# Current in the legs of a MOSFET

I'm currently doing an assignment and I've been asked to find the maximum power dissipation in a MOSFET. I've been told that this is equal to the the current through the MOSFET times the voltage across it

$P=I_D(V_D-V_S)$

simulate this circuit – Schematic created using CircuitLab

Where $V_D$ is 26V and $V_S$ is variable from 0 to 15V. My only problem is i'm not sure what the relation is for the current? I've so far assumed the current through the drain is equal to the current through the source.

The lowest value of $R_L$ is 22.7 ohms and the highest is 10,000 ohms. My initial calculation gave me roughly 15W using the lowest value for $R_L$. Is this anywhere close or is there a relationship i'm missing?

• The difference between source and drain current is just the gate current, and we know that's negligible, so for practical purposes source current == drain current. Jan 26 '14 at 16:32