Digital Circuit Boards: Mach 1 GHz

1.6 Current

The charges that create a static electric field are located on the surface of conductors. In every circuit with operating voltages, there are surface charges. These surface charges must move if the voltages change value. The pattern of charge motion can best be appreciated by noting the pattern of electric field lines that terminate on a conductor. Charges tend to concentrate where there are sharp bends or transitions. This means that if the field pattern changes, current flow must occur when the pattern changes. If there are no field lines terminating on a part of a conductor then there will be no current flow on that part of the conductor. This means that parts of a ground plane are often not used for current flow. Figure 1.1 shows the E field pattern around a typical trace over a ground plane.

Figure 1.1 The electric field pattern around a typical circuit trace.

The field pattern shows that the charges concentrate on one side of the trace and on that portion of the ground plane under the trace. No current flows on the other side of the ground plane. This charge pattern is essentially independent of frequency. Every E field line terminates on a unit of charge. The unit of charge is selected to best show the shape of the field. Note that the electric field lines terminate perpendicular to conducting surfaces. The lines that are shown parallel to the ground plane represent ...