N-Channel MOSFET Selection - SOA & determination of VDS - LED Application
How do I determine the VDS of the MOSFET in my application?


For power dissipation, given you are using an external part (series resistor) to limit the current, you just need to know the Rds(on) of the FET and the load current. Then Vds in the "on" state is just \$I \times{} R_{ds}(on)\$.


  the MOSFET will never be exposed to 36v because each LED diode has a forward voltage drop and the MOSFET itself will be at the end of the chain. Am I being led astray with this assumption?


As horta points out, you also have to consider the case when the MOSFET is "off". Then there is no current through the LEDs and the Vds on the MOSFET is the whole 36 V.

About Safe Operating Area (SOA)

NXP app note AN11158 explains the reason for the limits on different parts of the SOA curve:



Because you are using an external current limiting device, you should be in part (1) of the curve, limited by resistive self-heating.

When the LEDS are off, you will be in part (4) of the curve, limited by breakdown of the body diode or other junctions in the device.