Unwanted return current can cause several side problems like ground bounce, EMI and ringing.
In this video I will cover what is return current, how it propagates and circulate down the transmission
line, and what happens if return current gets disturbed.
And finally, I will discuss what happens if we increase the frequency on the current distribution for
a PCB track, along with a couple of demos of cadence spikes.
So let's get started.
In the first section of this video, we will try to visualize the return current, how it is created,
what is the direction of propagation and direction of circulation for a return current?
We can ask very first question why does current return?
And its answer is very simple as per Kirchhoff law, which states that the current entering into a node
or junction must be equal to the current flowing out, or algebraic summation of a current should be
equal to zero, or we can say the current enter is equal to current exit.
That means from q is equal to it.
Every charge in and out should be equal to zero.
Let's see the quick application of Kirchhoff's law in transmission line, and I amount of current going
in and returning back to the source.
That means it is flowing in loop.
What do you think in a transmission line?
How much time it take current to get back to the source?
And the answer is it returns immediately.
I will show you how.
If you remember our discussion on zeroth and first order model of a transmission line, you can watch
it by clicking over the I button.
If you forgot about it.
As soon as signal launch, it sees a bunch of tiny capacitor between signal and return plane underneath.
We will just take one capacitor from the transmission line and try to understand the return current
on it, and it will be similar for all other capacitor because it is a uniform transmission line.
We all know the working principle of a capacitor.
If the voltage across the capacitor is constant, there will be no current flow.
But as signal starts travelling, there will be a change in voltage which will cause an amount of current
will be flowing through it.
That current flows till the transition is happening on a capacitor.
Then signal will reach to another capacitor due to signals dynamic property.
Let's understand how current is flowing through a capacitor.
As you can see, capacitor on a transmission line is connected to signal and its return path.
And due to voltage transition, positive charge will accumulate on first plate of a capacitor.
And negative charge was already there on the other plate.
Due to this, electrons will move toward the positive charge to neutralize it and result you will see.
Current will flow through the capacitor.
Same funda will work throughout the uniform transmission line, which will cause same amount of current
flow.
And we call it displacement current.
As you can see on the above figure.
Current is moving forward as well as returning back through coupling capacitor.
In this case, current is moving forward.
That means its direction of propagation will be forward and current is also moving clockwise.
So that means its direction of circulation will be clockwise.
Let's see couple of examples on this one when the direction of circulation will be anti-clockwise.
First draw a transmission line.
But this time we will give negative VDC voltage.
Let's see on a single capacitor due to minus VDC transition.
Negative charge will accumulate on first plate and positive on the second plate.
An electron will flow downwards due to opposite electron flow.
Direction of circulation will be anti-clockwise.
Till now we have learnt how to visualise return current flow from coupling capacitors.
Point of view.
Now before going for simulation, let's see how electromagnetic field contribute to return current.
So as soon as we pass an alternate current in an conductor, it creates an electric field around the
conductor which will spread to the closest conductor.
In this case, it will be our return plane.
Then these electric fields give rise to electric potential difference between signal and return plane
underneath and hence the current flows.
This change in electric field will generate magnetic field around the conductor and for direction of
magnetic field will follow right hand rule.
Let's see a quick demo on piecewise.
For that open Orcad capture.
And here I have created a Spice model of transmission line, where I'm using three transmission line
of 50 ohm characteristic impedance and 0.5 nanosecond time delay.
I am terminating those transmission line with a 50 ohm resistor, and at the source side I am using
a step response of one volt amplitude and its source resistor is 200 ohm.
So before running the simulation, I just wanted to get the expected results.
For that, I'm going back to our whiteboard.
And here let's draw the voltage divider circuit.
And it is very simple circuit where I'm giving a one volt step response which is connected to 200 ohm
source resistor.
And it is further connected to a 50 ohm transmission line.
So here we'll apply Ohm's law which is V is equal to IR.
And we know the voltage.
We know the total resistance in series.
Then current will be is equal to four milliamps.
So expected results from the Spice simulation will be three waveform on each transmission line.
Return current and delay between those waveform will be Will be 0.5 nanosecond and amplitude will be
four milliamps.
Let's see the results.
And as you can see, green one is transmission line one.
And on that it is going to four milliamps.
And after 0.5 nanoseconds we'll see the current waveform of second transmission line.
And after that the return current of third transmission line which is again after 0.5 nanosecond.
Now let's see what are the factors that add discontinuity on the return path.
As a designer, what are the precautions we should take?
First one is slot on a return path.
So in this example you can see the return current is flowing from point A to point B, and due to plane
cutout there will be reflection.
Second one is shared return path on a mixed signal board.
In this example, let's suppose we are.
We have a analog and digital circuit and a power supply on the analog section.
So that means the current or return current will be flowing from digital toward the power supply will
be shared with analog and which will cause interference in analog signals.
So the right way to do it, we have to place power supply outside of analog and digital circuit, and
its return path will be separated.
Now let's see a quick demo of impedance discontinuity and its effect on return current.
For that open sources.
And here I did one change.
That is I have replaced 50 ohm transmission line to 80 ohm characteristic impedance.
That means there is impedance discontinuity.
Let's run the simulation.
And in simulation you can clearly see for first 50 ohm transmission line its current is four Ma.
As we have calculated earlier.
But again when it see a impedance discontinuity of 80 ohm the current drop to 3.5 milliamps.
And again it will see a 50 ohm transmission.
Line it again going for four milliamps.
And there is few more reflection due to termination resistor.
So as expected we are getting the same values here.
And we got those values using Ohm's law only.
Now another topic we're going to discuss is what is the effect of frequency on the return current and
the distribution of return current.
Let's talk about effect of frequency on return current distribution.
So first we will see from skin depth point of view lower the frequency.
Higher will be the penetration of charge in a conductor and vice versa.
But we will try to understand from impedance point of view or resistance point of view.
For low frequency, the ground current takes up the path of least resistance, but in case of high frequency,
the ground current takes up the path for least inductance, and that's why it flow under the signal
because of capacitive coupling between signal and its return path.
Let's summarize this video.
Till here we have learned what is return current, how displacement current propagate and circulate
down the transmission line, along with few p spice demos of return current and effect of impedance
mismatch on it.
And we have seen the relation between frequency and current distribution.
In the next video, we will discuss what happens when we place a power plane between signal and its
return path.