Logic-level MOSFET help

Discussion in 'Analog & Mixed-Signal Design' started by Razor Concepts, Jan 16, 2010.

  1. Razor Concepts

    Thread Starter Senior Member

    Oct 7, 2008
    I used 8 FDS6680AS logic-level MOSFETs to create a dual h bridge motor driver. Schematic below:


    I am using a 6-cell nimh battery supply (~7.2 volts) and I am delivering 5v to the gates of the MOSFETs. The motors (small 280 motors with gearboxes) move significantly slower than if connected directly to the battery. Also the MOSFETs get warm.

    Even though the MOSFET is logic-level, will I need a MOSFET driver to get the full current to the motors?
  2. SgtWookie


    Jul 17, 2007
    Yes, you'll need drivers.

    You can't wire your high-side and low-side gates together like that.

    Your high-side MOSFETs are acting like voltage followers. Their source terminals can't go higher than the gate voltage less the threshold level.