Estimating and mitigating radiated emissions from PCB heatsinks.

摘要

The advance of VLSI technology requires heatsinks to take away the heat generated by ICs and keep the temperature below an acceptable limit. These metal heatsinks radiate and cause EMI problems when electromagnetic noises are coupled to them. Thus it is important to study the possible radiation of a VLSI heatsink and the effective methods to reduce the radiated emissions.;This dissertation includes three chapters on estimating and mitigating VLSI heatsink radiation. In the first chapter, a closed-form expression is derived for determining the maximum possible radiated emissions from a heatsink over a printed circuit board or chassis plane as a function of the maximum voltage between the heatsink and plane. The relevant parameters are the dimensions of the heatsink. The closed-form expression is validated by comparing its results to full-wave simulation results. This analysis was done for rectangular heatsinks, but the results can be applied to other heatsink shapes.;The second chapter discusses a method to damp the unintended radiated emissions from PCB-chassis (or heatsink-PCB) resonances with lossy posts mounted near the four corners of the rectangular cavity formed. A simple closed-form expression was derived for determining an optimal series resistance for damping these cavity resonances over a wide range of frequencies. A similar analysis could be done to determine the optimal resistance values for other cavity shapes and mounting post locations. For the 4-post configuration, shorting one or more of the posts does not affect the optimum resistance value for the remaining posts.;The third chapter discusses the reduction of a tall heatsink radiation by using shorting posts that bypass some of the noise current to the PCB ground. At high frequencies, the size of a tall heatsink may be comparable to a quarter-wavelength and the heatsink/board geometry can be an efficient antenna. The effectiveness of shorting posts was examined for reducing heatsink radiation. The use of lossy components for damping LC resonances is also discussed.