Why are the simulation results different after updating the program?

You didn’t update Qspice for long time? There was an update if value in IGNORED > TSTOP months ago, may be that causing the different.

If you go to HELP > Simulator > Command Reference > Non-Linear Transient Analys(.tran), Qspice offers two .tran syntax, in which syntax depends if you have a 2nd numerical input.

  • .tran 1.2u means transient simulation to stop at 1.2us, but no maximum timestep specified
  • .tran 0 1.2u 0 1n is Berkeley Syntax, time to start recording waveform is 0s and stop at 1.2us, with maximum timestep 1ns

Qspice is adaptive step size algorithms. If you define a maximum timestep, each simulation step is force within MAXSTEP value. Therefore, if you set MAXSTEP to 1ns in this simulation, it run simulation more precisely. Downside is that it increases simulation time. If you find no MAXSTEP result is good enough, you don’t have to include MAXSTEP in your simulation.

Another method to specify MAXSTEP without using Berkeley Syntax is

.tran 1.2u
.options MAXSTEP=1n

Advantage of this syntax is that, you can comment .options to quickly switch between MAXSTEP and no MAXSTEP in your work. You can press “;” to comment a directive, or right click on text to select comment it.