Hello
I am trying to make sure I understand the thermal characteristics on the data sheet of a mosfet.
I am planning on using a D-Pak P channel Mosfet (IRLR9343PbF) as a switch, using the part as it is available (used on a previous design)

Link to part Pdf on IR website:
http://www.irf.com/product-info/datasheets/data/irlr9343pbf.pdf

The data sheet indicates Pd @ Tc 100C = 39W.
The power generated when the switch is on should be 4.25W

If the part is mounted on a PCB with no heat sink should I use the PCB Mounted JA thermal resistance to calculate the Junction temperature?
Eg:
Tj = Ta + Pd*Rja
Tj = 272.5C =60C + 4.25*50C/W
272.5C is a lot higher than the maximum junction temperature.

Questions:
Will the Mosfet need a heat sink (I am assuming it will)?
Would it be possible to get away with a large metal area on the PCB and are there any formulas for this?
If I use a heat sink how do I use the Rjc on the datasheet?
Why is the Pd stated for Tc not Ta (I was assumed it was Ta until I started looking at the thermal resistance values), Pd at Ta I think is more relevant.

I have never really done any thermal calculations for a design (always been well with the Pd of the device); please correct me if I am missing anything obvious.

Thanks

 

Without heatsink you can dissipate a power:
Pd = (Tj_max - Ta )/Rja = 1.3W
Or if you mounted on 1" square PCB, then
Pd = 2.9W
So yes you need a heatsink (PCP in not enough)
Rth = (150°C - 30°C ) / 4.3W = 27 C°/W

http://sound.westhost.com/heatsinks.htm
http://www.irf.com/technical-info/appnotes/an-994.pdf