Hello everyone!
Welcome to stream PCB YouTube channel.
My name is Avital and in the previous video we talked about return current and its modeling and simulation
on cadence piecewise.
The purpose of this video is to give you better understanding of return current path.
For example, if there is power plane or number of power planes between signal and its return path,
we will try to understand which path return current will follow.
Apart from that, we will try to understand what will happen to instantaneous impedance due to power
plane.
We will use couple of simulations and modeling on cadence piecewise.
So let's get started.
First we will try to understand the problem of changing reference plane.
In this very first example where we have signal and its return path.
And as we discussed in the previous video, there is capacitive coupling between them.
Now as signal travels down the transmission line, return current will flow through these tiny capacitors.
Now in another case, let's suppose we are dealing with multilayer PCB and we have introduced a power
plane or floating plane between signal and its return path.
Now we don't have any idea how current will return to source.
So let's talk about it.
But before we try to understand different ways to visualize return current, let's recall two important
points I have discussed earlier.
First point is for DC current.
DC current follows the path which has lowest resistance and then it return to source.
But similarly for AC current, it follows the path which has lowest impedance or inductance and then
is return to source.
These two points will help us to understand what will happen to return current if we add a power plan.
Now let's try to understand how return current flows in this case.
So first method is capacitive coupling waves like a two layer board return current flows through capacitive
coupling between signal and return plane underneath.
Similarly, in this case, return current flows from signal layer to layer two due to capacitive coupling,
and then from layer two to layer three due to their capacitive coupling, as you can see on this figure.
Another way to think about return current while switching between reference plane is eddy current way.
So eddy current is a loop current which induces in nearby conductor due to change in magnetic field.
And the direction of current will be opposite of that.
Let's try to see that in our three layer model, in which as soon as signal starts travelling, due
to that, there will be change in electromagnetic field which will induce loop current on adjacent conductor.
Now same thing will happen on the other side of the source, which will cause another loop current flow
at the other surface of the plane.
And when they meet at the corner of the plane, it completes the circuit.
Now in the next step, we are going to talk about what will be the effect of that capacitive coupling
between planes and signal layer.
Either it should be low or it should be high.
So for that I'm going to use a piecewise model.
So as you can see on the screen on this Spice model I'm using 50 ohm.
0.5 nanosecond time delay transmission line, which are connected in series.
And here we have a step response of one volt which is further connected to 200 ohm source resistor.
And I have terminated this model with a 50 ohm resistor.
Now here you are seeing couple of more transmission line.
So you can imagine that as a plane underneath this signal layer.
So this is our 50 ohm signal layer.
And underneath I have created a very less impedance plane which has almost zero time delay.
And another plane that you can imagine on this model is ground plane which is here.
So I'm talking about coupling between signal layer which is this capacitive coupling which is ten nanofarad
here and plane underneath.
And then coupling between plane and ground plane which is C10.
In, so everything will change when we vary C8 and C10 on these two waveforms.
So this is the current waveform of signal.
And here you will see the return current which is going back to source.
All right so this example is low capacitive coupling between planes.
So what what should be the expected results of low capacitive coupling.
First one is impedance discontinuity.
So if there is low coupling there will be high impedance which will cause.
So in case of impedance discontinuity we will see reflections on the line.
Now out of these two waveform which one has more reflection.
Obviously the second one has more reflection compared to this one.
Alright so let's see the results.
So as you can see this is our low coupling capacitor model.
And we can see.
So this is transmission line which is on the signal layer.
So green waveform is little bit stable.
But blue waveform has so much reflection on the line due to low coupling.
Now let's see the other example.
So I've created another model.
And here I have increased the capacitive coupling to one microfarad from ten nanofarad.
Alright.
Due to that increment in coupling, the current will be flowing from signal layer to power plane or
plane underneath and from that plane to ground and going back to our source.
So due to this there will be very less impedance discontinuity and which will cause low reflection.
So let's check out that as well.
So as you can see and this is the time delay that we have introduced.
As you can see green is the waveform of our signal.
And red is the waveform of return current through capacitive coupling.
And you can see the clear difference.
There is more reflection on return current due to capacitive coupling.
And this is our signal return current.
All right.
And there is also some reflection because we have added plane underneath.
We'll try to understand that in more detail.
What will happen or what driver will see when we add a plane under the signal layer.
All right.
So from that we got to know the capacitive coupling between planes should be higher.
Or you can say there should be tight coupling between the planes and signal layer.
Right.
So then we'll get more positive results compared to the low capacitive coupling.
Again I would recommend just play along these values.
Either change the value of capacitance, change the value of time delay impedance, add more transmission
line on this model and see what will happen on the return current waveform.
I will attach this project.
You can download it from description and let me know in the comment section in case of any question.
Now after discussing models and two methods.
Let's talk about what impedance the driver will see into the transmission line.
If there is a plane between signal and ground.
So from the two methods, one is the eddy current way and coupling capacitors way that we got to know
the current will be flowing in series.
So that means the total impedance between layer one and layer three will be is equal to the impedance
between layer one and layer two, plus the impedance between layer two and layer three.
All right.
So till here we got to know about return current path if we insert a plane.
Now what we'll do with all this information.
What is the application of that.
So let's talk about the very first point.
So first point is by choosing the coupling between these layers we can control the width of the impedance
of impedance control track.
For example the smaller the impedance between plane two and three or Z.
Two, three.
The thicker the signal track will be.
So sometimes we face this problem while we are routing some very thinner tracks which are not manufacturable
in real world.
So to make them thicker, we have to implement this method.
We have to reduce the coupling between planes so we can make them more thicker and manufacturable.
So this is the first application.
And the second application is selection of layer stackup.
So as you can see we have two layer stackup.
And the clear difference is one.
One has the little bit thicker Prepreg and Stackup two has little bit thinner prepreg which is 0.1 and
0.2mm.
So the case one is let's suppose we are going to route our high speed track or impedance control track
on the top layer.
So in that case which layer Stackup is good.
Obviously the second layer stack up.
So in this case second layer Stackup is good for Routing high speed track.
If you are going to route it on the top layer, because there will be more coupling between top layer
and inner copper layer, which is ground.
All right.
So generally for four layer we use this configuration ground signal power and signal.
All right.
Now another case is let's suppose I am planning to route high speed tracks or impedance controlled track
on the bottom layer.
So for that case which layer Stackup is better.
So we have to simply add up this thickness and this thickness.
And we can compare which one is higher.
So if we add up this thickness it will be approx 1.2 something.
And this will be approx 1.3.
So obviously in that case we'll going to if we are routing on the bottom layer.
Then first layer stack up is good for that.
All right.
So this is another application where you can implement this.
Whatever we have learned in this video to estimate if you are routing on inner layer, top layer or
bottom layer, which Stackup is better.
Right.
If you have a power plane or, you know, couple of more planes between signal and ground.
Now let's talk about how we can estimate the impedance between planes.
So for that we use simple formula which is z naught is equal to 377 divided by square root of epsilon
r which should be effective dielectric constant multiplied by height or gap between the planes divided
by its width.
So using this formula we can able to estimate the impedance between the planes.
Now let's conclude this video with this statement that to maintain rail collapse or to minimize rail
collapse, and to put return current path close to signal layer, these middle planes or power planes
should be tightly coupled with ground.