WEBVTT 00:00.480 --> 00:01.200 Hello everyone! 00:01.200 --> 00:03.000 Welcome to stream PCB YouTube channel. 00:03.040 --> 00:09.000 My name is Avital and in the previous video we talked about return current and its modeling and simulation 00:09.000 --> 00:10.360 on cadence piecewise. 00:11.000 --> 00:15.000 The purpose of this video is to give you better understanding of return current path. 00:15.240 --> 00:20.560 For example, if there is power plane or number of power planes between signal and its return path, 00:21.280 --> 00:24.160 we will try to understand which path return current will follow. 00:24.520 --> 00:29.400 Apart from that, we will try to understand what will happen to instantaneous impedance due to power 00:29.440 --> 00:30.440 plane. 00:30.440 --> 00:34.160 We will use couple of simulations and modeling on cadence piecewise. 00:34.160 --> 00:35.400 So let's get started. 00:35.960 --> 00:40.120 First we will try to understand the problem of changing reference plane. 00:40.120 --> 00:43.920 In this very first example where we have signal and its return path. 00:43.920 --> 00:49.160 And as we discussed in the previous video, there is capacitive coupling between them. 00:49.160 --> 00:55.680 Now as signal travels down the transmission line, return current will flow through these tiny capacitors. 00:55.680 --> 01:01.280 Now in another case, let's suppose we are dealing with multilayer PCB and we have introduced a power 01:01.320 --> 01:04.920 plane or floating plane between signal and its return path. 01:05.200 --> 01:08.000 Now we don't have any idea how current will return to source. 01:08.200 --> 01:09.480 So let's talk about it. 01:09.680 --> 01:15.640 But before we try to understand different ways to visualize return current, let's recall two important 01:15.640 --> 01:17.360 points I have discussed earlier. 01:17.920 --> 01:19.480 First point is for DC current. 01:19.520 --> 01:24.520 DC current follows the path which has lowest resistance and then it return to source. 01:25.040 --> 01:31.160 But similarly for AC current, it follows the path which has lowest impedance or inductance and then 01:31.160 --> 01:32.560 is return to source. 01:33.080 --> 01:38.720 These two points will help us to understand what will happen to return current if we add a power plan. 01:38.760 --> 01:41.880 Now let's try to understand how return current flows in this case. 01:42.120 --> 01:48.960 So first method is capacitive coupling waves like a two layer board return current flows through capacitive 01:48.960 --> 01:51.800 coupling between signal and return plane underneath. 01:52.240 --> 01:58.400 Similarly, in this case, return current flows from signal layer to layer two due to capacitive coupling, 01:59.040 --> 02:04.210 and then from layer two to layer three due to their capacitive coupling, as you can see on this figure. 02:05.410 --> 02:10.890 Another way to think about return current while switching between reference plane is eddy current way. 02:10.930 --> 02:16.970 So eddy current is a loop current which induces in nearby conductor due to change in magnetic field. 02:17.170 --> 02:19.770 And the direction of current will be opposite of that. 02:21.570 --> 02:27.330 Let's try to see that in our three layer model, in which as soon as signal starts travelling, due 02:27.330 --> 02:34.770 to that, there will be change in electromagnetic field which will induce loop current on adjacent conductor. 02:34.810 --> 02:40.330 Now same thing will happen on the other side of the source, which will cause another loop current flow 02:40.770 --> 02:42.570 at the other surface of the plane. 02:43.410 --> 02:47.570 And when they meet at the corner of the plane, it completes the circuit. 02:48.130 --> 02:53.250 Now in the next step, we are going to talk about what will be the effect of that capacitive coupling 02:53.250 --> 02:55.730 between planes and signal layer. 02:55.770 --> 02:58.490 Either it should be low or it should be high. 02:59.130 --> 03:01.930 So for that I'm going to use a piecewise model. 03:02.010 --> 03:07.290 So as you can see on the screen on this Spice model I'm using 50 ohm. 03:07.450 --> 03:13.410 0.5 nanosecond time delay transmission line, which are connected in series. 03:13.610 --> 03:20.290 And here we have a step response of one volt which is further connected to 200 ohm source resistor. 03:20.770 --> 03:24.290 And I have terminated this model with a 50 ohm resistor. 03:25.050 --> 03:27.850 Now here you are seeing couple of more transmission line. 03:28.450 --> 03:32.850 So you can imagine that as a plane underneath this signal layer. 03:32.930 --> 03:35.210 So this is our 50 ohm signal layer. 03:35.730 --> 03:42.690 And underneath I have created a very less impedance plane which has almost zero time delay. 03:43.250 --> 03:48.290 And another plane that you can imagine on this model is ground plane which is here. 03:49.290 --> 03:56.890 So I'm talking about coupling between signal layer which is this capacitive coupling which is ten nanofarad 03:56.890 --> 04:00.370 here and plane underneath. 04:00.650 --> 04:05.930 And then coupling between plane and ground plane which is C10. 04:05.930 --> 04:13.930 In, so everything will change when we vary C8 and C10 on these two waveforms. 04:14.290 --> 04:16.370 So this is the current waveform of signal. 04:16.650 --> 04:21.210 And here you will see the return current which is going back to source. 04:21.970 --> 04:26.810 All right so this example is low capacitive coupling between planes. 04:28.890 --> 04:33.650 So what what should be the expected results of low capacitive coupling. 04:34.210 --> 04:37.850 First one is impedance discontinuity. 04:37.970 --> 04:43.250 So if there is low coupling there will be high impedance which will cause. 04:43.370 --> 04:48.850 So in case of impedance discontinuity we will see reflections on the line. 04:48.890 --> 04:53.410 Now out of these two waveform which one has more reflection. 04:53.450 --> 04:57.130 Obviously the second one has more reflection compared to this one. 04:57.170 --> 04:58.970 Alright so let's see the results. 05:00.050 --> 05:03.370 So as you can see this is our low coupling capacitor model. 05:03.970 --> 05:04.970 And we can see. 05:05.850 --> 05:08.940 So this is transmission line which is on the signal layer. 05:08.940 --> 05:11.100 So green waveform is little bit stable. 05:11.500 --> 05:16.660 But blue waveform has so much reflection on the line due to low coupling. 05:17.100 --> 05:18.820 Now let's see the other example. 05:19.420 --> 05:21.180 So I've created another model. 05:21.180 --> 05:27.140 And here I have increased the capacitive coupling to one microfarad from ten nanofarad. 05:27.420 --> 05:28.060 Alright. 05:29.940 --> 05:37.540 Due to that increment in coupling, the current will be flowing from signal layer to power plane or 05:37.620 --> 05:43.300 plane underneath and from that plane to ground and going back to our source. 05:43.980 --> 05:50.220 So due to this there will be very less impedance discontinuity and which will cause low reflection. 05:50.340 --> 05:51.980 So let's check out that as well. 05:52.980 --> 05:56.620 So as you can see and this is the time delay that we have introduced. 05:56.620 --> 05:59.860 As you can see green is the waveform of our signal. 05:59.860 --> 06:05.060 And red is the waveform of return current through capacitive coupling. 06:06.060 --> 06:07.660 And you can see the clear difference. 06:07.780 --> 06:13.580 There is more reflection on return current due to capacitive coupling. 06:13.700 --> 06:16.100 And this is our signal return current. 06:16.220 --> 06:16.620 All right. 06:16.660 --> 06:20.380 And there is also some reflection because we have added plane underneath. 06:20.380 --> 06:22.140 We'll try to understand that in more detail. 06:22.140 --> 06:29.420 What will happen or what driver will see when we add a plane under the signal layer. 06:30.020 --> 06:30.500 All right. 06:30.500 --> 06:36.820 So from that we got to know the capacitive coupling between planes should be higher. 06:36.980 --> 06:41.220 Or you can say there should be tight coupling between the planes and signal layer. 06:41.900 --> 06:42.260 Right. 06:42.300 --> 06:47.540 So then we'll get more positive results compared to the low capacitive coupling. 06:47.580 --> 06:51.180 Again I would recommend just play along these values. 06:51.340 --> 06:57.020 Either change the value of capacitance, change the value of time delay impedance, add more transmission 06:57.020 --> 07:02.100 line on this model and see what will happen on the return current waveform. 07:02.260 --> 07:03.540 I will attach this project. 07:03.540 --> 07:07.900 You can download it from description and let me know in the comment section in case of any question. 07:08.740 --> 07:12.190 Now after discussing models and two methods. 07:13.230 --> 07:18.070 Let's talk about what impedance the driver will see into the transmission line. 07:18.310 --> 07:21.030 If there is a plane between signal and ground. 07:22.350 --> 07:28.790 So from the two methods, one is the eddy current way and coupling capacitors way that we got to know 07:28.990 --> 07:31.390 the current will be flowing in series. 07:31.830 --> 07:38.510 So that means the total impedance between layer one and layer three will be is equal to the impedance 07:38.510 --> 07:43.150 between layer one and layer two, plus the impedance between layer two and layer three. 07:44.070 --> 07:44.630 All right. 07:44.750 --> 07:48.790 So till here we got to know about return current path if we insert a plane. 07:49.110 --> 07:51.590 Now what we'll do with all this information. 07:51.750 --> 07:53.390 What is the application of that. 07:53.550 --> 07:56.110 So let's talk about the very first point. 07:56.510 --> 08:03.910 So first point is by choosing the coupling between these layers we can control the width of the impedance 08:04.510 --> 08:06.590 of impedance control track. 08:07.550 --> 08:12.830 For example the smaller the impedance between plane two and three or Z. 08:12.910 --> 08:13.470 Two, three. 08:13.870 --> 08:15.670 The thicker the signal track will be. 08:16.470 --> 08:22.710 So sometimes we face this problem while we are routing some very thinner tracks which are not manufacturable 08:22.710 --> 08:23.550 in real world. 08:23.910 --> 08:27.590 So to make them thicker, we have to implement this method. 08:27.590 --> 08:33.430 We have to reduce the coupling between planes so we can make them more thicker and manufacturable. 08:33.910 --> 08:35.630 So this is the first application. 08:36.670 --> 08:40.270 And the second application is selection of layer stackup. 08:40.990 --> 08:43.070 So as you can see we have two layer stackup. 08:43.430 --> 08:45.630 And the clear difference is one. 08:45.630 --> 08:53.510 One has the little bit thicker Prepreg and Stackup two has little bit thinner prepreg which is 0.1 and 08:53.510 --> 08:54.390 0.2mm. 08:54.910 --> 09:01.470 So the case one is let's suppose we are going to route our high speed track or impedance control track 09:01.470 --> 09:02.590 on the top layer. 09:03.750 --> 09:06.470 So in that case which layer Stackup is good. 09:06.510 --> 09:08.310 Obviously the second layer stack up. 09:08.510 --> 09:14.670 So in this case second layer Stackup is good for Routing high speed track. 09:15.270 --> 09:21.270 If you are going to route it on the top layer, because there will be more coupling between top layer 09:21.550 --> 09:24.870 and inner copper layer, which is ground. 09:24.910 --> 09:25.150 All right. 09:25.190 --> 09:31.310 So generally for four layer we use this configuration ground signal power and signal. 09:31.470 --> 09:32.070 All right. 09:33.590 --> 09:40.270 Now another case is let's suppose I am planning to route high speed tracks or impedance controlled track 09:40.270 --> 09:41.350 on the bottom layer. 09:42.590 --> 09:45.070 So for that case which layer Stackup is better. 09:45.110 --> 09:49.990 So we have to simply add up this thickness and this thickness. 09:49.990 --> 09:52.110 And we can compare which one is higher. 09:53.070 --> 09:58.350 So if we add up this thickness it will be approx 1.2 something. 09:58.630 --> 10:00.990 And this will be approx 1.3. 10:01.470 --> 10:07.670 So obviously in that case we'll going to if we are routing on the bottom layer. 10:10.870 --> 10:14.150 Then first layer stack up is good for that. 10:14.630 --> 10:15.200 All right. 10:15.200 --> 10:19.440 So this is another application where you can implement this. 10:20.000 --> 10:26.040 Whatever we have learned in this video to estimate if you are routing on inner layer, top layer or 10:26.040 --> 10:27.800 bottom layer, which Stackup is better. 10:28.000 --> 10:28.320 Right. 10:28.320 --> 10:34.600 If you have a power plane or, you know, couple of more planes between signal and ground. 10:35.440 --> 10:38.920 Now let's talk about how we can estimate the impedance between planes. 10:39.800 --> 10:46.320 So for that we use simple formula which is z naught is equal to 377 divided by square root of epsilon 10:46.320 --> 10:53.760 r which should be effective dielectric constant multiplied by height or gap between the planes divided 10:53.760 --> 10:55.080 by its width. 10:55.880 --> 11:01.760 So using this formula we can able to estimate the impedance between the planes. 11:01.800 --> 11:07.760 Now let's conclude this video with this statement that to maintain rail collapse or to minimize rail 11:07.760 --> 11:14.760 collapse, and to put return current path close to signal layer, these middle planes or power planes 11:14.760 --> 11:17.160 should be tightly coupled with ground.