At first glance, the SIMPLIS transient analysis appear to be just like a SPICE transient analysis. Both simulators have start and stop time controls and a way to control the number of data points.
Behind the scenes, the SIMPLIS transient simulation actually bears little resemblance to the SPICE transient simulation. In this topic you will learn how and why the SIMPLIS transient simulation is different from a SPICE transient simulation.
In this topic:
This topic addresses the following key concepts:
In this topic, you will learn the following:
To get started you will open a schematic and examine the transient analysis settings.
The SIMPLIS Transient analysis settings are divided into two groups:
There is a distinct difference between the Analysis parameters and the Plot data output parameters. The Analysis parameters, in particular the Stop time, define the time interval which SIMPLIS will collect switching instance data. Switching instance data is the state of each topology as the circuit switches from one PWL topology to the next. The switching instance data is internal to SIMPLIS and is not output to the waveform viewer as vectors.
The Plot data output has only one parameter Number of plot points, which sets the number of additional plot points added to the output waveforms. By increasing the Number of plot points parameter, you increase the fidelity of the waveforms displayed on the waveform viewer, but this doesn't change the accuracy of the simulation.
You will now run the transient simulation from 0 to 100μs. Before launching the simulation, you will clear the messages from the SIMPLIS Status window. To run the simulation,
TIME-DOMAIN TRANSIENT ANALYSIS New topology #7 New topology #8 New topology #9 New topology #10 New topology #11 New topology #12 New topology #13 New topology #14 New topology #15 New topology #16 01 New topology #17 New topology #18 02 New topology #19 New topology #20 New topology #21 New topology #22 New topology #23 New topology #24 New topology #25 New topology #26 03 New topology #27 04 New topology #28 New topology #29 05 New topology #30 New topology #31 06 New topology #32 New topology #33 07 08 09 10 11 12 13 14 15 16 17 18 New topology #34 New topology #35 New topology #36 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 Elapsed time : 0 hr 0 min 1 sec CPU time : 0 hr 0 min 0.17 sec Simulation time: 1.000000000000e-004 sec Writing pertinent data files ... Leaving SIMPLIS.
There are primarily two types of messages output to the SIMPLIS Status window during this transient simulation:
These messages are output in sequential order, for example, between 6 and 7% complete, the simulator found New topology #33. New topologies are described next.
During a SIMPLIS simulation, the PWL circuit will transition between multiple unique PWL topologies. For example, in a typical buck converter, there exists an state where the high-side switch is on, and energy is both delivered to the output and stored in the inductor. This is followed by a freewheeling phase where the low-side switch is on, and energy stored in the inductor is delivered to the load. This simple example represents two PWL topologies. As the SIMPLIS simulation progresses, SIMPLIS identifies these topologies and stores the topology information for later use. The first time a unique topology is encountered, SIMPLIS declares this to be a new topology. In plain terms, SIMPLIS learns the circuit as the simulation progresses.
The first time SIMPLIS encounters a particular topology, the topology is declared a new topology, a number of computationally intensive tasks are performed, and the information is stored for future use. This topology is now a known topology and information about the topology is resident in memory for future use. As the simulation progresses through topological changes, SIMPLIS adds new topologies to the memory store, and retrieves known topologies from memory. This machine learning process is one characteristic of SIMPLIS which is different than SPICE.
Since it takes significant computational power to store and retrieve the topology information, the more topologies a circuit transitions through, the slower the SIMPLIS simulation will be.
If you examine both the waveforms and the status window, you will see that the last New topology was found at around 18% completion. This corresponds to time=18μs on the transient analysis graph. After this time, the circuit is settling into a final, steady-state, and SIMPLIS has learned all topologies in this circuit.
At this point you know SIMPLIS has the ability to very closely save the state of the circuit for future use. By default SIMPLIS saves the state, or Snapshot, of the PWL circuit at time=0, at the Stop time, and at 10 roughly equally spaced time intervals between the start and stop times. These snapshots are available for use in a future simulation. In the previous exercises, you have told SIMPLIS to ignore any previous state information by checking the Force new analysis check box, this option tells SIMPLIS to not load snapshot information.
Consider the following real-world simulation situation:
You are developing a PFC controller and would like to closely examine the startup behavior of the circuit. Your first task is to examine the first two complete line cycles (40ms at 50Hz Line), then move onto the successive line cycles. Since PFC converters are switching at a high frequency and have a line frequency input which is much lower frequency, these converters are computationally intensive. To save time, it would be nice to be able to store the state after any number of line cycles as a snapshot, and pick up that snapshot in a future simulation. In the next exercise you will load a snapshot for a PFC converter.
An exercise will demonstrate loading the snapshot from a previous simulation. To get started,
The simulation will take approximately 15-20 seconds to complete. The waveform viewer will open to display the control signals:
Now you will tell SIMPLIS to load the state of the PFC circuit at 40ms, and continue to simulate for another 60ms to the stop time of 100ms.
The waveform viewer now shows two sets of curves, one each from each simulation run. The second simulation starts where the first simulation left off, at 40ms.
If you look carefully in the SIMPLIS Status window, you will see a message (you may have to scroll up to find the message near the top):
Reading the state/snapshot of the simulation at t = 4.00000000e-002 sec
This message indicates that at the start of the transient analysis, SIMPLIS has loaded the previous Snapshot information at time=40ms.
A number of items will prevent SIMPLIS from loading snapshot information:
To save additional snapshots in between the default Snapshot at time=0 and at the Stop time,
( not covered in class )
Go back through exercise number 3 with the Snapshot interval set to 1ms, and the maximum number of snapshots set to 50. Run the first simulation to 40ms, then re-run the simulation loading the Snapshot starting from 39ms.