Revolutionizing SPICE model creation.
by Tim McCune, consultant, and Siddharth Mohan, senior design engineer at Qorvo
SPICE (Simulation Program with Integrated Circuit Emphasis) is a widely used software tool for simulating electrical circuits. Like all SPICE-based simulation software, QSPICE (which is free for both commercial and educational use) requires accurate models for each component in the schematic. These models consist of parameter sets derived from technical datasheets that describe the components’ electrical behaviors.
How are SPICE models used?
When designers conceptualize a circuit, they first create a schematic in a SPICE-based simulator to assess its performance. Every component used in the schematic must have an associated SPICE model, which enables the simulation to analyze the circuit’s functionality.
For example, consider an audio amplifier circuit designed by renowned amplifier engineer Nelson Pass. The schematic capture includes an N-channel JFET (J1) for receiving the input signal and two MOSFETs (M3 and M4) for output. Each component symbol in the schematic contains an embedded QSPICE model.
A circuit designer can begin with a generic JFET (e.g., “J1”) and later replace it with a specific part, such as the InterFET IF170B, to optimize the circuit’s performance. QSPICE provides a built-in library of parts but also allows users to import additional models for different components. The QSPICE selection guide simplifies this process.
Where do SPICE models come from?
For some components, QSPICE includes “native elements,” which let users input electrical parameters to create a basic SPICE model. However, for more complex components, manufacturers develop SPICE models through their application support teams or hire specialists to generate them. Traditionally, creating SPICE models has been a labor-intensive process, requiring extensive engineering time and expertise.
What does a SPICE model look like?
A SPICE model consists of parameter names and data specific to a component, formatted as a string beginning with “.model.” Here is an example for the AO4262E MOSFET:
.model AO4262E NMOS (Level=3 VTO=-1.2 KP=50m RD=0.5 RS=0.5)
This notation defines electrical characteristics that determine the behavior of the MOSFET in simulation.
The model creation process
The method for generating SPICE models depends on the type of component. The QSPICE Model Generator specifically creates models for discrete MOSFETs, JFETs, and diodes. This process involves the following:
● Testing component samples: Engineers analyze a representative group of components, measuring key parameters and plotting electrical characteristics.
● Extracting data from datasheets: A well-structured datasheet contains all the necessary specifications required to construct a useful SPICE model.
● Mathematical modeling: Traditionally, engineers apply complex equations to derive accurate parameters. Noted circuit design expert Bob Cordell dedicates more than 30 pages in his book, Designing Audio Circuits and Systems, to SPICE model creation.
● Time investment: Experienced engineers may spend more than 12 hours developing a single SPICE model, in addition to the time required for data collection and verification.
Transforming the process
While testing and data collection remain essential, the QSPICE model generator automates the mathematical computations required for SPICE modeling. This tool dramatically reduces the time required to create a model from more than 12 hours to just minutes.
Users simply input datasheet parameters into designated fields and replicate datasheet curves in the model generator. The software then performs the necessary calculations, outputting a fully functional QSPICE model.
By streamlining the traditionally complex and time-consuming SPICE model creation process, the QSPICE Model Generator empowers engineers to focus on designing and refining circuits rather than getting bogged down in mathematical modeling. This innovation enhances efficiency, accuracy, and accessibility in circuit simulation.
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