Jean-Louis Malinge - EEWeb

TECHNICAL ARTICLE
Q
With a decrease in temperature
the system dissipates less
power so that the temperature
rises and equilibrium is restored
Figure 2: Power dissipation versus temperature
also ensures that engineers give themselves adequate
margins so that devices’ continued operation is ensured.
Figure 2 shows power dissipation on the vertical axis and
temperature on the horizontal axis. The gently sloped
red line is referred to as the ‘device operating line.’ This
represents a device whose power dissipation increases
with temperature. The blue diagonal line describes 1/
θJx as set out in equation 3. This is referred to as the
‘system line.’ It describes how increases in the device’s
operating temperature go with increasing amounts of
power that may be successfully dissipated from that
device. The intersection of these two lines gives the
nominal steady-state operating point. Thus, to the right
of the steady-state operating point, more power leaves
the system than the device produces, so it cools; to the
left, less power leaves the system than is introduced, so
it heats up. Either way, the imbalance in power causes
TJ to move back toward the steady-state operating point.
Once this stability is lost, however, the device is at risk of
thermal runaway. This will happen if the slope of the blue
line is less than that of the red line
In summary, the whole business of ensuring device
level thermal management has become increasingly
System
Line
As temperature rises,
more heat may be dissipated
Device
Operating
Line
T X T J T
difficult with the advent of more compact, powerdense,
functionally-complex devices enclosed in plastic
packages. Inside the package, there are multiple heat
paths that need to be taken into account; the idea that the
package’s thermal properties can be represented by a
single number is, at best, naive. Simultaneously, outside
the package, specific boundary conditions dictate
how heat flow from the device takes place. Engineers
need to be fully aware of the thermal issues involved if
their system designs are to achieve the reliability and
performance levels they require.
About the Author
Roger Stout received his BSE in Mechanical Engineering
at ASU in 1977, and went on as a Hughes Fellow to earn his
MSME at the California Institute of Technology in 1979.
He then joined Motorola in the equipment engineering
side of the semiconductor business, which after about
four years evolved into factory automation and control
engineering. In about 1990, he took on the responsibility
for thermal characterization of ASIC products. Roger
holds six patents, and has been a registered Professional
Engineer (Mechanical) in the state of Arizona since 1983.
■
EEWeb | Electrical Engineering Community Visit www.eeweb.com 19
TECHNICAL ARTICLE