Understanding model parameters and applications limitations (Zetex)

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SPICE was originally developed as a simulation tool for Integrated Circuit design (SPICE being an acronym for Simulation Program with Integrated Circuit Emphasis) by the University of California in the 1970s. It was quickly improved into the SPICE2 version from which all commercial “Spice” programs are derived. It has since been enhanced (in terms of faster more robust algorithms and graphics handling capabilities for circuit input, and ease of analysing simulation output) and marketed by a number of companies for use within the PC environment - commercial versions being PSPICE, HSPICE, IsSPICE, SPICEAGE, Microcap etc.

Though originally intended for IC design, the availability of low cost computing, and the push towards robust design has introduced many other circuit and system designers to the advantages offered by analog circuit simulation. This has led to the requirement for device models for the active components under consideration, and so now many semiconductor companies provide appropriately targeted SPICE models as part of the technical support function.

These models can be extremely useful when used as a design tool, but care must be exercised. Any simulation software text will warn against sole reliance on the software’s predictions. Models are (by definition) a compromise, and are essentially based on a device’s common features. An appreciation of the model derivation, the model parameters, and it’s inherent limitations should be sought, and would assist in the interpretation of simulation results and their application to the real world.

Zetex have created SPICE models for a range of semiconductor components. Many of these models are for the higher performance Bipolar and MOSFET transistors, but models for RF transistors, variable capacitance diodes, switching diodes and standard small signal parts are also available. These models are available through any Zetex sales office or agent.

Appendix A includes a printout of the introductory text file included in version 2 of the Zetex SPICE models disc. This file provides some background to how the model files are organised and a brief overview of the model parameters for each type of device model.

This application note is a guide to the understanding and use of Zetex SPICE models. It includes sections on how some of these models are derived - the measurements/ optimisation necessary how to customise models for those cases where a model is not available and the limitations to be aware of.

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