Just For Starters
power multiplied by the thermal resistance through which the power is flowing.
If a component is provided with a
highly conductive thermal path to the
ambient environment, its θ is low and,
consequently, its temperature rise is
low for a given power dissipation.
Finding θ for a given component
is usually done by consulting the manufacturer’s data sheet. You can devise
an experiment to obtain θ empirically,
but such efforts are rarely necessary.
You can calculate a component’s
maximum operating temperature when
you know its maximum power dissipation, its thermal resistance to air, and
the maximum ambient air temperature:
TMAX = TAMBIENT + θPMAX. If
TMAX exceeds the manufacturer’s
maximum rated operating temperature
... “Houston, we have a problem!”
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NUTS & VOLTS
So what can you do when you
run the numbers and TMAX is too
high? It is always nice to have the
luxury of redesigning a circuit for
lower power dissipation. However,
redesign is not always possible or
practical. It would also be nice to
lower TAMBIENT by only operating
your system in Arctic temperatures.
Again, this is not always practical.
Generally speaking, you must find a
way to reduce thermal resistance.
Two common methods of reducing
thermal resistance are increasing
airflow over the hot component and
introducing a heatsink of some form
into the system. When you place a fan
near a hot component, the increased
air flow results in more molecules of
cooler ambient air passing over the
heat source in a given time interval.
There is more contact between hot
and cold, resulting in a lower thermal
resistance. You can observe this
behavior at work in your computer,
which has a fan in the AC power
supply and probably has another fan
positioned close to the CPU.
Metal is a far better heat conductor
than air. Copper and aluminum are
commonly used to fashion efficient
heatsinks. Aluminum is the preferred
material because it is less expensive
than copper. You will find copper
heatsinks in leading edge circuits
where the heat problem is so severe
that copper’s higher cost is justified.
The basic purpose of a heatsink
is to efficiently conduct heat away
from a dense source and spread that
heat over a much larger area, where it
can be properly carried away by the
ambient air. A fan’s presence increases
the effect of a heatsink because the
moving air more rapidly conducts
heat carried by the heatsink.
The overall thermal resistance
from the hot component to the air is
what matters in our analysis. This total
resistance is the sum of all the heat
conductors and their interfaces. The