Design expressions for the trace-to-edge common-mode inductance of a printed circuit board

The parasitic ground-plane inductance is responsible for common-mode radiation, as one of the major unwanted radiation mechanisms of printed circuit boards. For the computation of the common-mode inductance simple relations are known for the case that the trace is centered above the ground plane. In this paper the increase of the ground-plane inductance for arbitrary trace-to-edge distances is studied. From the exact analytical result obtained by complex analysis a much simpler real-valued relation is derived which allows one to set up dimensioning equations for the minimum distance of the trace to the board edge. The inductance increase correlates quite well with published measurement data for the common-mode current increase. A parameter study for different dimensions of the board provides a quantification of the potential electromagnetic interference (EMI) level.