MOSFET heat sink calculation: I am designing H bridge to drive brushed DC motor . The MOSFET I'm using is IRFB4620PbF. I calculated the MOSFET losses which are 7.8W per MOSFET. I'm not sure if I am doing the calculations correctly for heat sink selection.
Tj = Pd (RthJC + RthCS + RthSA) + Tamb
Tj junction temperature : 150C
Pd = dissipated power (RMS current through the part x voltage drop) + switching losses = 7.8W
RthJC = thermal resistance, junction to case (from the device data sheet) = 1.045C/W RthCS = thermal resistance, case to heat sink (from the device data sheet) = 0.5C/W RthSA = thermal resistance of the heatsink to air.
Tamb = ambient temperature immediately adjacent to the module in your application = 25C
Susbstituting the above values I get the RthSA as 14.48C/W.
Could you please let me know if the calculations are correct or am I missing something. Thank you for your help.