# What is SIMPLIS?

SIMPLIS (SIMulation of Piecewise LInear Systems) is a circuit simulator specifically designed to handle the simulation challenges of switching power systems. Like SPICE, SIMPLIS works at the component level but typically can perform a transient analysis of a switching circuit 10 to 50 times faster. For switching power systems, the piecewise linear (PWL) modeling and simulation techniques employed by SIMPLIS result in qualitatively superior convergence behavior compared to SPICE.

SIMPLIS achieves its speed by modeling devices using a series of PWL straight-line segments rather than by using the SPICE technique of solving nonlinearities such as exponential expressions. By modeling devices in this way, SIMPLIS can characterize a complete, nonlinear system as a cyclical sequence of piecewise linear circuit topologies. This method generates an accurate representation of a typical switching power system where the semiconductor devices function as high frequency switches.

For a comparable accuracy, however, a PWL system can be solved more rapidly than the nonlinear system of equations that SPICE handles. The end result is an accurate and extremely fast simulation that can model complex topologies that would not be viable with SPICE-based approaches.

SIMPLIS has three analysis modes: Transient, Periodic Operating Point, and AC.

• The SIMPLIS transient analysis is similar to that of SPICE but typically runs 10-50 times faster.
• The Periodic Operating Point (POP) analysis is a unique analysis mode that finds the steady-state operating waveforms of a periodic switching system.
• The SIMPLIS AC analysis finds the frequency response of a switching system without requiring a derivation of average models. This approach means that SIMPLIS does not need to make assumptions implicit in the derivation of averaged models, such as a constant switching frequency and limitations on the relative magnitude of the ripple on the controlled output signal being. As a result SIMPLIS AC analysis is not limited to circuit topologies or control schemes where small-signal, averaged models have already been derived.

SIMetrix/SIMPLIS is the most widely used simulation tool for the design of closed-loop switching power supplies, primarily because SIMPLIS makes the analysis of large and complex systems practical for real-life design  based on its ability to do the following:

• Quickly find the steady-state Periodic Operating Point.
• Perform AC analyses on the full nonlinear time-domain switching circuit.
• Perform time-domain transient analyses 10 - 50 times faster than SPICE.
• Exhibit superior convergence behavior.

Because nonlinear device characteristics are modeled using a sequence of piecewise linear straight-line segments, SIMPLIS models for such devices differ from the corresponding SPICE models. Device manufacturers typically provide SPICE models of their products. Taking advantage of the fact that SIMetrix/SIMPLIS has both the SIMetrix SPICE and the SIMPLIS PWL simulation engines available, SIMetrix/SIMPLIS has the ability to convert certain types of two- and three-terminal SPICE models into a SIMPLIS model format. This conversion occurs when the device is placed on the schematic.

Devices currently supported are MOSFETs, IGBTs, JFETs, BJTs, Zener Diodes, and Diodes.  During the conversion process, SIMetrix/SIMPLIS runs a series of SIMetrix-SPICE simulations using the SPICE model to generate the appropriate characteristic curves for the device. Curve-fitting algorithms then calculate the corresponding PWL segments for the SIMPLIS model.

In order to obtain the maximum benefit from using SIMPLIS, it is important to fully understand three critical concepts that are unique to SIMPLIS.

In addition to the core SIMPLIS features, the SIMetrix/SIMPLIS Design Verification Module (DVM) can be added to any SIMPLIS version, allowing you to do the following:

• Run a design schematic through a series of simulations
• Record the measured results.
• Automatically generate a test report detailing the performance of the circuit.

Although DVM comes bundled with built-in test suites, its strength lies in its support for user-designed testplans. Using a relatively simple syntax, you can configure a schematic to use with the DVM. In less than five minutes you can ...

• Create a test definition
• Set circuit parameters
• Assign component values
• Specify analysis directives
• Select waveforms to include in the output report

DVM also includes the SIMPLIS Multi-Tone AC Analysis, which is useful for characterizing control loops for converters that do not have a true periodic operating point.