60 September 2017
To remove higher amounts of heat, a heatsink
is required to lower qca. A heatsink can be anything
sufficiently massive and thermally conductive to conduct
heat away from the component so that it can be dissipated.
For example, the metal enclosure of many handheld radios
acts as the heatsink for the output amplifier, getting rather
warm during prolonged transmissions.
4 — Select an Appropriate Heatsink
Heatsinks are specified by their thermal resistance
in °C/W. The rating tells you how much the surface
temperature of the attached component will rise per watt
of heat flowing through that surface. This figure generally
assumes natural convection, assuming that the only air
movement is caused by warm air rising.
To select a heatsink, you need to specify the maximum
body or junction temperature. Calculate the amount of
power the component must dissipate. Estimate the ambient
temperature. Remember that qjc is fixed, so you calculate
the maximum case temperature. The required thermal
qca = (Tcmax - Tambient) / P where Tcmax = Tjmax - P × qjc
Now, you can pick a heatsink design that fits your
component and your available space.
Be careful when using a heatsink inside an enclosure
or where free air flow is not guaranteed. Thermal resistance
can be significantly higher just by preventing hot air from
moving away from the heatsink. In these situations, a fan
or blower is used (or a heat pipe as mentioned in the
sidebar). Manufacturers of heatsinks often have application
notes or guidelines on their websites to help you in these
A good technique to get around not having free air
flow in an enclosure is to mount the heat dissipating
component directly on the enclosure wall. Using the
enclosure as the heatsink allows the heat to flow directly to
the outside air.
Be aware that the longer leads involved with
connections to the power dissipating device may become
significant at higher frequencies. At RF, this technique
generally requires the entire amplifier module or circuit be
mounted on the enclosure, and a coaxial cable connection
be made to any internal circuit board.
A Heat Management Experiment
Nothing instructs like putting a fingertip on a hot
component, so let’s get cracking!
Three-terminal voltage regulators used in power
supplies often require heatsinking. One of the most
common applications is dropping a 12V input voltage to
5V with a 7805 for use with digital logic. (You can find
the 7805 datasheet at www.sparkfun.com/datasheets/
Components/LM7805.pdf.) This circuit is shown in Figure
3. Construct it by tacking the components together
since the current levels will be too high for a solderless
To test this circuit, you’ll need a beefy load that can
draw up to one amp. A 5Ω/5W resistor can either be
purchased or constructed from a combination of resistors
in series or parallel. It will get warm!
1. Start with a light load of 100 ohms and no heatsink.
Apply 12V to the regulator input pin. The total power
dissipated in the load is V2 / R = 25 / 100 = 1/4W. The
(12-5V) times the current through it, Pd = 7 x 0.1 = 0.7W.
In this case, the regulator dissipates more power than the
Why Use Thermal Compound?
If you take a power transistor or IC off its heatsink, there is
usually a thin coating of a white or clear grease-like substance
between the component case and the heatsink. What is that
Surprisingly, it’s not particularly thermally conductive!
However, it is better than air.
The function of this thermal compound is to fill in any gaps
between the case and the heatsink. Air is a bad conductor
of heat, as we find out when we buy insulation for our homes
which has an R-rating in equivalent inches of air. Any gaps,
warps, or pits in the surface of the case or heatsink cause big
increases in thermal resistance.
Thermal compound fills in those imperfections and even
though it’s not as good a conductor of heat as metal, a thin
layer of it is better than just bolting the surfaces together. The
key word is “thin.” All you need is a few thousandths of an inch
of compound to do the job.
Put a thin coating on either surface, press the surfaces
together, twist or slide them a little bit to even out the coating,
and then tighten the mounting screw or clip. Yes, it does get all
over everything, including your fingers — that’s its job!
The much cleaner thermal pads or washers also work, but
not as well as the compound.
A Pipeful of Heat
Inside a PC, the many watts of heat dissipated by the
main processor (and some graphics processors in high-performance PCs) are conducted away from the processor
by a heat pipe. One end of a tube is mounted directly to the
package of the processor, and the other is mounted on a
heatsink or formed into a heatsink.
The working fluid in the pipe flows from the hot end
(on the processor) to the cold(er) end at the heatsink, then
back again to the processor. This is a simple form of a heat
engine that drives a mechanical motion (fluid flow) by using a
The larger the temperature difference, the faster the fluid
flows — just like voltage driving a motor.