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Other Parts Discussed in Thread: SFRA

Hi everyone and thanks in advance for your help.

I am going to perform an SFRA analysys in a BLPFC board, and was wondering if somebody may explain me a little more about what does the SFRA library do, as the technical documents I have read does not go into this kind of details.

For instance, I have doubts on which kind of signal does the SFRA inject. Is this an step signal with the amplitude of the SFRA1.amplitude variable? is it a sinewave? Any further information about this woudl be appreciated.

Besides, as the input of this board is AC voltage, how does the SFRA block differentiate between the changes introduced at the output by the SFRA block itself, from the changes introducuced by the change in the AC voltage input? Does this mean that I cannot go very low in frequency analysis because both signals would be similar and unable to differentiate? Is it better to make this test with DC input, instead of AC voltage, to avoid the above situation?



  • Hi Maximiliano,

    SFRA injection and collection works on similar basic principles that commercially available frequency response analyzers work on. The injection amplitude is very small compared to the parameter under analysis. There are limitations to this with the SFRA but mainly at higher injection frequencies because of the processing that needs to be done inside the sampling loop.

    I hope this helps.
  • Thanks Hrishi,

    I was looking to other forums post, that basically ask very similar question (how does the inject function work) and ti claim it is under propietary conditions, but they are prety old.

    Maybe some of this nondisclosure topics can be commented a bit more now. My only concern is on how to be sure on what the inject function does, so this way I can understand better what to config on the amplitude of injection variable.

    If you can comment a little bit on which is the type of injection (sinusoidal, step signal, or somehting like that) it would be appreciated


  • Sure. The injection is sinusoidal. Please let me know if you have any other specific questions on the SFRA or the injection function.
  • Thanks Hrishi

    I now have another doubt.

    I am about to test this SFRA in a BLPFC project. First of all I want to test the H plant transfer function in open loop (just controlling duty cycle and measuring the corresponding current).

    As I need to power the board with AC voltage (50hz), how does the system differentiate between the variation on the output current produced by the AC voltage change, from the variation of the current produced by the change of duty cycle (through the inject function).

    Is it better if I test with a DC input voltage?

    Thanks again
  • If you use AC input your frequency response data at 120Hz (assuming 60Hz AC)  may not be correct because of 120Hz ripple and injection signal ripple . But this is ok since the loop BW is much higher than 120Hz. Ideally, You would use DC input for boost PFC freq response. But this is not necessary. 60Hz AC input is also ok.