POWER DESIGN
Performing Thermal Analysis in System
By Tom Ivins, Applications Engineer, Semtech, Camarillo, Calif.
I
Many miniature MLP devices have θJA ratings as low as
30°C/W specified in their data sheets. In practice, however,
θJA can be much higher than the manufacturer specification
due to various system design limitations. To avoid putting
false hope in data sheet thermal specifications, an in-circuit
thermal analysis should be performed to verify that the actual
θJA is low enough to support the application. This simple test
can be the difference between a robust system design and
one that temperature cycles or self-destructs due to poor
thermal management.
Step one: To determine the true thermal resistance of
your IC in its application, start by identifying the maximum
TJ of the device allowed by the OT protection circuit. Typically, this is around 150°C. It is a good idea to measure the
OT activation temperature in a temperature chamber with
the device in a minimum power state under no-load conditions. Increase the temperature until the device protection
is triggered. At this point, TJ = ambient temperature (TA)
since power dissipation (PD) is negligible. Record the result
as TJ .
Step two: Find the application circuit’s maximum power
dissipation. First, force the ambient air to a high temperature
to make it easier to reach OT through self-heating when a
load is applied to the IC. Set TA somewhat higher than the
maximum system application ambient. Usually TAMAX
=
MA
85°C, so try 100°C and record this value as TA. Set PD for
the device and increase PD very slowly until the OT circuit
ntegrated circuit (IC) manufacturers normally publish
a thermal resistance rating (θJA; thermal resistance from
die junction-to-ambient air) on their data sheets. This
information is useful for estimating a device’s operating
temperature. The θJA rating is usually based on an industry-standard method using a standardized layout and test
procedure. In practice, the actual θJA may be much different
than the rated value when used in a layout that is suitable for
the application but is not optimal for heat dissipation.
A simple test described here may be performed to verify
the actual in-system thermal performance to ensure a reasonable operating temperature. With this method, the junction
temperature (TJ) of an IC may be measured in the application, provided the device includes an overtemperature (OT)
protection circuit.
One of the biggest concerns in portable power management designs is heat dissipation. Because of the limited air
flow and printed circuit board surface area in a portable
application, components such as linear battery chargers, low
dropout regulators, charge pumps and dc-dc converters can
reach excessive junction temperatures. Thermally efficient
package types such as the micro leadframe package (MLP)
and leadless leadframe package (LLP) are now used with
the aim of keeping the IC’s operating temperature within
acceptable levels. The best weapon these packages have for
thermal management is a thermal-die attach pad in the
center of the device.
Thermal test chamber
Ambient
temperature
measuring
equipment
–
+
DC power
supply
Power
(+)
Power
(–)
Thermal-die
attach pad
Printed circuit board
TA
MLP
Power
test
point
Power
test
point
Via
DC power
measuring
equipment
(voltage,
current)
Keeping
gap small
improves
accuracy
Cu plane
The setup shown here can be used to determine the true thermal resistance of an IC in its application.
Power Electronics Technology May 2006
54
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POWER DESIGN
activates. An OT circuit with hysteresis will temperature cycle
very slowly when the PD is slightly over maximum. Slowly
reduce the PD until the temperature cycling stops. Record
this as the maximum PD for the application.
Step three: Calculate thermal resistance using the following equation:
θJA = (TJ – TA) / PD.
A good layout should result in a value close to the data
sheet specification for θJA.
Use the resulting θJA from this step to determine the operating TJ for the intended circuit conditions
and worst-case TA and PD . If TJ is close to
the maximum allowed by the manufacturer,
one or more of the following actions may
be necessary.
● Improve the layout of the pc board by
increasing the copper area connecting to the
thermal slug.
● Decrease the power dissipation of the
IC. Typically, the only feasible way to reduce
power dissipation is to reduce the maximum
load current.
● Decrease the operating temperature
range of the system application.
To ensure the most accurate results
possible, these suggestions should also be
followed: Use a thermal-insulating mat on
the bottom side of the board to prevent
room temperature air from influencing
the measurement. Also, attach a thermal
enclosure to the thermal-induction system
for full emersion of the device and application board. And finally, use an oven instead
of a thermal-induction system and measure
TA only near the board, since ovens tend to
have hot spots.
This method of IC thermal evaluation
uses the OT protection circuit to find TJ. θJA
is then calculated, relying only on measuring
TA at no load and TA with load. The method
does not rely on direct temperature measurement of the case or junction. TJ and TC are
normally very difficult to measure accurately
in the application.
When applying this method, be aware
of the possible sources of error. First, triggering of the OT protection circuit may
vary with input voltage (V IN). However,
this should not be an issue if VIN is held
constant. There are two ways to address the
VIN-related error. During step one of the
evaluation, set VIN to the maximum used by
the application and vary the load current
to change PD. Or, during step two, vary VIN
to change PD and then remeasure the OT
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trip point at no load with the final VIN from step two.
Error can also result from the way thermal-induction
systems may force an ambient temperature on only one side
of the pc board. Since there is a thermal slug conducting heat
through to the lower layers of the pc board, a long temperature
chamber soak time (several minutes) may be required. The
goal should be total emersion of the pc board and IC at the
forced temperature. Furthermore, note that actual IC power
dissipation includes adding in the factor (VIN  IQ) to the ideal
PD. Neglect VIN  IQ only when appropriate.
PETech
55
Power Electronics Technology May 2006