There’s an “R” in Varistor
This presentation is about how to model a varistor surge performance in SPICE (Simulation Program with Integrated Circuit Emphasis). For those whose interests are just concerned with varistors, rather than modelling, there is a general review of the current varistor standards and the new IEC 61051-1 ED3 (2018), Varistors for use in electronic equipment – Part 1: Generic specification.
A varistor VI characteristic should be regarded as three segments, one for d.c., one for a.c. and one for surge conditions, although under surge conditions the shape of the characteristic varies with di/dt. SPICE has a simplistic varistor model and the convenience and shortcomings of this model are discussed. Manufacturers often supply “locked” SPICE models of their varistors, which stops any examination of component tolerance. Creating a varistor SPICE model can be done from a list of V and I characteristic values that can be interpolated or from an equation linking V and I. This presentation concentrates on the equation approach, and from a V and I data list finding the best correlation is with the varistor resistance and current. Three equation types were examined and, using the best fit equation, a varistor SPICE bidirectional sub-circuit is developed.
- Varistor standards
- Standard varistor SPICE models
- Curve fitting quality to varistor VI characteristics
- Standard equation
- Chinese equation
- Modified standard equation
- Description and performance of varistor SPICE equation model