check the datasheet for thermal de-rating factor - it might be a surprisingly high number (a 100W mosfet can easilly become 40W max at high temperatures)
Checking the datasheet, I cannot see anything directly called thermal de-rating factor - unless I am overlooking something.
The IXTH30N50L2 is quoted as a 400W device (when Tc = 25 deg C).
If I take the quoted SOA: 200W @ Tc = 75 deg C.
RthJC is 0.31 deg C/W.
200 * 0.31 = 62 deg.
Therefore: 75 + 62 = 137 deg C for the junction temperature when the case is 75 deg C and I am driving it at 200W.
I plan to drive the device at a maximum of 150W.
Assuming 137 deg C is within the SOA and working backwards...
150 * 0.31 = 46.5 deg C.
137 - 46.5 = 90.5 deg C.
To remain within SOA, I can allow the case temperature to be as high as 90.5 deg C @ 150W.
Now add to this the RthCS of 0.25 deg C/W --> 150 * 0.25 = 37.5 deg.
90.5 deg (case) - 37.5 deg = 53 deg.
If my math above is correct...
I can allow my heatsink face to get to 53 deg C if I am driving the FET at 150W.
Have I understood this correctly?