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By Chris Hill
Senior Applications Engineer
Philips Semiconductors
Stockport , United Kingdom
It’s no secret that the mobile device market is the primary segment driving innovation in the electronics industry today. But just as mobile devices are creating market opportunities, the increasing functionality and smaller size of today’s mobile devices are creating new thermal management challenges. The lack of space in today’s mobile devices reduces internal airflow and external temperature requirements are usually quite strict. So while the thermal design of mobile devices has traditional been carried out with hand calculations and physical testing, thermal simulation is increasingly becoming necessary for leading-edge mobile designs.
Thermal simulation of mobile devices follows the same general procedures as stationary devices but there are some significant differences. For example, accurately modeling the material properties of the case is much more critical than usual because the lack of airflow increases the importance of the case in cooling and because of the need to accurately predict external temperatures. Another challenge facing engineers is that as package size is reduced the thermocouple used for physical measurements begins to act as a heat sink, reducing the accuracy of physical testing and increasing the importance of simulation. This article will discuss the thermal management challenges of the new generation of smaller, more powerful mobile devices and provide some suggestions for identifying and solving thermal problems prior to the prototyping phase.
Higher functionality, smaller size means thermal problems
A major reason for the rapidly increasing popularity of mobile devices is that they are delivering rapid increases in functionality in smaller and smaller packages. For example, a typical new mobile phone features a 3.2 Megapixel camera, video recording and playback, a digital music player, and a full web browser. Each of these new functions adds heat sources to the device. At the same time the physical size of mobile devices is rapidly shrinking, further reducing the space available to provide airflow that can assist in cooling the device. Mobile devices almost never use forced air cooling and there are usually strict limits on openings in the case so the enclosure properties have a much bigger than normal effect on thermal management.
In the past, when thermal management of mobile devices was less of a concern, hand calculations were usually sufficient to evaluate mobile device cooling. These calculations were generally based on the thermal resistances found on device data sheets, which are analogous to electrical resistances, and used to quantify the flow of heat energy along a pre-defined path. Thermal resistances are defined as follows:
Figure 1: Definition of R th j-mb for power semiconductor
A full description of the test methodology for determining thermal resistances may be found in JEDEC specifications JESD51-1 through JESD51-10. These values are generated under very specific conditions which are unlikely to match those found in real applications. This means that the accuracy of this method in realistic applications is open to question. In particular, this method of measuring R th j-mb does not account for differences in the size, shape and composition of PCBs from those defined in the test specification; the effects of other nearby heat sources; and the influence of the enclosure. Further, the concept of R th j-mb is of limited use on its own since it is only one component within a much larger, more complex network of thermal resistances.
Simulation provides flexibility and accuracy
The increasing challenges of thermal management for mobile devices has created the need for a more accurate and flexible approach to determining device operating and external temperatures. Thermal simulation software is already widely used by designers of stationary electronic devices to analyze complex thermal scenarios involving coupled heat transfer by conduction, convention and radiation. These same methods can be readily applied to mobile devices as long as engineers consider a number of subtle but still significant differences between thermal management in the stationary and mobile device spaces.
The first step in simulation is collecting all of the information relevant to the device being simulated. In the case of a typical mobile device, this would include the specifications of all of the 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. Various types of primitive shapes or cuboids are used to recreate the geometry of the device with relevant physical properties attached to each shape. Properties such as material composition, surface properties, thermal properties, and radiation attributes, can be attached to cuboids. The construction and composition of the enclosure are a much more important factor in mobile device thermal management. In a mobile device, it’s particularly important to obtain detailed material properties of the enclosure. The properties of many common and exotic materials are already included in software libraries provided by thermal simulation software providers.
Vendor-supplied models save modeling time
It’s important to note that accurately simulating components with intricate geometry such as semiconductors, heat sinks, etc. can be time-consuming using these methods. Fortunately, many components vendors are developing behavioral models that predict the temperature of the package at critical points such as the junction, case and board. Many components vendors provide their customers with models which can be dropped into a full thermal design simulation and used to model the behavior of the component in their system environment with minimal effort.
For example, SmartParts3D is a free web database that provides certified analysis models and datasheet information for a wide range of semiconductors, printed circuit boards, power supplies, heat sinks, enclosures, capacitors, grilles and many other components. Design engineers can now download information from many different vendors, search and compare parts in the library by attribute and performance characteristics, and drop different parts into their model to compare their performance from a thermal standpoint.
Semiconductors with high power dissipation have the most impact on simulation results. An example is N- or P-channel MOSFETs used to switch one or more power rails in many mobile devices. As functionality is added to mobile devices, MOSFETs are expected to handle more power in an environment where relatively few options are available for cooling them. An example is the Philips PMN23UN which provides on-resistance of 28 milliohms in a TSOP6 package of 9.3 mm squared. The device is designed for use in applications such as a load switches and driver FETs for DC/DC converters in mobile devices.
Mobile device simulation example
The following example shows how the thermal performance of one of these devices was simulated using Flotherm software from Flomerics as a charge controller for a mobile phone. The simulation takes into account other sources of heat in the device that contribute to temperature rise inside the handset. We used Flomerics software in this application because its popularity means that many pre-built models are available, reducing the time required for analyzing thermal performance of a mobile device.
Figure 2: Simulation model.
A prototype of this device was built and tested with steady state MOSFET power dissipations of 0.5W, 0.75W, and 1W. Corresponding MOSFET junction temperature rises were measured for each of the three power dissipations, using the body diode thermometry method described in JEDEC specification JESD51-1. Then a second heat source was added in the form of a resistor dissipating 1W. Three simulations were carried out with the resistor in three different positions and the MOSFET power dissipation kept constant at 0.75W. The results showed that the simulation was able to predict the rise in junction temperature with an error ranging from 1.1% to 1.9%.
Figure 3: Surface temperature plotted onto model.
Figure 4: Comparison of measured and simulated results
Although the TSOP6 package used in this device was found to work acceptably in the chosen application, it was clear that temperature rises associated with this package were becoming an issue as power dissipations increased. To address the needs of the future, a radically different package type was needed. As packages have been reduced in size the proportion of footprint area occupied by leads has become more significant. The nanoPAK package was developed to reclaim board space by eliminating leads while enhancing thermal performance. The new package makes contact with the PCB through a copper pad rather than the relatively thin rails used in the TSOP6. Thermal simulation was used to predict the performance of prototype nanoPAK packages before any time and money was spent on prototypes. The simulation predicted that nanoPAK packages would have substantially better thermal performance than TSOP6 packages. Physical testing results demonstrated that R th j-mb values for the nanoPAK package were about half the level of the TSOP6.
It’s important to keep in mind the limitations of thermal simulation. Simulation results are only as good as the data, such as material properties and enclosure geometry, that were used to create them. Obtaining properties of less common materials used in the device can be particularly challenging. 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, it’s important to consider the potential for error in physical measurements as well as in simulation. For example, both thermocouples and thermal imaging cameras have inherent potential inaccuracies that may contribute to variances between measured and simulated temperatures.
Yet, even after taking these limitations into account, there are major advantages in simulating the thermal design of mobile devices. Designers can relatively quickly evaluate concept designs and correct thermal problems long before the prototype phase. Many companies that perform thermal simulation on a regular basis at the concept design phase discover that it is almost never necessary to build additional prototypes to solve thermal problems. The elimination of the need for additional thermal prototypes can reduce design costs and, often even more important, makes it possible to bring products to market earlier by reducing the number of prototypes required. Thermal simulation software also provides much more information than physical testing, including detailed 3D graphical information on pressures, temperatures and airflows throughout the design, which helps engineers iterate much more quickly to an optimized design.
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