2.1 Buck Converter Circuit and Inductor

This section of the tutorial introduces the Buck converter circuit that will be used, goes over the basics of the Multi-Level Lossy PWL Inductor (Version 8.0+) in SIMPLIS and introduces the new Level 2 inductor model.

In this topic:

Key Concepts

This topic addresses the following key concepts:

What You Will Learn

In this topic, you will learn the following:

  • Review the model levels of the Multi-Level Lossy PWL Inductor.
  • How to identify if SIMPLIS MDM is available on your system by looking for the Level 2 model in the Edit Multi-Level PWL Inductor dialog.
  • How the Level 2 model differs from a Level 1 model.

2.1.1  Open the Schematic

Open the schematic 2.1_SIMPLIS_MDM_tutorial_buck_converter.sxsch from the directory of example schematics you previously downloaded. You should see the following:

Feel free to examine the circuit by double-clicking on the various symbols. The input voltage is set by V1 to 12V. The output voltage of this buck converter is regulated by a 3-pole, 2-zero voltage mode compensator. The DC output load current is currently set to 5A, with a maximum design load of 10A. In this tutorial, you will modify the inductor design only; therefore, do not change any parameter values in the circuit, except as instructed in this tutorial. You will be changing the inductor model only.

The example schematic set up for a steady-state analysis using the SIMPLIS Periodic Operating Point (POP) Analysis. The POP analysis finds the steady-state operating point of the circuit and displays an integer number of steady-state switching periods in the Waveform Viewer. To review the types of the various SIMPLIS simulations, please refer to section 3.0 Simulating the Design of the SIMPLIS Tutorial.

There are no SIMPLIS MDM models currently implemented in this schematic.

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2.1.2  Examine the Inductor Model

Symbol L1 in the schematic is the Buck converter's output filter inductor which you will be designing in this tutorial. As mentioned earlier, it is a Multi-Level Lossy PWL Inductor (Version 8.0+). To review the concept of multi-level models as used in SIMPLIS, please refer to section 2.3 Edit Multi-Level Models of the SIMPLIS Tutorial.

To examine the inductor model, double-click the symbol L1.
Result: The Edit Multi-Level PWL Inductor dialog will open:

You will see that this inductor has a Level 1 model specified for it. Prior to the release of SIMPLIS MDM, this inductor part only had two model levels: 0 and 1. To review:

  • A Level 0 model consists of a piecewise-linear (PWL) inductance defined as a current vs. flux linkage curve, and a shunt resistance. No resistive loss is modeled.
  • A Level 1 model consists of a PWL inductance defined as a current vs. flux linkage curve, a shunt resistance, and an equivalent series resistance (ESR) to model losses.

You can examine and modify the PWL curve that defines the inductance by clicking the Define... button in the above dialog.

SIMPLIS MDM introduces the Level 2 model. If you do not have a valid license for SIMPLIS MDM, you will not be able to see the "Level 2" radio button in the above dialog. If you are able to change to the Level 2 inductor model, then you are using a version of SIMetrix/SIMPLIS which supports MDM and you have a valid MDM license. If you are reading this tutorial and would like to evaluate MDM, please send an evaluation request to: support@simplis.com.

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2.1.3  Level 2 Inductor Model

Click on the Level 2 radio button in the Edit Multi-Level PWL Inductor dialog. You should see the following:

The model status box is red and the message "ERROR: No Model Has Been Created For This Inductor" is shown. This is because this inductor does not currently have a SIMPLIS MDM model. Until you create a model with SIMPLIS MDM, you cannot close the dialog while the level is set to 2.

The Level 2 model, in terms of what is used to simulate the schematic, is identical to the Level 1 model in structure: it contains a PWL inductance, a shunt resistance, and an ESR. However, unlike with the Level 1, these parameters cannot be entered or edited by the user directly. They are extracted from the physical model of the inductor which the user builds using MDM.

Note the Write MDM generated model parameters to all lower model levels? checkbox. When you create a level 2 model with this box is checked, the PWL inductance and shunt resistance will automatically be written into this symbol's Level 0 and Level 1 models, and its ESR will be automatically written into this symbol's Level 1 model. Any user-defined parameters that exist at the Level 0 and 1 models would thus be overwritten. Checking this option makes the different model levels consistent with one another, and ensures that changing model levels only adds or subtracts parts of the model, but does not change the parts which are common to two or more model levels. For example, if you then switch from a Level 2 to a Level 0 model, you will no longer be modeling resistive loss when you simulate the schematic, but the inductance will remain the same.

Click Cancel to close the dialog, discarding any modifications you may have made.

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2.1.4  Run the Simulation

Run the simulation (press F9, click the appropriate button on the toolbar, or from the menu bar select Simulator > Run Schematic). The Waveform Viewer should appear, and you should see, from the top, three steady-state switching cycles of the output voltage, inductor current, and switching node waveforms:

Since you are using a PWL inductor in this schematic, a single inductance value is not displayed anywhere. To see the PWL inductance curve,

  1. Right-click the symbol L1
  2. In the resulting menu, select Plot PWL points for R, L, C.
    Result: A new tab, PWL Inductor Points will appear in the Waveform Viewer:

You can see that the characteristic is linear from -7A to 7A. This means that the inductance over the operating range simulated is constant. In other words, the inductor has not entered, or indeed significantly approached, saturation. The inductance is the slope of this linear portion of the curve. You can calculate the inductance in the following manner:

  1. Click anywhere on the Waveform Viewer. The menu bar contents will change to show the Measure menu.
  2. Select Measure > Maximum.
  3. Select Measure > Minimum.
  4. In the lower right corner of the Waveform Viewer the measurement results will appear, showing the maximum flux linkage at the maximum current and the minimum flux linkage at the minimum current.
  5. Subtract the minimum flux linkage from the maximum linkage, and divide it by the difference between the maximum and the minimum current. You will get an inductance of approximately 670 nH.

Therefore, for this circuit, you will use SIMPLIS MDM to design an inductor with an inductance of approximately 600 to 700 nH. The goal is to replace the existing Level 1 model with a Level 2 model with approximately the same characteristics in the electrical (circuit schematic) simulation.

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