Chris Hill
Senior Applications Engineer
NXP Semiconductors
Stockport, U.K.
It's no secret that the mobile-device
market is the primary driver of innovation within the electronics industry.
But the small size of modern mobile
devices also creates thermal-management challenges. Cramped corners in
mobile devices reduce internal airflow
and lower the maximum allowed external temperature.
Thermal simulation helps identify
and resolve cooling problems in leading-edge mobile designs. For example,
some of the latest mobile phones feature a 3.2-megapixel camera, video recording and playback, a digital music
player, and a full Web browser. Each function generates heat. At the same
time, the physical size of the phone is
rapidly shrinking, limiting airflow and
reducing the surface area available for
cooling the device.
In addition, engineers may have a
tough time measuring how well their
designs dissipate heat. Thermocouples
can act as heat sinks on miniaturized
packages, so recorded temperatures
may be inaccurate. Thus, thermal simulation may give more accurate temperature readings than what's possible
with simple thermal probes.
Thermal simulation of mobile devices follows the same general approach as that for stationary
devices, but there are some significant differences. For example,
mobile devices almost never use
forced-air cooling and there are
usually strict limits on case openings. Accurate modeling of the
case material properties is critical for predicting external case
temperatures.
Traditional thermal design involved extensive manual calculations verified by physical tests.
The calculations generally used
the thermal resistances found on
device data sheets. Thermal resistance resembles electrical resistance, except it opposes heat
flow rather than electricity. The
value quantifies the flow of heat
energy along predefined paths by
calculating the change in temperature between two points divided
by the rate of heat flow.
Test methods used to determine thermal resistance are listed
in Jedec specifications JESD511 through JESD51-10. The thermal-resistance values generated
under these highly specific test
conditions are unlikely to match
those in real applications. Obviously, this raises questions about
their accuracy. In particular, the method of measuring the thermal resistance from the junction
to motherboard, Rth j-mb, makes no allowances for PCB sizes,
shapes, or compositions differing from those defined in the test
spec. It also does not account for
the effects of other nearby heat
sources and the influence of the
enclosure. Further, the concept
of Rth j-mb is limited because it only
considers one component within
a larger, more complex network
of thermal resistances.
Thermal-simulation software
already helps analyze complex
thermal scenarios in stationary
electronics that involve coupled
heat transfer by conduction,
convection, and radiation. These
same methods apply to mobile
devices as long as engineers consider several subtle but still significant differences.
As for any engineering analysis, the first step in a simulation is
to collect all information relevant
to the simulated device. For a typical mobile device, this includes
the specs of all components on
the PCB including their power
dissipation, the construction and
orientation of the PCB, the size,
shape, and composition of the enclosure, the anticipated ambient conditions, and any other
relevant physical attributes. Engineers model the component
geometry using a variety of primitive shapes or cuboids with relevant physical properties attached
to each shape. Properties such
as material composition, surface
properties, thermal qualities, and
radiation attributes, attach easily
to the cuboids.
Enclosure construction and
material details are important for
analyzing thermal performance.
Fortunately, the software libraries for thermal-simulation software include properties of many
common and exotic materials.
It can be time consuming to
accurately simulate components
with intricate geometry, such as
semiconductors and heat sinks.
So many component vendors
have developed behavioral models that predict the temperature
of the package at such critical points as the junction, case, and
board to help save time. The models drop into a full thermal design
simulation with all parameters already programmed.
For example, SmartParts3D is
a free Web database that holds
certified analysis models and
data-sheet information for a wide
range of semiconductors, printedcircuit boards, power supplies,
heat sinks, enclosures, capacitors, grilles, and many other components. Design engineers can
download the information from
many different vendors, search
and compare parts in the library
by attribute and performance
qualities, and drop different parts
into a model to compare thermal
performance.
Semiconductors that handle a
lot of power impact simulation results the most. A typical example is
the N or P-channel MOSFETs used
to switch power rails in many mobile devices. The MOSFETs work in an environment where relatively
few cooling options are available.
An example is the NXP PMN23UN
that provides an on-resistance
of 28 mΩ in a TSOP6 package of
9.3 mm2. The device finds use in
such applications as load switches
or driver FETs for dc/dc converters. Just a 42% jump in load current more than doubles the power
dissipation within the device.
An example shows how thermal simulation typically proceeds. Consider a charge controller for a mobile phone. A simulation with Flotherm software from
Flomerics Corp. takes into account other sources of heat in the
device. The Flomerics software
possesses many prebuilt models
that cut the time needed to set up
the thermal simulation.
A prototype was built with
steady-state MOSFET power dissipations of 0.5, 0.75, and 1 W.
Designers measured the corresponding MOSFET-junction temperature rise for each of the three
power dissipations using the body-diode thermometry method
described in Jedec specification
JESD51-1. Then they added a second heat source in the form of a
resistor dissipating 1 W. Three
simulations took place with the
resistor in three different positions while the MOSFET power
dissipation stayed constant at 0.75 W. The resulting simulation
was able to predict the rise in
junction temperature with an error ranging from 1.1 to 1.9%.
Though the TSOP6 package
worked acceptably, clearly its temperature rise would become an
issue as power dissipations rose.
Future needs appear to dictate a
radically different package type.
The proportion of footprint area
occupied by leads becomes more
significant as packages shrink. In
that regard, the nanoPAK package
was developed to reclaim board
space by eliminating leads while
enhancing thermal performance.
The new package contacts the
PCB through a copper pad rather
than the relatively thin rails used in the TSOP6. A thermal simulation predicted the performance
of nanoPAK packages before any
engineers invested time or money
on prototypes. The simulation
predicted that nanoPAK packages
would perform substantially better thermally than TSOP6 packages. Physical testing results
demonstrated that Rth j-mb values for the nanoPAK package were
about half the level of the TSOP6.
Always keep in mind that thermal simulation has limitations.
The simulation results are only
as good as the data used to create them. It may be tough to get
the properties for less common
materials. When thermal data is
not available, engineers may be
forced to make educated guesses
based on the properties of other
similar materials. Finally, when
comparing simulations to empirical data, consider the potential
for error in physical measurements as well as in the simulation. For example, both thermocouples and thermal-imaging cameras have inherent potential
inaccuracies that may contribute
to variances between measured
and simulated temperatures.
Even with these limitations,
there are advantages to simulating the thermal design of mobile
devices. Designers can quickly
evaluate concept designs and
correct thermal problems long
before the prototype phase. Many
companies that perform thermal simulation regularly at the
concept design phase discover
it is almost never necessary to
build additional prototypes to
solve thermal problems. Thermal-simulation software also
provides much more information
than physical testing. Detailed 3D
graphical information on pressures, temperatures, and airflows
help engineers quickly optimize
their design.
MAKE CONTACT
Flomerics Inc., (508) 357-2012, www.flomerics.com