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System period length in AC sweep?

+1 vote
Hello everyone,

As you have noticed I asked many questions related to AC sweep(PLECS BLOCKSET) and I start to get familiar with it.

my question here is related to the value of the System period length, in the help it described it as follow:1- The period length of the unperturbed system.

I found as well another explanation but not sure if it is the referring to it exactly :

2-System period
The system period is the least common multiple of the periods of all sources (signal or electrical) in the model. If the parameter setting does not reflect the true system period or an integer multiple thereof, the analysis will yield a meaningless result or fail to converge altogether. A system period of 0 is equivalent to defining the system as non-periodic. When set to auto, which is the default, PLECS will try to determine the system period automatically.

in my design of the DC-DC boost converter with peak current mode control, when I run the AC sweep (Verr to Vout ) the default value was 1e-5 and my bode plot was meaningless and  the value of Vout does not reach the steady state (I expect 16V as Vout and Vin is 5V) and stays around 3V which means never reached the steady state where the perturbation must be happen.

I have changed the value to 1e-2 and I can see the difference and the steady state of Vout=16V are reached and I see the oscillations of different frequencies.


my questions how does this value affect the frequency response because of it still not clear to me?

how do I obtain a fast bode plot as it takes very long time(I use a switching frequency of 3Mhz and my frequency range is from 100 to 1Mhz)?

best regards,
asked Mar 7 by EMdude (54 points)
edited Mar 8 by EMdude

1 Answer

+2 votes
With a simple peak current mode controlled buck converter, the system period field of the dialog should be set to auto. I'm confident PLECS will determine the system period (which should be the switching period in your case) correctly. You can check it in the log, where it will tell you the automatically calculated value.

I would suggest to use the Impulse Response Analysis instead of the AC Sweep. It gives almost instant results.

See the attached model. I modified the peak current mode buck demo. Varying the perturbation amplitude has no effect on the bode plot, which is a good sign.
answered Mar 7 by Falk Kyburz (107 points)
Hi Mr. Kyburz,

Thank you very much for your information, it helped to understand more. however, i am sorry I did not tell that I am using the PLECS Blockset and I did as you said in the dialog box, I set the system period length to auto but shows errors and does not recognize it.

you have suggested me to use the Impulse Response Analysis, I have tried it now, it is just showing another error related to the frequency of the system as you can see in the in the picture?i believe there is something I don#t understand related to periodic system.

I am going to try besides the standalone PLECS.


best regards,