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|Handheld Mobile Device Cooling Design and Control|
|Keywords: handheld, passive, cooling|
|The design process for the cooling of passive handheld devices is very different from that of larger computer systems, and a different mindset is necessary for success. A properly designed passive system will normally be limited by the touch temperature of its surface, not the junction temperature of its components. Overall power dissipation capability will be limited by the ability to spread heat throughout the system and bring each surface uniformly to its maximum allowable temperature. Marketing and Industrial Design will insist on thin sleek systems, and this can make it especially difficult to add dedicated heat spreaders to a system. Radiation typically will reject nearly as much heat as convection. A thermal design efficiency number may be calculated by comparing the maximum heat dissipation of a specific system design to that of a system with each surface uniformly at its maximum temperature. It may be acceptable for some areas of a system to become hotter than others. The size of hot spots also must be considered, lest a tiny hot area be the limiting factor for overall temperature control and cause excessive limiting of system performance. For thermal design and temperature control considerations, the system must balance 1) touch temperature limits, 2) power dissipation, 3) computing performance, and 4) ambient temperature expectations. If a defined use case can be determined for a system, with known power v. time, then the use case can also be considered in the thermal design and for thermal tuning purposes. In use, temperature control software will reduce system power – and performance – to maintain acceptable touch temperatures. It falls to the thermal design engineer or thermal control engineer to explain this during the design phase: lower cooling capability will at some point result in lower computing performance. Different materials will produce different perceptions of temperature to the touch, and system thermal design details must comprehend this. The perceived temperature of an object is influenced more by heat flux to the observer’s skin than the temperature of an object. Thus, a high conductivity object with high heat capacity will transfer more heat to the observer’s hand and therefore feel much hotter than a low conductivity low heat capacity material at the same temperature. Surface temperatures of the system during use are calculated from the readings of temperature sensors, such as thermistors, placed on the main board. By placing the sensors wisely, their temperature will follow the time response of the temperature of the system surface. Thermal performance is aided by the use of heat spreaders such as the main PCB, system structural elements, and add-ins like graphite, heat pipes, and vapor chambers. Even so, the system must also accommodate things like areas for battery attachment, which can interrupt low resistance thermal conduction paths. Antenna windows and cameras can also limit the extent of a thermal spreader. Unsolved problems in this space include that users commonly add an aftermarket cover to their systems for personalization or protection. These covers can considerably impede heat transfer from a system and thereby reduce computing performance.|
|Mark Carbone, Thermal Engineer
Santa Clara, California