Things you must know to get started:
. Tamb Maximum ambient air temperature of the environment
. Tjunc Maximum allowable junction temperature of the pass element.
. RthetaJC Rated thermal resistance from junction to case of the
. pass element (regulator).
. Pmax Maximum power that will be dissipated by the regulator.
.
Now, Let
. RthetaJA = Maximum allowable Thermal resistance from junction to ambient air
. (You are going to calculate this below).
. Rthetahs = Maximum allowable thermal resistance of the heat sink.
. (You are going to calculate this below).
. Tj = Tamb + (Pmax X RthetaJA)
. Solve for RthetaJA:
. RthetaJA = (Tj - Tamb) / Pmax
. Solve for Rthetahs:
. Rthetahs = RthetaJA - RthetaJC
. (This is the maximum heat sink thermal resistance that you can use).
.
If you are using a commercially available heat sink, the Rthetahs value will be a published value.
.
If you are using the copper on a PCB, things are a little harder. You can do one of the following:
. Use published tables of thermal resistance vs surface area
. Measure the copper thermal resistance by:
. Mounting a resistor on the copper, applying a known power P, and measuring
. the temperature rise Delta T. Then
. Rthetacopper = (Delta T)/P Deg/Watt.
.
I hope this helps,
Kral