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How is the return current path in a PCB formed?
To get a sense of how the return current path in a PCB is formed, we’ll first look at what happens with DC current in a simple example. The top view in the example layout below shows traces on the surface layer of a PCB leading to an IC. The bottom half of the figure below shows the interior ground plane in the second layer.
Where does the DC current go in a PCB?
(Note: this is actually the reason that every trace in a PCB is really a transmission line; more on this in an upcoming article). A DC current that originates in the top layer (at the +5 V point) travels directly along the trace, which has the path of least resistance.
Can a power layer be used as a return path?
The power layer will still be used as a return plane, but the return path can still move back to ground as a displacement current. The situation where a power plane or power rails are used as a return path should generally be avoided in higher speed boards with dense component layout and routing.
Should you use your power plane as a return path?
A slightly better arrangement is to use signal-power-ground-signal within your layer stack. The power layer will still be used as a return plane, but the return path can still move back to ground as a displacement current.
Where does the displacement current go in a PCB?
With any time-varying signal (either a pulse, digital, or analog signal), the situation is different. Because the voltage and current are changing in time, the signal can induce a displacement current through the lumped capacitance in the substrate, which then travels through the ground plane.
Where does the current go after leaving the IC?
After the current leaves the IC, it enters the interior layer through a via and travels along the ground plane; it then returns to the supply point on the surface layer through another via.
Why does the return current travel through the ground plane?
Because the voltage and current are changing in time, the signal can induce a displacement current through the lumped capacitance in the substrate, which then travels through the ground plane. This means that the return current is produced in the ground plane below the trace.