Power MOSFET Selection

Discussion in 'General Electronics Chat' started by Federov, Sep 21, 2010.

  1. Federov

    Thread Starter New Member

    Sep 14, 2010
    I have built a voltage controlled current source for a project at school and am currently trying to spec the components I need to actually build the circuit. The circuit is shown below.


    I realize I should just be using an op-amp but this isn't allowed in the design nor is a voltage regulator. P.S. The small 1u resistors are just to show me current through the MOSFETS.

    This is a run down of current through the mosfet / power dissipated:

    X1 52mA 55mW
    X2 53.4mA 1.04W <---PROBLEM
    X4 26.7mA 535mW
    X5 26.7mA 354mW
    X3 26.7mA 25.8mW
    X6 26.7mA 510mW
    X7 500mA 3.75W <---PROBLEM

    Since this is a differential amplifier X4 and X5 should match (ideally) as well as X3 and X6. The idea would be to buy a single N-MOSFET for X1,X2,X4, and X5 and a single P-MOSFET for X3 and X6. Obviously X7 is the largest problem with X2 a close second. The only thing i have thought of doing is adding MOSFETS in parrallel to disperse the power. The problem is even doing this I will need over 5 MOSFETS to compensate for it and thats being optimistic. I know there are power mosfets rated for 2W + but I also know this is if you had an infinite heat sink which I currently dont have ;). If someone could point me in the direction of a MOSFET that can handle this kind of power and ones to handle the smaller requirements that would be perfect. Thanks in advance!
  2. Federov

    Thread Starter New Member

    Sep 14, 2010
    Sitting here trying to figure out a solution myself I think i have determined i can lower the power dissipation in MOSFETs X1-X6 by simply lowering the bias current. The negative effect is that I risk not saturating the MOSFETs, but I think I can find a fair balance just need to keep the threshold voltage of the MOSFETs around .7V at 1mA for X1 -X6. Suggestions on a good MOSFET for this application would be appreicated along with still the major problem MOSFET X7.
  3. SgtWookie


    Jul 17, 2007
    Try adding a power resistor in the current path that limits maximum current. Let the power resistor take the heat.