[BrianG]

Is that 150*F figure heatsink temperature or device temperature? The temps at the FETs (or the surrounding traces) can be substantially higher than the heatsink temp, especially if it's cold outside. Also, depending on what kind of temp probe you have, they can be inaccurate if you don't calibrate it. I'm talking mainly IR types here where you have to set the emissivity based on the material and color you are taking the measurements off of.