I am trying to calculate the junction temperature of a D2PAK FET that is dissipating 5W. The junction-to-case thermal resistance is 0.4 C/W. The junction-to-ambient (PCB mounted, steady state) resistance is 40 C/W. It seems that D2PAK datasheets don't give a case-to-sink figure.
I am using this sink which has a thermal resistance of just 3 C/W with forced air flow. My question is, when calculating the junction temperature do I include the junction-to-ambient figure in the calculation?
Tj = Pd(Rth(jc) + Rth(ja) + Rth(heatsink)) + Tambient
Or can I ignore the ambient figure when using a heatsink?