368 Chapter 13
The primary heat transfer driving force is the
temperature difference between Tmaxcase and Tmaxambient
modified by the conductive delta T losses in the inter-
face and any extraordinary hot spot offsets and
spreading losses. If heatsinks are mounted over spread
hot spots these last conductive losses are not sufficiently
large to consider. They really only become significant
when considering very unusual arrange-
ments—high-watt density loads such as typical LEDs.
13.1.1.3 Radiation
Radiation is the third and least important method of heat
transfer for audio system heatsinks. Radiation has a
maximum 20–25% impact in natural convection appli-
cations with a negligible impact after 200 lfm applica-
tions. Radiation is a function of the fourth power of the
absolute temperature difference between the hot side
and surrounding cooler surfaces that look at each other
and their respective emissivities. In the real world in
which we live, these are not significant enough to
suggest a lot of effort to understand and optimize.
Aluminum extruded heatsinks were typically made
black in an anodizing process at a significant cost to get
an emissivity of 0.95 (dimensionless). The typical
aluminum surface forms an oxide film in less than a
second after machining with an emissivity of about
0.30–0.40. Nominally, almost half the benefit will come
free so that our advice is “Leave the radiation effects
alone.” What you get beneficially you were going to
largely get anyway, free from Mother Nature.
In review, heatsinks use all three methods of heat
transfer to produce the desired effect of cooling the
typical electronic component in the typical audio
system.
13.1.1.4 Summary
Convection is usually the most significant method, and
it depends on having sufficient fin surface area in direct
contact with the surrounding air and design features to
minimize the insulating effects of boundary films. Aero-
dynamic shapes and adequate open fin spacing that
allows free air movement are critical design issues.
Conduction is the first step in the heat transfer chain
in that conduction transfers the heat from the device
into heatsink, then through the heatsink to the fin
surface where convection takes over. Some heatsinks
need conduction enhancements such as heat pipes to
keep the conduction temperature gradients to a value
that is low enough to allow the convection to complete
the heat transfer without exceeding the application
temperature limits.
Radiation is a secondary level effect that is always
present, marginally significant in natural convection,
but not economical to control.13.1.2 New Technologies to Make Things Fit More
EasilyThe range of technologies, materials, and fabrication
processes available to the thermal designer today is
quite impressive. The primary goal when employing
these advanced technologies, materials, and fabrication
processes is to increase the effective density of the of
the resultant heat transfer system. Technically, we are
increasing the volumetric efficiency of the thermal solu-
tion proposed for a given application. In “man speak”
the required heatsink gets much smaller in size and
therefore fits more easily into the ever-shrinking
product envelope. A smaller heatsink has a decreased
conductive thermal spreading resistance and therefore a
smaller conductive temperature gradient. In this section
we are going to assume that we have a convective solu-
tion defined for a baseline heatsink. The baseline heat-
sink is fabricated from an extruded aluminum alloy
(6063-T5). The following paragraphs will describe a
technology, material or fabrication process and give a
volume ratio or range of volume ratios that can be
applied to the existing solution to quickly see the
benefit of applying this technology, material, or fabrica-
tion process to the audio application at hand. Ratios that
are less than 1.00 are indicating a reduction in heatsink
volume.
Thermal solution problem solving is an iterative
process balancing the application boundary specifica-
tions against the affordable technologies/mate-
rials/fabrication processes until a system compromise
solution is defined.^1 For example; marketing has
directed that only a natural convection solution is
acceptable but the heatsink is too big. One solution
might require the Tmaxambient be reduced by 5°C and the
heatsink be fabricated from copper, C110 soldered
together. This could reduce the size of the heatsink by
25–35%. The penalties would be the weight would
increase between to and three times and the unit cost of
the heatsink increase by three to four times. There are
software systems^2 that specialize in defining these
trade-offs rapidly, allowing a real-time compromise to
be made, even during the design review meeting with
marketing.