Reg:Heat Sink Calculation

[sakthivel.eee]

Dear All,
I have few doubts regarding heat sink calcution. Please help me
Heat sink Thermal resistance is given by
Rsa=((Tj-Ta)/Q)-(Rjc+Rcs)
1. How to find Q for specific ambient temperature? I hope we should not use maximum power dissipation. (Datasheets has curve only case temperature Vs Power dissipation)

Rcs= p*t/a
p= resistivity of interface material
2.Interface material means insulating between heat sink and board or heat sink material's resistivity?
3.How to decide the size of heat sink?

4.For SMD packages, We will give pad in opposite side of the PCB. How to decide the size of that pad?
5. Does anyother parameter should be considered for selecting heat sink?

Thanks in Advance,

Thanks & Regards,
Sakthivel.S

 

[eem2am]

there is a limit to how much heat you can get out of a device with copper on a pcb.

there is a good app note on it.....something like onsemi.com app note

 

[sakthivel.eee]

Dear Sir,
What is the applicatin note name or else could u post the link

Sakthi