# Power MOSFET Selection

#### Federov

Joined Sep 14, 2010
26
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!

#### Federov

Joined Sep 14, 2010
26
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.

#### SgtWookie

Joined Jul 17, 2007
22,227
Try adding a power resistor in the current path that limits maximum current. Let the power resistor take the heat.