2.1.2 Back Annotating a Schematic

Exercise #1: Schematic Back-Annotation

1. Open the schematic 2.2_PFC_Critical_Conduction_Mode.sxsch.
2. Press F9 to run the simulation.
   Result: The waveform viewer opens with the simulation data plotted over the time interval of zero to 40ms.
   
3. From the schematic menu, select Simulator > Initial Conditions > Back-annotate
   Result:

   • The information in the .INIT file is parsed and the initial condition information is written to the individual symbols.
   • A message is output to the command shell: Back annotation complete.

The next time the simulation is launched, the circuit will be in the same state as it was at time=40ms in this simulation.

Exercise #2: Run the Back-Annotated Circuit

You have already back-annotated the schematic, all you need to do is close the .INIT file and run the simulation.

1. Close the initial conditions file by clicking on the X toolbar icon, or with the menu File > Close.
2. Press F9 to run the simulation.
   Result: The simulation starts at the back-annotated initial conditions. The waveform viewer now has two sets of waveforms - the first set from the initial simulation, and the second from the back-annotated simulation.
   

In the above graph it is easy to see that in the second simulation, the soft start voltage, SS starts at the same voltage where the first simulation stopped. In the next exercise you verify the measured voltage on the soft start capacitor is the same.

Exercise #3: Measure the Soft-Start Voltage

This schematic is setup to use fixed probe measurements for the three probes SS, Vctrl, and FB which measure the first and last values of the simulation vector. In this section you will verify that the starting voltages for the second simulation are the same as the final voltages for the first simulation.

1. On the waveform viewer, note the green SS curve from the first simulation has two measurement values:
   • First Value=1.275V @ 0Secs
   • Last Value=1.4384852V @40mSecs
   These are the initial and final voltages for the SS waveform from the first simulation.
2. Next, look at the teal SS (simplis_tran2) curve curve from the second simulation. This curve has the following two measurement values:
   • First Value=1.4384852V @40mSecs
   • Last Value=1.6019602V @40mSecs

Although your measured value might be different from those above, the Last value from the initial simulation should match the First value from the last simulation.

Exercise #4: Initialize the Schematic by Including a .INIT File

SIMetrix/SIMPLIS has the ability to easily include text files in the simulation. The .INIT file is a sequence of SIMPLIS commands which tell the simulator to initialize each circuit device. Because the file is already in the correct format for SIMPLIS, all you need to do to initialize the circuit is to include the .INIT file with a .INCLUDE statement.

In this example you are going to include the auto-generated .INIT file. This means that each successive simulation will use the final values generated during the previous simulation. Each time you simulate, another two line cycles of soft-start will be displayed on the waveform viewer. To automatically include the init file,

1. You first need to disable the initial conditions on the schematic. From the schematic menu, select Simulator > Initial Conditions > Disable
   Result: The schematic values for the initial conditions disappear.
2. Select the schematic window and press F11 to open the command (F11) window.
3. Scroll down to the bottom of the F11 window and add the .INCLUDE statement as the last line:

   .INCLUDE ./SIMPLIS_Data/2.2_PFC_Critical_Conduction_Mode.deck.init

4. Run the simulation.
   Result: The simulation starts from the initial conditions generated when the second simulation in exercise #2 finished. The waveform viewer shows three sets of curves:
   

You can easily prove to yourself that each successive simulation continues where the last simulation left off. For example shown below is the results of the next two simulations. These were generated after closing the waveform viewer, so two sets of curves appear:

What Can Go Wrong?

The number of problems with loading initial conditions is quite small. By far the most common problem is when a circuit has duplicate or multiple initial condition statements for the same component. Each schematic symbol must be designed to handle initial conditions, and the method used by each symbol varies with the underlying device model. To avoid duplicate .INIT statements, you disabled the schematic back annotation when you included the .INIT file in the third exercise.

.INCLUDE ./SIMPLIS_Data/2.2_PFC_Critical_Conduction_Mode.deck.init 
.INCLUDE ./SIMPLIS_Data/2.2_PFC_Critical_Conduction_Mode.deck.init