MOSFET

Discussion in 'Electronic Design' started by BrianK, Mar 5, 2007.

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  1. BrianK

    BrianK Guest

    Hi, I have been reading the posts on high side MOSFET drivers, and
    still have some questions. I just want to drive 12V, 40amps, using a
    PWM signal. I drive 12V, the other end of the load is connected to

    1. Would you use an N-channel mosfet because they are cheaper, better,
    etc. If I do, I would need a high side driver and a charge pump to
    generate at least 18 volts. I was looking at the IR2117 to drive it
    but its current (+200mA/-420mA) seems low to drive a P75NF75 mosfet. I
    ran some calcualations for a gate driver and it came up around 6 amps.
    I thought MOSFET drivers needed to drive higher currents because of
    the gate capacitance. Are there any high side drivers with the charge
    pump built in?

    2. If I use a P-channel MOSFET, I would still need a high side driver,
    what would you recommend? I have seen examples just using a 2N2222.
    Again, isn't that current low (500ma). If I use a series resistor to
    drive the gate, won't that slow it down even more?

  2. maxfoo

    maxfoo Guest

    Not 40A, but this safely handles 10A using a BUZ11, just search digikey for
    higher amps.
  3. Mook Johnson

    Mook Johnson Guest

    It depends on the swithcing frequency as to how much gate charge current is
    neded to switch it fast enough to not dissipate power. You could make an
    isolated DC supply and optocouple to the input a 1A or better driver to
    drive a large high side N-FET.

    A more reasonable solution is to put the N-fet on the low side with the
    source referenced to ground. Then you can use the many low side drivers
    and drive it as hard as you desire.

    The series resistor close to the gate is to prevent undershoot and overshoot
    (shic could get as serious as oscillations in some cases) gaused by the gate
    capacitance, stray inductance and high Di/dt diring the charging edge. It
    usually only takes 10 ohms or so to damp it. Yes it will slightly slow down
    you switching but this can be a good thing as a compromise between the
    radiated EMI and the MOSFET power dissipation. Fast edges and wires are
    good RF tramsmitters.
  4. Most n-channel high-side MOSFET drivers include a provision for a
    flying-capacitor gate-driver supply voltage, using the PWM signal
    itself for the charge-pump action. You have to add the capacitor,
    and sometimes a diode as well. Some include a low-voltage disable.
    The IR2117 is meant for high-voltage offline use. There are lots
    of good choices for low-voltage high-side drivers, and most have
    much higher current capability. Check out TI, MicroChip, LTC, etc.
    ST's 75nf75 is a modestly-serious MOSFET, and 400mA gate-drive
    current would be a bit on the weak side, but 6A will likely be
    more than you need. Anyway, there are drivers to 12A and beyond.
    There are a few that have internal self-oscillating charge pumps,
    so they can be used at DC, but they're rarely used for PWM.
  5. Guest

    I would recommend you use an IX6R11S3 -- high side/low side gate
    driver with 6Amps peak output current. You can get info off the IXYS
    web site --

  6. John Fields

    John Fields Guest

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