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8 DESIGN007 MAGAZINE I NOVEMBER 2025 P ower delivery on modern systems is an impor- tant and challenging problem facing hard- ware designers. These challenges are even more apparent as device operating speeds and current demands increase. Modern FPGAs, processors, and other devices feature fast switching currents and demand precise voltage levels. Engineers often focus on discrete decoupling capacitors placed near the switching devices in hopes of providing the required capacitance for these high current demands. One of the more over- looked items of the power distribution system is the capacitance formed by the power plane cavities, and how it contributes to the power distribution system's ability to decouple the switching devices. Through F E AT U R E A RT I C L E by K i r k F a b b r i , S i e m e n s E DA a series of simple simulation experiments, this arti- cle will demonstrate a basic principle regarding the effectiveness of the capacitance formed by PCBs. Basic PDN Model A basic power delivery network (PDN) includes the voltage regulator model (VRM), the discrete decou- pling capacitors, the power plane cavities, and any on-die capacitance formed on the ICs or devices themselves. The edge rate of this switching current is extremely important when trying to calculate how effective the PDN will be in suppressing the ripple voltage. A tra- ditional method of estimating these currents is to sim- ulate the I/O and look at the driver current spectrum to better understand the switching activity of these devices. 1 From this frequency spectrum, one can gain a better understanding of the load dynamics, and how to gauge the effective decoupling radius. A simple way to represent power plane cavities is with a distributed RLC network. From Equation 1, capacitance can be calculated. From this equation, we see multiple ways to increase the capacitance. Where: C = capacitance in pF eo = permittivity of free space (0.225 pF/inch) er = dielectric constant A = area (in 2 ) S = spacing between the planes (inches). 2 Power Integrity: Effective Decoupling Radius