10
HOW TO SELECT A HEAT SINK
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Find a heat sink to keep a TO-220 device below the maximum
150 °C junction temperature in forced convection at 400 ft/min.
Device must be electrically insulated and mounted with a labor
saving clip.
Given:
P
D
= 12 watts
R
θJC
= 2.5°C/W (from semiconductor manufacturer)
T
J
max = 140°C (from semiconductor manufacturer)
T
A
max = 50°C
A Hi-Flow® pad works great with clip mounting and provides the
necessary electrical insulation.Thermal resistance for
Hi-Flow® at low pressure is 1.15°C/W (from page 87).
Using equation 1, solve for R
θSA
R
θSA
= 140 – 50 - (2.5 + 1.15) = 3.85°C/W
12
Many styles are available. If board space is a concern,
533202B02551G (pg 55) meets the requirements.
According to the above graph, an airflow of 400 ft/min results
in a thermal resistance of 3°C/W.This is less than the required
thermal resistance of 3.85°C/W and is therefore acceptable
under these airflow conditions.
If height is a concern, 533702B02552G would meet the
requirements and is only 1.0”tall
Find a space saving heat sink to keep a TO-220 device below the
maximum 150°C junction temperature in natural convection. Device
will be screw mounted with an electrically conductive interface.
Given:
P
D
= 6 watts
R
θJC
= 3°C/W (from semiconductor manufacturer)
T
J
max = 150°C (from semiconductor manufacturer)
T
A
max = 65°C
A Kondux
TM
pad is a good choice for electrically conductive
applications.Thermal resistance for Kondux
TM
can be determined
from the following graph.
Example B
Example A
How To Select a Heat Sink
At 2 in-lb of torque the thermal resistance
is approximately R
θCS
= 0.5°C/W
Using equation 1, solve for R
θSA
R
θSA
= 150 – 65 - ( 3 + 0.5) = 10.7°C/W
6
The Index by Heat Sink Style on page 8 lists space saving heat sinks.
Several models are in the 10 °C/W range. Choose the one that best
fits the application and verify thermal resistance from graph.
Part number 593202B03500G shows
a 60 °C temperature rise at 6 watts.
R
θSA
= 60 = 10.0°C/W
6
Which meets the above requirement in natural convection.
