MOSFET Driver vs op amp or comparator

Discussion in 'The Projects Forum' started by roxetz, May 1, 2008.

  1. roxetz

    Thread Starter New Member

    Mar 6, 2006
    5
    0
    Hello,
    I am looking for something to interface a control signal at +3.3V to drive the gate of a power MOSFET at about +16V for my power inverter in an interconnected H Bridge topology. I have been looking around for high and low side MOSFET drivers, but I just wanted to know what the real difference is if I chose to drive it with a simple non-inverting op amp instead with the appropriate gain. Along those lines, wouldn't it be okay to use a fast comparator as well with rails at 16V and ground? What is unique for the MOSFET drivers? Thanks in advance!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, why would you want to do that?

    Current requirements can get pretty high if you're driving your bridge above a few kHz. Then you're getting into power opamps, and their bandwidth is somewhat limited - pretty soon you're driving the gates with triangle waves as your MOSFETS begin to glow in the dark. Got marshmallows?

    The MOSFET drivers are designed to have a wide bandwidth. They don't have all the whistles and bells that comparators and opamps do, but they don't need that functionality, either. They just charge and discharge gates, as quickly as possible.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Some power MOSFETs need to be turned off by something able to sink a couple Amperes. Most op-amps or comparators just can't do that.
     
  4. roxetz

    Thread Starter New Member

    Mar 6, 2006
    5
    0
    Thank you guys for your reponses, to follow up:
    The system will operate at a rate 100W at 60Hz. So I don't think BW will be an issue...
    To help with the power MOSFET turn off I've put Schottky's in parallel with them to eliminate dead time.
    I'll probably go with the MOSFET drivers, though. Which means I need to decide the difference between high side drivers and low side. I am confused because sometimes these switches (MOSFETS) will either be connected to Vcc, and other times to GND. In that case, should I go with High side or Low side?
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The switches connected to Vcc are driven with high-side drivers. The switches connected to GND are driven with low-side drivers. Many manufacturers make affordable full bridge drivers and half bridge drivers - low side and high side all in one package.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    That is not the bandwidth we are talking about. There are two ways to operate a MOSFET.
    1. Fast turn-on and turn-off to avoid excessive power dissipation from running the device in its linear mode. This requires a device that can move a boatload of current very quickly. A 741 just won't cut the mustard.
    2. Operation in the linear mode requires a really big heatsink with liquid Nitorgen cooling channels. Don't believe me? Figure the current, square it, and multiply by the worst case rds(ON) of the device.
     
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