Power bus noise prediction in multilayer printed circuit boards requires an accurate estimation of the power bus impedance. We have previously developed a fast algorithm for efficiently and accurately calculating the impedance of a rectangular power bus structure, based on a cavity mode model. In this paper, we show that the fast algorithm can also be easily utilized for a right-angled triangular power bus structure. Together with a segmentation method, the fast algorithm can then be extended to calculate the impedances of more complicated power bus structures whose patterns consist of several segments of rectangles and/or right-angled triangles. Fast computation of the S-parameters and good agreement between the calculated results and measurements for various boards demonstrated the efficiency and accuracy of using the fast algorithm together with the segmentation method as a powerful technique to estimate the power bus impedance.