calculating heatsink for LM350At..need help!

i am looking for a heatsink. I am new to this henc i am not sure on how to calculate.

Input supply ( from solar cells) : 15.6v, 1.33A ( max)

ouput of the regulator is 12.85V.

Power dissipation: ( 15.6-12.85)* load current ( approx 1.3) = 3.575w

Everytime i run the lm350 it gets very hot. I had tested on the dc supply , the regulator switch to its thermal protection by limiting the current.

jc- junction to case, cs- case to heatsink, sa- heatsink to ambient

the junction temperature must not exceed 125 degrees.

can i neglect Rcs?

T(junction)- T(ambient) = Power loss ( Rjc + Rcs + Rsa)

125- ( how do i determine?) = 3.575( 3+ Rsa)

Rsa is to determine he minimum thermal resistance of the heatsink?.

 

Too much math. Take a 5W or 10W power resistor, the ceramic kind that has an aluminum flange for mounting on a flat surface. Or better yet, one like this. Temporarily screw it to a 4" square of aluminum, about 1/8" thick or thicker. Hook the resistor to a power supply, adjust the voltage until the resistor is dissipating ~4W. E=sqrt(4/R).

In the final application, can you mount the heatsink vertically or horizontally? Set the aluminum plate in the same orientation. Let it reach thermal equilibrium (might take a few minutes). Measure the temperature of resistor body and the plate near where the resistor is mounted.

If the body of the resistor body is hotter than ~75 degC, you need a bigger heatsink, or it needs to be placed vertically instead of horizontally. If the temperature is < 50 degC, then you can make the plate smaller.

If you have a heatsink in the junk box, you can use this test on that, too.

 

thanks for the link..will try to understand the concept.

 

I had shortlist a few heatsinks but not sure whether i had done it correctly.

http://singapore.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2342299

Tj = Ta + P( Rjc + thermal resistance of heatsink) < 125 degrees.

Ta- operating temperature of air eg sunny day take as 34 degrees

Tj = 34+ 3.575(3+9.9) = 50 plus degrees < 125 therefore it will not activate the thermal shutdown of the ic chip.

correct me if i am wrong..thx...

 

noted..had purchased some mica.. any negatives to screw the ic with a nut to the heatsink?.with the mica in between to provide insulation?..

will be switching the heatsink once i completed my testing on my breadboard..thx..

 

noted thanks