3.2 SIMPLIS Monte Carlo Analysis

Exercise #1: Assign Uniform Distribution to the Input Source V1

1. Select the input voltage source V1.
   Result: The symbol changes from red to blue indicating the symbol is selected.
2. Use the keyboard shortcut Shift+F7 to open the literal value editing dialog.
   Result: The literal value editing dialog opens:
   
3. Enter the uniform distribution: {310*Unif(0.0333)}
   Result: The configured dialog should appear as follows:
   
4. Click Ok.
   Result: The property value for the input voltage changes to the uniform distribution of 310V +/- 3.33%.
   
5. From the schematic menu, select Monte Carlo > Run Monte Carlo
   Result: The SIMPLIS Monte Carlo Analysis executes and the waveform viewer opens with the curves from the original 4 steps with a constant Input voltage, and the additional 4 steps from the second Monte Carlo Analysis with input voltage variation.
   

Exercise #2: Verify L4 Values

1. Double click on L4.
   Result: The Define PWL Inductor: L4 dialog opens:
   
2. Note that each Flux Linkage value is multiplied by a constant value L4_Tol. Since the Flux Linkage values are the "y-axis" values, and the slope of each PWL segment represents the inductance of that segment, every PWL segment changes inductance by the same percentage.
3. Click Cancel on the Define PWL Inductor: L4 dialog.
4. Press F11 to open the command (F11) window. Scroll down to where the L4_Tol is defined:

   .var L4_Tol={Gauss(0.20)}
   { '*' }
   { '*' } L4's tolerance : {L4_Tol}
   { '*' }

   The first line assigns the return value of the Gauss(0.20) function to the variable L4_Tol, which is in turn, used to parametrize each y-axis point for L4. The other three lines were purposely placed in the F11 window to allow us to easily see the value returned by the Gauss(0.20) function. Each line is a comment, with the middle line also displaying the value of L4_Tol.
5. As you did in section 3.0.2 What Actual Device is Simulated in SIMPLIS?, you look in the deck file to ascertain exactly what values are used for L4_Tol, and the PWL inductor L4. From the menu bar, select Simulator > Edit Netlist (after preprocess).
   Result: the SIMPLIS Deck file opens in the netlist editor.
6. To search for the L4 tolerance comment line:
   1. Use the shortcut key Ctrl+F to open the search dialog.
   2. Type L4's
   3. Click on the Find Next button.
      Result: The deck file scrolls to line # 13 where the debug comment is located:
      
7. Finally, to search for the actual L4 device instantiation line:
   1. Use the shortcut key Ctrl+F to open the search dialog.
   2. Type !L$L4
   3. Click on the Find Next button three times.
      Result: The deck file scrolls to line # 49 where the L4 instantiation and its .MODEL line are located:
      

Piecewise Linear Inductors, such as L4 in this example, are implemented with a .MODEL statement, which in this case, starts on line #50. The PWL points are listed as X,Y pairs starting at X0 and Y0. Notice the X values have the same numerical values you saw in the dialog. The Y-values are all multiplied by the constant value L4_Tol. The L4_Tol is constant because the value is evaluated once per Monte Carlo run, and from that point onwards, the same constant value is used in the expressions for L4_Tol. The value of L4_Tol will almost certainly be different for the next Monte Carlo simulation step.

The value of the L4_Tol variable is 1.0696640600968, which represents a 6.97% increase in inductance values. Calculating the change in inductance is left as an independent study exercise.

Exercise #3: Enable Multi-Core

This exercise assumes your computer has more than one physical core and that you are using a SIMetrix/SIMPLIS Pro license.

1. Close the waveform viewer.
2. From the schematic menu, select Monte Carlo > Setup Monte Carlo...
   Result: The Define SIMPLIS Monte Carlo Analysis dialog opens with the Monte Carlo configuration information.
   
3. Change the Number of cores to 4, the maximum allowed by the Pro license.
   Note: Note: if your machine has either a single core or two cores, the dialog will limit your Number of cores entry. If you have a single core, you can skip this exercise.
4. Change the Number of steps to 32.
   Result: The configured dialog will appear as follows:
   
5. Click Run.
   Result: The Monte Carlo simulation runs 32 steps on the maximum number of physical cores.
   

Exercise #4: Plotting Histograms

Plotting histograms is different from plotting time domain curves because the histogram function is expecting a single measured value per Monte Carlo step. Compare this to plotting the converter output voltage after a transient simulation - there may be 10,000 points in the output voltage vector, each one representing the voltage at a particular simulation time. If you were to average the output voltage over the simulation time span, you would have just one measurement per Monte Carlo step. This is exactly what a histogram is - the frequency of a particular scalar measurement over a statistical sample.

Therefore, every histogram will require a function to operate on the voltage or current, and this function must return a single scalar value for each Monte Carlo step. For the above mentioned example, the function used to return the average value of a voltage or current from a time-domain simulation is Mean1(). For this example you will plot the frequency of the converter frequency using the Frequency() function.

1. From the schematic menu, select Monte Carlo > Plot histogram....
   Result: The Define Performance Analysis dialog opens:
   
2. In the Expression field, type Frequency( and leave the cursor positioned immediately after the opening parentheses.
   Result: The configured dialog should appear as follows. Note the cursor is placed immediately after the opening parentheses. The position of the cursor is important for the next step!
   
3. On the schematic, move the mouse over to the output of the POP Trigger schematic device.
4. Click the left mouse button once.
   Result: The POP Trigger output net, :31, is copied to the Define Performance Analysis dialog:
   
5. Type a closing parenthesis ).
   Result: The final configured dialog should appear as follows: The final expression is Frequency(:31).
   
6. The default number of histogram bins is defined by the equation: , which for a 32 run simulation will result in only 2 bins. For this exercise, you will change the number of histogram bins on the Define Performance Analysis dialog to 8. To change the number of histogram bins, follow these steps:
   1. Uncheck the Use default checkbox.
   2. Enter 8 in the Number of histogram bins field.
   3. Click Ok.
      Result: The histogram for the frequency of the converter is output to the waveform viewer:
      

Some Histogram Tips and Tricks

• The Define Performance Analysis dialog remembers previously plotted expressions. If you execute the schematic menu Monte Carlo > Plot histogram..., you can access the previous expressions with the pull down menu as depicted below. This will save you time if you wish to re-plot a histogram.
• A list of functions which can be used to define the histogram expression is available by clicking on the Functions... button.
• More information on using this dialog to generate plots is available by clicking on the Help button.

Exercise #5a: Find the Run Number Which Produced the Lowest Frequency

On the waveform viewer window:

1. Select the transient waveform tab if the histogram is the current tab.
2. Press the keyboard shortcut C to turn on the cursors.
   Result: A hint box may open, depending on your preferences.
   
3. Click on the Close button to dismiss the hint box.
   Result: The cursors are displayed on the graph.
4. There are two cursors, labeled REF and A. Move your mouse over the A cursor until the cursor changes shape to a 4-way arrow as shown below.
5. Press and hold the left mouse button while moving the mouse to the right-hand most Inductor Current curve. This is the curve with the lowest frequency.
6. Release the mouse button.
   Result: The cursor snaps to the nearest curve.
   
7. If you moved the REF cursor in the previous step, repeat for the process for the A cursor.
   Note: The exact cursor location on the curve is not important. What is important is the A cursor is on the right-hand most curve which is the step with the lowest frequency.
8. From the graph menu, select Curves > Show Curve Information.
   Result: the curve information, including the Monte Carlo run number is output to the SIMetrix/SIMPLIS command shell:

   Curve name: Inductor Current (simplis_pop_primary_1_1)
   Source group: simplis_pop_primary_1_1
   Curve id : 17 
   Run number : 22 

In this exercise, you have found the run number which produced the lowest converter switching frequency. In the next exercise, you will find the seed value and the simulated parameter values from the Monte Carlo Log file.

Exercise #5b: Find the Parameter Values Used for a Particular Run Number

You now know that run number 22 produced the lowest switching frequency. To find the seed value which corresponds to this run number, you open the Monte Carlo Log file. To open the Monte Carlo Log file, follow these steps:

1. From the schematic menu select Monte Carlo > View Log File.
   Result: The Monte Carlo Log file opens.
2. Scroll to run #22:

Note the seed value 1988583953 which was used for this run. In this converter, the switching frequency is highly dependent on the inductance value. Not unexpectedly, the evaluated parameter value for the inductance is higher than the default value, which requires longer charging and discharging times, and hence a lower switching frequency results.

In the next exercise, you will run a single step simulation using the same parameter values which generated the lowest frequency simulation run. You do this by running a Monte Carlo simulation using the seed value found in this exercise.

Exercise #5c: Run a Single Step Using a Known Seed Number

1. From the schematic menu, select Monte Carlo > Setup Monte Carlo....
   Result: The Define SIMPLIS Monte Carlo Analysis dialog opens with the Monte Carlo configuration information.
   
2. Make these three changes to the dialog values:
   1. In the Monte Carlo Seed field, paste the seed value: 1988583953
   2. Change the Number of steps to 1
   3. Change the Number of cores to 1
      Result: The configured dialog should appear as follows:
      
3. Click Run to run the known seed value of 1988583953.
   Result: A single Monte Carlo Step is run, reproducing the Monte Carlo run with the lowest frequency:
4. Press the Home key on the keyboard to zoom the waveform viewer to see the full waveforms: