Influence of temperature control limits on passively cooled computing system performance
Passively-cooled electronic systems such as tablet computers and cell phones commonly are thermally limited by the allowable touch temperature of their surfaces. These systems typically cannot cool maximum processor power in steady state and simultaneously meet surface temperature restrictions. The primary surface temperature control method for thin mobile devices is to limit power, typically resulting in lower computing performance. Improved thermal designs allow higher power dissipation and therefore increased computing performance. Thermal and mechanical design engineers typically operate in terms of temperature for a given power, and this is not usually translated to computing performance. It can be difficult to justify a better thermal design if it is not quantified in its utility to the user. This paper quantifies the tradeoffs between thermal solution performance/temperature control limits and computing performance for two tablet computers and one cell phone that all use the same type of SoC processor.
Package thermal challenges due to changing mobile system form factors
Mobile platforms have driven semiconductor package form factors from several times the die area to sizes approaching die size. To make matters even more challenging, the package thickness, traditionally on the order of 1–3 mm, has been reduced to thicknesses less than 0.40 mm. In mobile applications, it is size that drives package design. There has been a price paid for the package shrinkage in terms of its thermal performance. Copper, previously used for power and ground planes, and multiple layers of traces, has been replaced with extremely fine traces built in fewer layers. The body size previously used to promote the spreading of heat from the die is now left with two options for heat flow: either out the top (immediately above the die) or through the bottom (below the die) of the package. Improvements to the chip architecture in terms of power efficiency is one of the few remaining options for thermal enhancement at the package-level. Further thermal enhancement should focus on the system level, as this is where the greatest opportunities exist. While many papers have focused on the thermal challenges associated with the system-level, few have translated these constraints to challenges at the package level. This study investigates the historical evolution of mobile platforms and their impact on packaging thermal challenges. Metrics for evaluating the optimization of packages for the mobile space will also be discussed.
Numerical tools applied to power reactor noise analysis
In order to be able to calculate the space- and frequency-dependent neutron noise in real inhomogeneous systems in two-group theory, a code was developed for the calculation of the Green's function (dynamic transfer function) of such systems. This paper reports on the development as well as the test and application of the numerical tools employed. The code that was developed yields the space-dependence of the fluctuations of the neutron flux induced by fluctuating properties of the medium in the two-group diffusion approximation and in a two-dimensional representation of heterogeneous systems, for both critical systems and non-critical systems with an external source. Some applications of these tools to power reactor noise analysis are then described, including the unfolding of the parameters of the noise source from the induced neutron noise, measured at a few discrete locations throughout the core. Other concrete applications concern the study of the space-dependence of the Decay Ratio in Boiling Water Reactors, the noise-based estimation of the Moderator Temperature Coefficient of reactivity in Pressurized Water Reactors, the modeling of the beam- and shell-mode core-barrel vibrations in Pressurized Water Reactors, and the investigation of the validity of the point-kinetic approximation in subcritical systems driven by an external source. In most of these applications, calculations performed using the code are compared with at-power plant measurements. Power reactor noise analysis applications of the above type, i.e. core monitoring without disturbing plant operation, is of particular interest in the framework of the extensive program of power uprates worldwide.