transfer function problem (AC sweep analysis)

Question

Hi

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

Answer 1

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. 

Comments

ok thanks, anyhow this affect only the AC model not the time domain right?

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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.