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transfer function problem (AC sweep analysis)

0 votes

I'm simulating the tf of a thermal system, single pole RC filter with tau=2000. But the small signal model seem incorrect, the DC gain is infinite.

If I test the same with tau=10 everything is fine
asked Jul 9, 2019 by tinivella (96 points)

1 Answer

+1 vote
Best answer

PLECS is running into numerical truncation problems due to your choice of parameters:

  • large DC offset, i.e. your filter input (1330)
  • large time constant (2000)
  • very small "Rel. perturbation for Jacobian" in the "Steady-State Options" of the AC Sweep (1e-6)

As a consequence, when PLECS tries to linearize the model, it gets a plain 0 for the Jacobian matrix.

If you increase the "Rel. perturbation for Jacobian" to 1e-4 (which is the default), you will get the expected result. But if you then also increase the filter input or the time constant by a factor of 100, you will run into the same problem again. 

answered Jul 10, 2019 by Wolfgang Hammer (346 points)
selected Jul 10, 2019 by tinivella
ok thanks, anyhow this affect only the AC model not the time domain right?
Correct. This particular truncation problem occurs during the linearization of the model, which is specific to the AC analysis. A normal transient simulation is not affected by this.