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
