# Measure controlled system with AC sweep **URL:** \ **Category:** PLECS\ **Tags:** plecs, control, ac-sweep\ **Created:** [February 26, 2025, 10:17am UTC](https://forum.plexim.com/t/measure-controlled-system-with-ac-sweep/1833 "2025-02-26T10:17:54Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![raphael.baumeler](https://avatars.discourse-cdn.com/v4/letter/r/48db29/32.png) [@raphael.baumeler](https://forum.plexim.com/u/raphael.baumeler)\ **Post date:** [February 26, 2025, 10:17am UTC](https://forum.plexim.com/t/measure-controlled-system-with-ac-sweep/1833/1 "2025-02-26T10:17:54Z")
Dear plexim Team [PFC\_einphasig\_bidgless\_bipolar\_APFC\_Regelstrecke\_bestimmen.plecs](https://forum.plexim.com/uploads/short-url/79qN3rb7YHDiZQXtVEbcgkfp97S.plecs) (26.1 KB) In the attached model i want to measure the controled system with the help of an AC sweep. In comparsion to the buck converter example this model has a AC voltage source. When i try to measure the current response (iL) i get a unrealistic solution (amplitudes and phase response with many peaks). Is it possible to measure the controled system H(s) = iL/D like this? What do i have to adapt in my model that it will work? Thank you for editing this post. Best regards, Raphael Baumeler ---
**Author:** ![Bryan\_Lieblick](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/bryan_lieblick/32/10_2.png) [@Bryan\_Lieblick](https://forum.plexim.com/u/Bryan_Lieblick)\ **Post date:** [February 28, 2025, 3:34am UTC](https://forum.plexim.com/t/measure-controlled-system-with-ac-sweep/1833/2 "2025-02-28T03:34:47Z")
Hi Raphael, Small signal analysis conceptually requires operating around a linearized operating point. That is not the case in your model. You are operating at a constant duty cycle while the AC grid voltage changes. If we think of the ~iL/~D of a buck converter, it depends on D’ or 1-Vin/Vout so there isn’t really one transfer function for this system as you’ve implemented it. A common approach for analyzing PFCs would be choose a DC operating point on the AC curve for the controller design. Another approach would be to implement a controller and reformulate the perturbation such that the perturbation represents a “more linearized” system. Christopher Basso has good reference material on this topic. Regards, Bryan ---
**Author:** ![raphael.baumeler](https://avatars.discourse-cdn.com/v4/letter/r/48db29/32.png) [@raphael.baumeler](https://forum.plexim.com/u/raphael.baumeler)\ **Post date:** [February 28, 2025, 6:47am UTC](https://forum.plexim.com/t/measure-controlled-system-with-ac-sweep/1833/3 "2025-02-28T06:47:57Z")
Hi Bryan, Thanks for your reply. I understand that my model will not work. Can you show me how you would analyze this model with a DC operating point? Where can i find the reference material from Christopher Basso? Do you have an example? Thanks for your time and help. Best regards, Raphael ---
**Author:** ![raphael.baumeler](https://avatars.discourse-cdn.com/v4/letter/r/48db29/32.png) [@raphael.baumeler](https://forum.plexim.com/u/raphael.baumeler)\ **Post date:** [March 5, 2025, 5:39pm UTC](https://forum.plexim.com/t/measure-controlled-system-with-ac-sweep/1833/4 "2025-03-05T17:39:25Z")
Hi Bryan, Thanks for your reply. I’ve tried it with a DC Voltage Source and it works now. Thanks for your help. Best regards, Raphael ---
**Author:** ![Bryan\_Lieblick](https://dub1.discourse-cdn.com/flex006/user_avatar/forum.plexim.com/bryan_lieblick/32/10_2.png) [@Bryan\_Lieblick](https://forum.plexim.com/u/Bryan_Lieblick)\ **Post date:** [March 6, 2025, 9:42pm UTC](https://forum.plexim.com/t/measure-controlled-system-with-ac-sweep/1833/5 "2025-03-06T21:42:23Z")
Glad to hear you have things working. Regarding Basso’s work, I believe he had a recent tutorial at APEC on PFC simulation. He also has several books, but I’m not certain which one is most relevant to your immediate question.