Download EPE 2001

Regular Clinic
CIRCUIT
SURGERY
ALAN WINSTANLEY
and IAN BELL
Out consultant troubleshooters take a look at basic heatsink calculations this month
month we outlined some of the
L
basic considerations related to the
power and heat dissipated by components.
AST
Without doubt some parts (e.g. mains
transformers) can become very hot indeed
even in normal operation, especially if they
are mounted on plastic cases which don’t
dissipate heat very well at all. Hopefully,
adequate ventilation will be available to
help with the throughflow of air.
Obviously, bolting such parts to a steel or
aluminium chassis will help dissipate heat.
The point was made last month that
heatsinks are often necessary on certain
devices – regulator chips or power transistors for example – but if the heatsinks
become very hot it’s usually just a sign that
the heatsink is doing its job of conveying
heat from the regulator. If the heatsink system has been correctly designed, it will
prevent the semiconductor junction from
exceeding the manufacturer’s maximum
temperature rating.
Although a wide variety of parts (e.g. 3pin regulators) have built-in thermal overload protection, relying on this feature is
not a good way of compensating for the
use of inadequate heatsinking; thermal
cycling between extremes of temperature
may ultimately “stress” the device leading
to premature failure over time.
A TO-3 can style regulator sawn off to
reveal the i.c. chip inside.
698
Thermal Resistance
In this month’s Circuit Surgery we
describe the basic procedure for selecting
the correct heatsink for a device. The idea
is to carry heat away from the part that
matters most – the semiconductor junction
or i.c. die. Just as electric current flows
through a conductor, heat flows from a
semiconductor chip to the outside ambient
air also through a path of “conductors” –
air, metals and alloys which carry the heat
away through a thermal conduction action.
Hence, anything getting in the way of this
path adds to the system’s overall thermal
resistance, preventing the heat from being
conveyed so efficiently.
Thermal resistance is measured in
°C/Watt – it indicates how much the temperature will increase across a “thermal
resistor” for a given power dissipation. A
large, expensive heatsink rated at 1°C/W
increases temperature by only one degree
per watt of power dissipated, and is far
more efficient (because it has a much lower
thermal resistance) than a small heatsink
of, say, 25°C/W.
A typical heatsink system contains the
elements described in the photo. This
could represent any power transistor containing a junction, or say a voltage regulator chip such as a the TO-3 type opened up
for the photograph. The requirement is to
ensure that the semiconductor junction
temperature (symbol TJ) does not exceed a
figure of 125°C usually, a typical value
specified by data sheets.
The junction is at the heart of the package, but unfortunately for us, various elements of the system hinder the passage of
heat and have to be accounted for.
Everything possesses a thermal resistance,
symbol θ (theta), which can be sub-defined
as follows:
θJC – thermal resistance between
junction and the case;
θCS – thermal resistance between
the case and heatsink;
θSA – thermal resistance between
heatsink and ambient.
The electronics designer will hopefully
see a number of “thermal resistors” in
series: each resistor prevents heat from
flowing to ambient, so heatsink design
implies calculating to ensure that the overall thermal resistance of the system is not
so large as to prevent heat flowing away
efficiently, or this will result in the semiconductor junction producing more heat
than can be carried away effectively.
A typical assembly showing how the thermal resistance is divided between the
device and heatsink.
Everyday Practical Electronics, October 2001