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Design007-Mar2019

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64 DESIGN007 MAGAZINE I MARCH 2019 Key Points: • In a low-frequency or DC circuit, the return current takes the path of least resistance filling the cross-sectional area of the trace • As the frequency increases, the magnetic forces surrounding a trace become signifi- cant and the return current takes the path of least inductance • High-frequency return current distribution follows a tight path directly above and/or below the trace in the reference plane(s) • The skin effect is the tendency for magnet- ic fields to distribute current to a shallow depth around the perimeter of the trace • The proximity effect is the tendency for magnetic fields to distribute current around the perimeter of the trace in a non-uniform manner when referenced to a plane • It is important to have a clearly defined return current path and to know exactly where the return current will flow • Return path discontinuities tend to divert current increasing the loop area, induc- tance, and delay • If the reference planes are at the same DC potential, they can then be directly connected by stitching vias near the signal via transition to provide shorter paths for the return currents • If the reference planes are at different DC potential, then decoupling capacitors must be connected across the planes at these points to provide a return path • Two decoupling capacitors spanning split power planes is a better solution as this eliminates the transfer of power supply noise from one supply to another • The distribution of the current equation provides insight into where the return path current flows • The fundamental distribution of the cur- rent equation is also the basis for simple crosstalk estimates Further Reading • Olney, B. "Beyond Design: The Dark Side—Return of the Signal," The PCB Design Magazine, May 2017. • Olney, B. "Beyond Design: Effects of Surface Rough- ness on High-speed PCBs," The PCB Design Magazine, February 2015. • Olney, B. "Beyond Design: Mythbusting—There Are No One-way Trips!" The PCB Design Magazine, April 2014. • Olney, B. "Beyond Design: Plane Crazy, Part 1," The PCB Design Magazine, December 2015. • Olney, B. "Beyond Design: Controlling the Beast," The PCB Magazine, December 2011. • Olney, B. "Beyond Design: Return Path Discontinui- ties," The PCB Design Magazine, April 2017. • Johnson, H., & Graham, M. High-speed Signal Propa- gation: Advanced Black Magic, Prentice Hall, 2003. Barry Olney is managing director of In-Cir- cuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company de- veloped the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded from www.icd.com.au. To read past columns or contact Olney, click here. tive graphene aerogels. The results show that the smaller the particle, the less likely it is to crack or fracture upon lithiation The next steps are to develop technology for creat- ing silicon nanoparticles in a faster and less expensive way, making these tools more accessible for industry and technology developers. The paper was published in Chemistry of Materials. (Source: University of Alberta) Scientists believe that silicon could be the answer to your battery woes with the potential for a charge capac- ity 10 times larger than current lithium-ion batteries. Now, University of Alberta chemists have published research that studies the effect of nanostructuring the silicon within lithium-ion batteries to understand the importance of size. In their research, the researchers examined silicon nanoparticles of four different sizes within highly conduc- Tiny Silicon Nanoparticles Cement New Era for Ultra-high Capacity Batteries

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