PCB layout full-wave simulator and SPICE co-simulation - Page 1
Forum section: electronics / general

author: r0d3z1
author_rank: Regular Contributor
timestamp: December 14, 2017, 12:48:48 pm
content: Hello everyone,I have been impressed by this articles of texas instrumentshttps://www.ti.com/seclit/ml/slup344/slup344.pdfwhere there is a chapter about the Co-simulation of pcb layout using a full-wavesimulator and SPICE. They used CST to simulate the parasitic effect of track and plane on layout and then spice to simulate the signal in the various point of the PCB. Anyone here have experience on this topic ? I'm looking for some info to move the first steps in this field.

author: blackfin76
author_rank: Regular Contributor
timestamp: January 16, 2018, 09:17:59 am
content: The software they use is very expensive and requires a very skilled person to use. I'm not sure how many non professional people use this type of software to do analysis, most likely not many. However due to higher signal frequencies and the ever increase in in wireless connectivity in more applications it may become more relevant.What is your specific interest for the use of this technology?

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 16, 2018, 11:31:06 am
content: I want to learn the basic of post-layout simulation. I my case is more for education purpose, it would be nice to see the current flow at various frequency in a DCDC converter layout or other power electronics devices.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2018, 12:58:13 am
content: Right. Very expensive. It's out there, it definitely exists and works -- but if you aren't doing large scale commercial production... do you really care? There are cheaper methods to figure it out (including not needing to at all).I half-ass it by using:1. A layout approach which produces a simple inductor matrix, and2. approximating that matrix with estimated inductances in the simulation.Between any pair of connected pads on a PCB, there is a winding on a transformer; the coupling between all of these windings can be represented by an NxN matrix of coupling coefficients. Most of these will be near zero, meaning that the traces are very distant and do not interact significantly. These can be first approximated as zero, giving a sparse matrix.The matrix can be further reduced by assuming that ground inductances are negligible or zero, or can be moved to supply or device connections instead. This is a poor assumption, to be used carefully, but a useful one.If the only significant couplings are self-inductance, then you don't need any coupling terms at all, and can use lone inductors in the model.To achieve this in the layout, first and foremost you must have significant shielding between any traces for which coupling will be a problem. That means ground plane, solid, well stitched. The PCB does not need to be multilayer, but it cannot be an extremely dense layout on 2 layers: you must reserve enough space between components and subcircuits to fill and bond that ground around them."Ground" does not necessarily mean circuit or earth ground, but whatever the local return current path is in a given location.That key assumption I mentioned, has these implications:- Even for a solid ground plane, there are ground loop voltages induced in it. Ground will not be "ground" in some absolute sense, at any point on the board. It may be close, but not exactly (and things are worse at higher frequencies and currents).- Ground is a local concept: the local current return path, the local voltage reference plane. Design the circuit and layout around the concept of a subcircuit reading its inputs against local ground. If local ground will be at a different (AC or DC) potential than where the signal comes from, add local filtering, or differential sensing. (This concept hints at how one might connect a coaxial cable to a circuit, while avoiding traditional problems with galvanic connections (ground loop) or EMI susceptibility.)Otherwise -- if you meant in terms of how fields are distributed over the board, you can apply the same principles at design time: for example, a trace running over a ground plane will produce so-and-so current distribution. As long as it's not crossing other traces, you can take that assumption and run with it, no problem. But if you have to jumble traces together and you want to know where the signals are going, or if you're dumping a lot of power into some weird shaped polygons or planes, or you simply need a more accurate answer than a hand-wave; you can only solve that the hard way: with a simulator.Tim

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 17, 2018, 11:20:32 am
content: Quote from: T3sl4co1l on January 17, 2018, 12:58:13 amthis is exactly my case. However I find your ground concept in the previous answer very useful. Tnx.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 22, 2018, 04:27:43 pm
content: I would like to add to this that not just the software is expensive, but the hardware to run it on is too... These simulation problems grow more-than-linear, so going from a simple lumped-element SPICE to a FEM simulation or parasitic extraction (PEX) into spice takes a heck of a lot of time. I've use similar thing in IC design, and that simple 10-transisotr amplifier that took 5 minutes to simulate before suddenly takes 12 hours....CST has some stuff. Keysight also makes ADS which can do similar. HFSS can also do things but I've never used it for anything but hardcore EM-wave simulations for stuff like filters and waveguide components.But these are tools that are easily in the 6-figure pricetag. And they are not easy to use - Colleagues who have been using HFSS for years still have many, many help-tabs open at all times, and at least two copies of the manual.