circuit design

when they’re working with the company’s AC/DC Module, which is an add-on to the base Comsol Multiphysics package. That module reads a SPICE netlist and simulates the components it contains using ordinary differential equations (ODEs) to solve the S-parameter network. However, users can then create a 3D component that accounts for other aspects such as thermal effects or structural changes and insert it into the 2D circuit, using a FE boundary to couple

model its gain and radiation pattern. When it comes to signal integrity, SPICE models can only approximate effects such as coupling and crosstalk. Further, the majority of SPICE

component models are just narrowband RLC (resistor/inductor/capacitor) equivalent circuits, whereas in the signal-integrity world in the time domain you really need broadband analysis from DC to 20 GHz and above. A SPICE model becomes quite

ENGINEERS MIX AND MATCH CIRCUIT AND FE TECHNOLOGY, BECAUSE THE CIRCUIT EQUIVALENT MODELS ARE NOT ACCURATE

ENOUGH TO DESCRIBE THE BEHAVIOUR OF THE PHYSICAL COMPONENT THEY ARE MEANT TO REPRESENT

directly to the electrical circuit. This might be useful for determining the resistance of a special geometry and inserting it into the SPICE simulation, or finding the capacitance of a condenser microphone where that parameter changes depending on the frequency and the applied bias voltage. Another way to use this capability is, for

example, to describe a mechanical system in terms of electrical components and couple it to an acoustic FE domain. In such an analogy, a circuit resistance corresponds to mechanical losses and damping, inductance corresponds to the mass and capacitance relates to springs. By using lumped electronic circuit elements, it’s possible to reduce model size, get accurate results and see initial system response quickly. When doing so with the condenser microphone just mentioned, the small-signal electric model for the system is solved as a lumped (electric equivalent) model coupled to FE elements. The model is a true multiphysics problem that involves several physics interfaces: thermoacoustics, electrostatics, moving mesh plus an electric circuit model.

Sending FE models into circuit simulators While it’s always possible to use circuit simulation, these circuits are in devices or structures that influence circuit behavior. Thus, people are starting to do co-simulation and add FE models comments Markus Koop, product manager for electronics at Ansys. Engineers mix and match circuit and FE technology, because the circuit equivalent models are not accurate enough to describe the behaviour of the physical component they are meant to represent. SPICE also falls short in other areas. Take, for instance, an antenna, whereas SPICE models provide lots of information, but you can’t effectively

36 SCIENTIFIC COMPUTING WORLD

inaccurate when there are hundreds of traces and vias and planes over and through a design – if it can account for these things at all. A far more accurate model is an S-parameter model, but where do these come from? From an electromagnetic modelling program, for instance. Thus, Ansys’ approach differs from most

others; rather than import SPICE information into a 3D physics simulation package such as Ansys Multiphysics, it uses a 3D modelling package to create an S-parameter model that is imported into SPICE-compliant circuit-simulation environments. In this case the company’s Simplorer low-frequency design software and Designer RF/microwave design software. It does so using what is known as a reduced order model, which is basically a tabular listing of I/O parameters, a broadband spice model or scattering matrix (S-parameters) information. This data can be imported into Simplorer, or users can also create a dynamic link so that Simplorer grabs whatever data it needs at any given time. Further, these links are bidirectional so that as you change the stimulus in Simplorer, this then affects the 3D physics modeling. At this time, there are dynamic

links between Simplorer and other Ansys products including Maxwell (electromagnetic field simulation software), HFSS (3D full-wave electromagnetic field software), SIWave (for analysis of circuit boards and IC packages) and Q3D (a 2D and 3D parasitic extraction software tool), while Designer has links to HFSS, SI Wave and Q3D. Note that Ansys announced some time ago that ‘these links will be expanded to include Ansys Multiphysics in a future release’, but there is as yet nothing to announce. Koop also mentions another use for this

technology, and that is to use electronic analogies to model complex physical systems in a reduced manner. Here, the RLCs relate to physical parameters, and you can, for instance, model the human cardiovascular system using a reduced order model in Simplorer.

Hardware-description language When Mentor Graphics – which has a long history in IC and circuit simulation – acquired Flomerics and its CFD software about three years ago, it would seem a logical move to integrate the capabilities of these two technologies. However, that hasn’t been the case as yet. Even so, Mentor does have something to offer in this area. Says Mike Donnelly, principal engineer, system modelling and analysis, ‘We have a very effective “FEA + circuit/system simulation” capability. We’ve been collaborating with an FEA software provider, Infolytica, for several years.’ Their FEA tools target low- frequency electromagnetic components such as motors, linear actuators and transformers. Part of their analysis capability is to export a behavioural model of the machine using VHDL-AMS (a hardware-description language with analogue and mixed-signal extensions) based on FEA results. That model can then be directly imported into Mentor’s SystemVision, an integrated and scalable environment for circuit, system and mechatronics modelling. In that environment it can be connected to models of the drive electronics, mechanical loads, sensors, control software, etc. Donnelly adds that ‘we’ve had a number of customers use this capability and they are finding it to be strategically important. They can design and size a custom machine (i.e. choose the geometry, materials, winding patterns, etc.) and then virtually test it in the realistic system context in which it will operate.

Reference 1. Whitepaper, ‘The Role of Simulation in Innovative Electronic Design’, Ansys, 2011.

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