# Thermal Analysis( When to use heat sink)

#### sharath_412

Joined May 7, 2007
32
Hi,
Can anyone explain me about Thermal analysis of IC's. When to use Heat sink and all related stuff. How to calculate the temperatue rise etc. Any link .........

thanks & regards
sharath

#### eeboy

Joined Sep 27, 2007
90
You would use a heat sink in the case that power dissipated by the component in question causes you to exceed the maximum power dissipation listed on the data sheet. At this point you need something to conduct the heat away such that the junction temp remains below the absolute maximum. This can be achieved via heat sink, thermal pads, forced air.... many methods.

I've read several good application notes on the subject but I can't seem to locate one at this time. Try International Rectifier to see if you can find something from them.

It actually turns out to be fairly simple. Although I've read other documents which downplay the industry standard method at calculating the junction temperature. My take on it after reading quite a bit of info is that it's not exact and so one should build in a good safety factor.

#### nanovate

Joined May 7, 2007
666

#### John Luciani

Joined Apr 3, 2007
477
To determine the temperature rise you need to calculate the junction
to ambient thermal resistance (Tja) ---

Tja = Tjc + Tcs + Tsa all the dimensions are DegC/W

Tjc thermal resistance between the IC junction and IC case. This is
from the IC datasheet.

Tcs thermal resistance between the IC case and your heatsink. This
should also be in the IC datasheet. Usually you place a thermal
compound between the IC case and the heat sink to decrease the
thermal resistance. The decrease occurs because the thermal
compound fills the air gaps between the two surfaces and conducts
heat better than the air.

Tsa thermal resistance between the heatsink and ambient air. This is
in the heatsink datasheet. Most heatsink datasheets specify Tsa
with and without airflow. You can determine if you need a fan by
reviewing this number.

For a single heatsource the junction temperature would be

Tj = Tja * P + Tamb (DegC)

where P is the power dissipated in the IC and Ta is the ambient
temperature.

(* jcl *)