Heat Sink Design Questions

[Jim Chim]

Hi,

Is Thermal Resistance is only a model to describe the heat-transfer
phenomenon for a heat-sink? Or it have it's own physical meaning?

If I have a heat-sink, will the thermal resistance be constant value
in thermal test using different ambient temperature?
For example,
(1) T_ambient=34C, Rca=0.30
(2) T_ambient=12C, Rca=0.60
where Rca is the thermal resistance that calculated across CPU case
and ambient by using the equation
(Tcase-Heat_sink_inlet_temperature )/
CPU_power_dissipation
I have heard that the thermal resistance for a same heat-sink should
be the same value, it account for the performance of the heat-sink,
but I found out that the value of thermal resistance is varying by
testing in different ambient temperature. I rather belief that thermal
resistance is not constant for a heat-sink when tested in different
environment.

Thanks for you comments
Jim

 

[CFoley1064]

Subject: Heat Sink Design Questions
From: ksyqhk@yahoo.com.hk (Jim Chim)
Date: 3/30/2004 12:28 AM Central Standard Time
Message-id: <4f64af9b.0403292228.63a2e6db@posting.google.com

Hi,

Is Thermal Resistance is only a model to describe the heat-transfer
phenomenon for a heat-sink? Or it have it's own physical meaning?

If I have a heat-sink, will the thermal resistance be constant value
in thermal test using different ambient temperature?
For example,
(1) T_ambient=34C, Rca=0.30
(2) T_ambient=12C, Rca=0.60
where Rca is the thermal resistance that calculated across CPU case
and ambient by using the equation
(Tcase-Heat_sink_inlet_temperature )/
CPU_power_dissipation
I have heard that the thermal resistance for a same heat-sink should
be the same value, it account for the performance of the heat-sink,
but I found out that the value of thermal resistance is varying by
testing in different ambient temperature. I rather belief that thermal
resistance is not constant for a heat-sink when tested in different
environment.

Thanks for you comments
Jim

Take a look at this link -- it's very descriptive, and a good introduction to
the concept (calculator alert)
— —
HOW TO SELECT A HEAT SINK
http://www.thermalloy.com/technical/papers/pdfs/select.pdf

Good luck
Chris