Why Is My DC-DC
Converter Too Hot?
By Brian Narveson, Marketing Manager, and Geoffrey Jones,
Product Marketing Engineer, Texas Instruments, Dallas
To properly interpret thermal-derating curves
provided by power-module manufacturers,
engineers must determine the test conditions
under which the curves were derived.
T
he bulleted items on the front page of a dc-dc
power module’s data sheet often highlight electrical performance that the product cannot
actually deliver in system. This presents a challenge to system designers, who must compare
the electrical/thermal performance of different manufacturers’ modules. System designers must ensure the dc-dc
power modules chosen for their end equipment offer the
requisite electrical/thermal performance across the application’s full temperature range. Estimating the minimum
and maximum load currents a module can supply in an actual system environment can be the most important factor
for determining the cost and reliability of a power supply.
This helps designers choose a module that is capable of the
required output current at the most economical cost.
The electrical/thermal performance of a power module
is characterized by its thermal-derating curves. The curves
are the best and most commonly used metric for judging
the overall performance of a module. Power-module manufacturers conduct extensive thermal testing to generate the
curves, which are published in data sheets. The thermalderating curves in Fig. 1 show the maximum current a
module can deliver under various airflow velocities and
ambient temperatures. This defines the device’s safe operating area (SOA)—the operating condition where the maximum electrical output can be achieved without exceeding
the recommended thermal design limits.
Each point on a thermal-derating curve represents a combination of output current and an environmental condition
that causes the temperature of some component within the
module to reach a predetermined limit. In the example above,
an application may require 30-A load current. Environmental conditions include 50°C ambient temperature with
air velocities as low as 1 m/sec (200 lfm). Upon consulting
the module’s data sheet, the SOA curves (Fig. 1) may reveal
that a module with a 30-A maximum-output current rating can reliably deliver only 23 A continuously under these
conditions.
Thermal-derating curves tell the designer if a chosen
module will deliver the desired current at the desired ambient temperature, if additional airflow is needed, and how
much margin or reserve is available in case of clogged filters
or cooling-fan failure in an enclosure. Moreover, thermal
data tells the designer if he must either derate (operate the
module below its maximum-output rating), or supply increased amounts of cooling air or, in some instances, attach
a heatsink.
In actual applications, many dc-dc power modules do not
achieve the output current rating on the front of their data
sheets. One reason for this is power-module manufacturers’
specsmanship. Another is the fact that the power-module
industry has no standard thermal-derating characterization
90.0
80.0
TA (°C)
70.0
60.0
50.0
40.0
30.0
20.0
0.0
Natural convection
100 lfm
200 lfm
300 lfm
400 lfm
500 lfm
5.0
10.0
15.0
20.0
Output current (A)
25.0
30.0
Fig. 1. Thermal-derating curves specify the maximum current a
dc-dc converter can deliver at different airflow speeds and ambient
temperatures.
Power Electronics Technology June 2006
40
www.powerelectronics.com
THERMAL DERATING
Module
center line
Adjacent pc board
A
I
R
F
L
O
W
3.0"
0.50"
AIRFLOW
Air velocity and ambienttemperature measurement
location
Fig. 3. Smoke, represented here by the blue lines, indicates how airflow
across the module in the restricted SOA test setup increases from that
measured in front of the module.[1]
Air passage
center line
performance in increasingly creative ways.
Unfortunately, when it comes to interpretation of manufacturers’ thermal-derating data, a true comparison is not so
simple. The system designer should take into consideration
differences in derating test details. Differences such as airflow and ambient-temperature measurement method and
location, the maximum component temperature allowed,
board pitch and test fixtures have significant influence on
the derating curves. Because of these differences, the derating curves published by different manufacturers cannot be
easily compared without first understanding their measurement method.
Fig. 2. In a restricted SOA test setup, an adjacent pc board (used to
simulate a car- rack environment) forces air over the power module.
process for isolated and nonisolated dc-dc power modules.
System designers face the challenge of selecting modules from a large number of suppliers. As a result, the
dc-dc power-module business is highly competitive. An
aspect of the intense competition is specsmanship, which
has led power-supply manufacturers to describe product
Quarter Brick, Half Brick, Full Brick sizes.
Thermal-Derating Measurement
There is no industry standard for measuring thermal
performance. Two of the traditional approaches use an
air velocity measurement inside a wind tunnel. This setup
replicates the typical thermal environments in most modern
electronic systems with distributed power architectures. The
electronic equipment in networking, telecom, wireless and
advanced computer systems operates in similar environments and uses vertically mounted pc boards or circuit cards
in cabinet racks.
Fig. 2 shows a typical restricted SOA test setup. The
power module is mounted on a test board and is vertically
oriented within the wind tunnel. An adjacent board is used
to simulate a card-rack environment. The adjacent board
forces air over the power module. The spacing between the
two boards is typically twice the height of the module. The
wind-tunnel setup uses a probe to measure airflow and
ambient temperature at a single point.
Power modules are typically tested at their nominal input
voltage. Thermocouples or a thermal-imaging camera is
used to measure the temperature of key components, while
load current is varied from no load to maximum load. Data
is collected at several representative airflows, typically from
0 m/sec to 2.5 m/sec.
Smoke is sometimes used in wind tunnels to qualitatively
describe airflow. Fig. 3 is a representation of a wind-tunnel test
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Power Electronics Technology June 2006
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42
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THERMAL DERATING
Fan
Airflow
Overhead view
D.U.T.
Test
board
IR
camera
7.5"
Fig. 5. In an unrestricted test setup, there is relatively constant spacing
of the smoke filaments, represented here by blue lines, in front of and
across the surface of the module. [2]
Airflow and
ambient-temperature
sensor
must be monitored at the component’s case or tab.[3] Most
manufacturers use automatic measurement processes to
determine thermal performance. This is done by using a
thermocouple on all of the power components, such as
FETs, magnetics or a thermal camera that can monitor many
components under program control.
Thermocouples can affect the measurement of low-mass
components. Because of its metal construction, a thermocouple transfers generated heat away from the part it touches,
making it difficult to obtain an exact thermal profile. Thermocouples use a single point to measure temperature.
D.U.T.
End view
7.5"
IR
camera
Fig. 4. In an unrestricted SOA test setup, there is no parallel-facing
pc board to increase the air velocity across the module.
for the restricted test setup. This figure reveals how the restricted test setup reduces the spacing of smoke filaments across
the power module. This indicates that airflow across the
module has increased from the airflow measured in front of
the module. The parallel-facing pc board can increase airflow
from 1 m/sec to 2 m/sec. Manufacturers that use this method
claim it simulates a card-rack environment.
Fig. 4 shows an unrestricted SOA test setup, in which
the power module is soldered to a test board within the
wind tunnel. The setup has no parallel-facing pc board. The
unrestricted test setup allows air to move over the module
without restricting airflow. This does not decrease the crosssectional flow area (which would increase the air velocity)
as in the restricted test setup.
As shown in Fig. 5, there is relatively constant spacing
of the smoke filaments in front of and across the surface of
the module in the unrestricted test setup. This indicates that
airflow across the module is the same as airflow measured
in front of the module. In the restricted test setup, airflow
across the module is much greater, which will result in a
more aggressive SOA curve. (The module will deliver more
current at a given airflow.)
Temperature-Measurement Methods
Where the temperature measurements are taken is critical
to the accuracy of the SOA curves. Some manufacturers recommend measuring temperature at a spot on the pc board.
Rarely is this the hottest point in the circuit. For accuracy,
the hottest components (typically the FET, control IC or
magnetics) should be measured directly. FET temperatures
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43
Power Electronics Technology June 2006
THERMAL DERATING
90.0
80.0
80.0
70.0
70.0
60.0
60.0
TA (°C)
TA (°C)
90.0
50.0
50.0
40.0
40.0
30.0
30.0
20.0
0.0
5.0
10.0
15.0
20.0
Output current (A)
25.0
30.0
20.0
0.0
Natural convection
100 lfm
200 lfm
5.0
10.0
300 lfm
400 lfm
500 lfm
15.0
20.0
Output current (A)
25.0
30.0
Fig. 6. Thermal-derating curves for an isolated, 3.3-V, 30-A quarter-brick illustrate the difference between the unrestricted thermal-measurement
method (left) and the restricted measurement method (right).
Since heat patterns are difficult to
predict, it is not always possible to
know where to attach the thermocouples necessary to make measurements. For this reason, power-supply
manufacturers attach thermocouples
at multiple points. Wires connecting a
thermocouple to various points on a
power module can block airflow across
the part, causing it to run at higher
temperatures.
Many manufacturers now use thermal (infrared) imaging to help design
and characterize their products. The
thermal-imaging camera provides
an alternative to thermocouples for
measuring the temperature of key
components. Thermal imaging uses
multiple points for measuring thermal performance. It can be used with
either the restricted or unrestricted
test setup. As shown in Fig. 4, thermal
images of a power module are taken
through a window on the side of the
wind tunnel.
Thermal imaging is often used
where the power components are visible so that the surface temperature
of the individual components can be
measured. The images provide a good
overall thermal profile of the module
and can identify layout problems or
overstressed components. What’s
more, thermal images allow powersupply manufacturers to evaluate the
effectiveness of cooling and “shadPower Electronics Technology June 2006
owing” from adjacent heatsinks and
components.
The measurement of a component’s
surface temperature provides a direct
indication of its internal core temperature. Of significance is the junction
temperature of the semiconductors and
the winding temperature of the magnetic parts. By varying the temperature
limits placed on these components,
the module’s derating curves—and
its output rating at a specific ambient temperature and airflow—can be
manipulated.
Some manufacturers push their
module’s rating by setting higherthan-normal internal component
temperature limits. This contributes
to an improved thermal rating. As an
example, one manufacturer may operate FETs at junction temperatures close
to the component’s absolute maximum
rating, while another may limit it to a
lower, more conservative value. These
opposing design conditions can have
a major effect on overall power-module performance and reliability. For
instance, if the operating temperature
of a FET is increased from 115°C to
125°C and all other operating conditions remain the same, the module’s
reliability changes from a MTBF rating
of 929,368 hours (1076 FIT) to 822,368
hours (1216 FIT).[4]
Manufacturers use these higher
ratings to claim superior thermal
44
performance on their data sheets.
These performance claims and the
SOA graphs on the data sheet’s inside
pages lead designers to believe they can
reliably operate a particular module in
their systems at higher temperatures.
Designers don’t realize that the life of
the power module will be reduced if it
is used consistently in these operating
conditions.
There is no right or wrong way to
measure thermal performance. Each
approach has unique advantages. SOA
curves obtained from restricted test
setups are valid only in an environment similar to the test setup. SOA
curves obtained from unrestricted test
setups can be used in a wider variety of
environments. Because many applications do not use parallel boards with
restricted airflow, the unrestricted test
setup yields the most conservative
approach.
In addition to the SOA test setup,
there are several other factors that can
affect test results. Is the airflow measured by anemometer or by volumetric
calculation? Hot-wire anemometers
used to measure airflow directly in
front of the module ensure the highest
airflow accuracy. Is the airflow type
turbulent or laminar? Laminar airflow
is the more conservative approach.
Some of today’s dc-dc power modules are available in both horizontaland vertical-package styles. Some of
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THERMAL DERATING
the orientations yield better thermal
performance, which is typically highlighted in the module’s data sheet. The
designer must question the performance
at other orientations and whether or not
the derating curves are based on bestcase or worst-case orientation.
Thermal Test Results
Even though most thermal performance is calculated by using data from
thermal-imaging cameras, the actual
test setup and method of measurement
will have a significant impact on the
results.
Fig. 6. shows two sets of thermalderating curves for an isolated quarter-brick module rated to deliver a
3.3-V output at 30 A. The unrestricted
thermal-measurement method was
used to generate the thermal-derating
curves shown on the left in Fig. 6. The
restricted measurement method was
used for the curves shown on the right
in Fig. 6. The maximum component
temperature, mounting orientation
and airflow direction were the same in
both tests.
At 70°C and 1 m/sec airflow, the derating curve derived in an unrestricted
setup indicates that the module should
be operated at a maximum of 18 A, as
shown on the left in Fig. 6. When the
same module is measured in a restricted
airflow setup, the derating curves indicate it can be operated up to a maximum current of 23 A, as shown on the
right in Fig. 6. If the system designer’s
product configuration is not identical to
the restricted setup, there is significant
risk that the module’s internal components will run at temperatures much
higher than the manufacturer’s recommendation. This could lead to future
reliability problems.
System designers often find that the
output current rating on a data sheet’s
front pages do not match the actual
output current shown on the thermal
derating graph. This situation can
turn product comparisons into quite
a challenge. The bulleted items on the
front page often fail to mention the test
conditions under which the derating
curves were measured. This is why, before comparing thermal performance,
www.powerelectronics.com
the designer must look beyond the
data sheet’s front page, searching its
inside pages. In many instances, the
actual output current that a power
module can deliver is usually less than
what is claimed on the front of the
manufacturer’s data sheet. This is often
due to differences in the test setups and
operating conditions.
To understand a module’s thermal
performance, the system designer must
determine if temperature measurements were taken using a thermalimaging camera or thermocouple. The
system designer also must understand
if the temperature was measured at
a single point on the pc board or, for
greater accuracy, directly at multiple
components such as the FET, control
IC or magnetics.
Another consideration is the thermal test setup. Some manufacturers
use the unrestricted setup, while others
use the restricted setup, which results
in a more aggressive SOA curve. Lastly,
system designers must understand if
a manufacturer has allowed internal
component temperature to approach or
reach maximum limits when evaluating
thermal performance.
To eliminate confusion in the dc-dc
power-module selection process, system designers must beware of creative
marketing. Be sure to compare thermal
performance by carefully scrutinizing
thermal data and derating curves or,
better yet, by evaluating the module’s
thermal performance in the actual
application.
PETech
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References
1. Strassberg, D. “Tiny Titans: Choose
‘Em And Use ‘Em With Care,” EDN,
p. 41, May 2, 2002.
2. “Discover a New World without Baseplates,” SynQor Inc., p. 7, 2001.
3. “Thermal Derating Curves for LogicProducts Packages,” Texas Instruments
Inc., p. 4, 1999.
4. “Reliability Prediction of Electronic Equipment,” Military Handbook,
MIL-HDBK-217F, Dec. 2, 1991.
5. Narveson, B., and Jones, G. “Understanding Thermal Performance of
DC-DC Power Modules,” Applied Power
Electronics Conference (APEC) 2006.
45
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Power Electronics Technology June 2006