Understanding Return Current Path in PCB

Return Path PCB

Introduction

One of the Most Important Basic aspects of any Electronic Circuit is the current Return path. In any Electronic circuit, the return current path should be obvious. Most of the designers only think about where the signals flow and not concerning about path taken by the return current. Once a designer understands how the geometry and the characteristics of a signal affect the return path, it becomes easier to determine the return path for signals.

What is meant by Return path?

To understand about return path let’s take one example from our livelihood. If you are traveling from Home to one place eventually want to return home. So, like that in Electronics we send electrical signals from source to sink. So these signals always try to reach source, for this those signals require clear return path.

The incorrect Signal Return path in Electronic Designs are the main most common sources of EMI and noise coupling problems.

What Determines the Return Path?

The Return current always follows the least resistance path. but this is only applicable for DC
circuits. In time-varying signals, return current always follows the path of least reactance. Which is also the least impedance path. So Impedance of the circuit which carries Return current Determines the Return Path in PCB.

  • The impedance in circuits consist of two portions one is a resistive portion another one is the reactive portion
  • The resistive portion does not depend on the frequency
  • The reactive portion depends upon the frequency
  • In PCB any circuit behave as resistive, capacitive, inductive depending upon geometry, different components behavior and frequency of the signal
  • In reality case, circuits on board should be modeled, at minimum, as RLC Circuits; this is the standard approach with transmission lines even if the circuit does not contain any discrete capacitors or inductors.
  • PCB layout act as an RLC circuit due to adjacent conductors are separated by substrate (dielectric material) act as a capacitor which creates parasitic capacitance.
  • The inductive behavior arises Due to some magnetic permeability of the substrate and the path followed by the current. Thus, each circuit has some parasitic inductance
  • The natural Resistance of Conductors and parasitics contribute to the impedance when signals travel through the board.
  • So The geometry of traces and planes in PCB Determines the Return path that the Signals will follow when returning to the source.

How Return Path Formed Based on Signal Frequency?

In DC and Low Frequency signals take the direct short path (least resistance) from load back to source for the return current, in contrast return current in high Frequency signals always choose least reactance. Which is also path of least impedance (tries to follow the signal path).

Return Path in DC Circuits:

Before we know about how the return current path in PCB formed, we should know what happens in DC circuits. Below the images Figure 1 is Top View of PCB Shows Traces Routed on top signal layer of PCB carrying electrical signals from source to sink. Figure 2 shows the inner reference ground plane in the second layer. these two conductor layers are isolated by substrate material (Dielectric material). The yellow lines shows in below image is the return path

Figure 1: Top View of PCB

Figure 2: Side View of PCB

Here The two conductor layers are isolated by substrate material and this forming a capacitor structure, Now this is almost working as capacitor, it provides capacitance between the two layers. Note that the capacitor symbols shown in picture do not indicate the presence of discrete capacitors.

DC current that given in the top layer (at the source point) travels through the trace, which has the path of least resistance. Once it reaches to destination, After the current leaves the IC, it enters the inner reference layer through a VIA and travels along the Reference layer (Here GND is reference Layer), then returns to the Source Supply on the surface layer through another VIA. The DC current consist infinite impedance between the Signal layer and the Reference GND Layer, Due to this the current does not travel directly to the substrate through the lumped capacitance. Once the current enters the ground plane, it follows the path of least resistance back to the GND via. Here the path of least resistance is the shortest distance (the straight yellow line) between the two vias.

Return Path in Time varying Signals:

Below the images Figure 3 is Top View of PCB Shows Traces Routed on top signal layer of PCB carrying electrical signals from source to sink. Figure 4 shows the inner reference ground plane in the second layer. these two conductor layers are isolated by substrate material (Dielectric material). The yellow lines shows in below image is the return path

Figure 3: Top View of PCB

Figure 4: Side View of PCB

The signals which are varies its characteristics of voltage and current with respect to time are known as time varying signals. Otherwise known as AC. Due to less impedance between signal layer and the reference layer,The signal induces displacement current through the lumped capacitance in the substrate. Which then travels through the ground plane. This means, The return current produced in the ground plane underneath of trace. The return current corresponds to the least reactance path

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