The Power of Piecewise Linear Modeling


Previously Held on Oct 20th, 2016

Piecewise linear (PWL) resistors, capacitors, and inductors allow designers to create very complex models using only a single two terminal device. This one-hour webinar will start with the one of the most versatile SIMPLIS primitives: the PWL Resistor. The webinar uses real-world application circuits to define PWL Resistors as well as explain their limitations. Circuit examples include:

  • Constant Power Loads
  • ESD Diodes with Snapback
  • Soft-Start Blocks
  • Current Sources with Saturation
  • Multipliers and Dividers
  • MOSFET Driver Output Characteristics

The balance of the webinar will be spent on PWL capacitors and inductors. The primary applications for these PWL devices are junction capacitors, MLCC capacitors and saturating magnetic inductors and transformers.

The critical concept of how to determine PWL breakpoints to optimize both speed and accuracy will also be covered.

Link to Webinar Recording

The webinar recording can be viewed at this link: The Power of Piecewise Linear Modeling (55:35)


  1. Two types of PWL Resistors:
    • Voltage-Controlled (VPWL)
    • Current-Controlled (IPWL)
  2. How to define PWL resistors on the I-V plane.
  3. Limitations of PWL Resistors
  4. Capacitors - defined on Charge-Voltage plane.
  5. Inductors - defined on the Flux Linkage vs. Current plane.

Reference Materials

Schematics and presentation slides for the webinar can be downloaded here:

Questions and Answers

Q1: What's the difference between symmetric and arbitrary PWL definition?

A1: Symmetric devices are required to pass through 0,0 and are defined in the 1st quadrant and then the program mirrors the 1st quadrant definition into the 3rd quadrant.  Arbitrary definitions are not required to pass through 0,0. Symmetric definitions are common for PWL inductors used as filters in power supplies. Most PWL resistors are arbitrary as the PWL resistor can be used for just about any application.

Q2: How do I assign a tolerance for a PWL capacitor or inductor so that all segments change by the same percentage during a Monte Carlo simulation.

A2: This can be accomplished by assigning a tolerance variable in the schematic's F11 window. In the Advanced SIMPLIS Training course 3.2 SIMPLIS Monte Carlo Analysis topic, a PWL inductor is defined with a tolerance.

Q3: How do I define a current mirror in SIMPLIS which models output and rail-rail saturation?

A3: Although this wasn't covered in the webinar, a simple example can be found here (right click and save as...). This example uses two SIMPLIS Transistor Switches to create a current mirror. The saturation characteristics can be modeled by adjusting the transistor switch parameters.