questioning wrong -360 degrees in AC sweep results

anhnha
anhnha Altair Community Member
edited November 2023 in Community Q&A

From the article titled "AC ANALYSIS OF PEAK CURRENT MODE CONTROLLED BUCK CONVERTER," accessible through the provided link here. In the first section, "1. CURRENT LOOP TRANSFER FUNCTION," an AC sweep is configured to measure the current loop transfer function. Upon examining the Bode plot during this phase, it is evident that the phase starts at -360 degrees.

I am curious to understand why PSIM calculates -360 degrees instead of -180 degrees as this is a negative feedback. Is there a specific reason for this?

Somehow it added 180 phase shift in addition to the loop gain -GH with the minus sign due to negative feedback.

  image

 

The amplitude and phase plots of current loop transfer function

https://powersimtech.com/resources/application-notes/ac-analysis-of-peak-current-mode-controlled-buck-converter/

Tagged:

Answers

  • anhnha
    anhnha Altair Community Member
    edited November 2023

    Any idea on why it causes this wrong behaviour? 
    If you put two probles right before and right after the ac proble loop and then calculate the loop gain and phase from that you would get a different result for phase. 

  • NikosDimitrakopoulos
    NikosDimitrakopoulos
    Altair Employee
    edited November 2023
    anhnha said:

    Any idea on why it causes this wrong behaviour? 
    If you put two probles right before and right after the ac proble loop and then calculate the loop gain and phase from that you would get a different result for phase. 

    Hi Anhnha,

    Thanks for detecting this!

    You are right, there is an issue with this example and the differential AC probe. The phase plot should start from -180deg. This is logged in order for us to examine it and fix it.

    Until then, you can try to add single AC probes before and after the perturbation. I tried this and it works. 

    Best regards,

    Nikos

Categories