SIMPLIS Tutorial

3.3 Set up an AC Analysis

You are now ready to set up the AC analysis. Although you could run an AC analysis on the schematic as it is, that analysis would have no additional information because the schematic has no modulator.

For this exercise, you can use the last schematic you saved or open 3_SIMPLIS_tutorial_buck_converter.sxsch.

If the grid is not showing, go to the menu bar in the schematic editor, and select View ▶ Toggle Grid.

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3.3.1  Prepare the Schematic

To prepare the previous schematic for the additions, follow these steps in the Schematic Editor:

1. To select components to move to the right, place the cross hair at the top of the window between the V2 and R2 components and draw a box around all of the components to the right as shown below:
   
   Result: The selected components change from red to blue.
2. Place the cross hair on any blue line, and drag the selected components 20 grid points to the right.
3. To delete each of the four wires highlighted in blue below, place the cross hair on the wire, click the left mouse button, and then Ctrl+X or press Delete.
   
4. Click the V2 symbol and drag it over two grid points and down so it is even with V1 as shown below:
   

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3.3.2  Add the New Components

To add the new components, follow these steps:

1. From the Commonly Used Parts category, double click Power Supply, move the cross hair to the right of the V2 wave generator, and click the left mouse button to place it.
   
2. Repeat step 1, but place the cross hair above and to the left of the D1 symbol.
   
3. In the Commonly Used Parts category, double click AC source (for AC analysis), and place the cross hair above the V3 symbol and click to place the new symbol.
   
4. From the Parts Selector, click the + in front of Analog Functions, and then double click Comparator with Ground; place the cross hair just to the right of V5 so that the lower connection lines up with the R2 symbol.
   
5. At this point, click the line just above the U1 symbol text and drag the mouse to move it up 4 grid points.
6. With the cross hair, draw a box around all of the symbols on the left side (V1, V2, V3, V5, and U1), and drag them all to the left 4 grid points.
   
7. To place the SIMPLIS custom gate driver, follow these steps:
   1. In the Parts Selector, click the + in front of MOSFET Drivers.
   2. Double click Totem-Pole Output Driver.
   3. Place the cross hair between the comparator U1 and symbol  R2, and click to place the new symbol.
      
8. To add the Bode Plot Probe from the Commonly Used Parts category, double click Probe- Bode Plot - Gain/Phase - w/ Measurement, and move the cross hair to the top of the schematic between IL and L1; and then click to place it on the grid.
   
   

   Result: A dialog appears for you to add measurements to this Bode Plot probe.
   

9. Uncheck Gain Margin and Phase Margin, leaving only Gain Crossover Frequency checked, and then click Ok.

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3.3.3  Saving your Schematic

To save your schematic,  follow these steps:

1. Select File ▶ Save As.
2. Navigate to your working directory where you are saving your schematics.
3. Name the file 4_my_buck_converter.sxsch.

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3.3.4  Add Wiring to the Schematic

Use the following diagram to add the wiring to your schematic:

From the menu bar, select File ▶ Save.

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3.3.5  Change Parameter Values

With the additional components, you have repurposed the waveform generator V2 from the pulse source to the modulator ramp voltage source. In the procedure below, you will set the modulator ramp to 1V peak-peak so that the DC source (V3) has a DC value (0.11V) equal to the duty cycle (11%) of the converter.

To change the parameter values for this analysis, follow these steps:

1. Double click the V2 symbol.
   Result: The Edit Waveform dialog box appears.
   
2. On the right side of the dialog in the Wave Shape section, select the Sawtooth radio button.
3. Set the Pulse value to 1V.
4. Click Ok.
5. Double click the V3 symbol and change its DC Voltage to 110mV; and then click Ok.

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3.3.6  Run the AC Analysis simulation

To run the AC analysis simulation, follow these steps:

1. From the menu bar in the Schematic editor, select File ▶ Choose Analysis....
2. On the right side of the dialog box, click to check AC.
   

   Note: The POP check box is also selected and grayed out so that both POP and AC analyses are performed.

   
3. Click Run or click Ok and then press F9.
   Result: The graph data output from this run is shown below. As connected, the Bode plot probe generates the control-to-output response of the converter.
   
4. From the menu bar, select File ▶ Save.

A schematic saved at this state can be downloaded here: 4_SIMPLIS_tutorial_buck_converter.sxsch.

In the next section, you customize the graph output so that  this graph is easier to read.

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