EMC Question of the Week: April 20, 2020

Return Current Density on a Circuit Board Return Plane

At what frequencies do currents on a circuit board take the path(s) of least resistance?

  1. DC only
  2. kHz frequencies and lower
  3. MHz frequencies and higher
  4. all frequencies


The best answer is "b". Currents tend to take the path(s) of least impedance. At low frequencies, the impedance is dominated by the resistance. In this case, the currents will take all available paths with a density that is inversely proportional to the resistance of that path. Low-frequency currents that flow on a trace from point A to point B and return on a plane, will spread out across the plane. The highest current density will be along the shortest path between the two points, independent of the signal trace routing. 

At megahertz frequencies and higher, the path of least impedance on a circuit board will be determined by the inductance. Currents returning on planes will take the path that minimizes the energy stored in the magnetic field. For microstrip or stripline geometries, this will concentrate most of the current on the plane(s) in a relatively narrow path directly above and/or below the signal trace.

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