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CCS/TMS320F28335: Unwanted EPWM Interference.

David Bamgboje
Intellectual 770 points
Part Number: TMS320F28335


Tool/software: Code Composer Studio

Hello,

I need advise on something. I am operating a closed loop boost converter but I am getting some unwanted interference at my epwm terminals. I really don't know where the interference is coming from. Recently, I connected the GND terminal of pin 20, I tied it to my main ground briefly and I noticed some improvements but it is still not what I hoped for. This felt weird, so pls what is the proper way to handle the epwm GND terminal, should it even be touched? By the way, I specified other unused gpio pins as output.

The first 3 pulses in the figure below illustrates the nature of the interference I am experiencing. It's ripple effect can be seen in my final output current. By design, I am expecting only one pulse to be on at one time and ZERO from the other pulses during that time but some interference is triggering the other two pulses.

What do you think? #eZdspf28335

Thanks.

David.

  • 0
    TI__Mastermind 42195 points

    David,

    Is the 4th(green) signal a tripzone? If so, is it forcing the PWMs into a high impedance state? This could cause the signal on the respective PWMs to float up above 0V.

    Regards,
    Cody 

  • 0 in reply to Cody Watkins
    Intellectual 770 points

    Thanks for your response.

    The 4th signal is not a trip zone signal. I am just measuring the output current at the LED load end.

    What do you think?

    David.

  • 0 in reply to David Bamgboje
    TI__Mastermind 42195 points

    David,

    are you using the Trip Zones for anything? They have the ability to switch the PWM into a High-Z state which has been know to cause PWM's voltage to float up rather than being driven low.

    Please check if anything is forcing the PWM into high impedance mode, if not, we can look for other possible causes.

    Regards,
    Cody

  • 0 in reply to Cody Watkins
    Intellectual 770 points

    Thanks Cody.

    I am not using trip zone at all. And to guard against high impedance mode, I am now using a 10k pull down between the epwm output and gate driver input. I am seeing some improvements but it still not what I expected. Below is the image as of now. If you look closely, you will notice that the glitch is still there.

    Is there a better approach?

    Thanks again.

    David.

  • 0 in reply to David Bamgboje
    TI__Mastermind 42195 points

    David,

    the pull-down resistor will definitely help.

    As an experiment could you try setting TZCTL.bit.TZA and TZCTL.bit.TZA =11. Please verify that you have correctly written the registers by reading the value back as these registers are EALLOW protected. This will ensure that the trip zones are not the problem.

    I will look for other possible causes.

    Thanks,
    Cody

  • 0 in reply to Cody Watkins
    Intellectual 770 points

    Thanks for the advice Cody. I did as you said to be 100% sure but no change.

    I hope to hear from you soon.

    kind regards.

    David.

  • 0 in reply to David Bamgboje
    TI__Mastermind 42195 points

    David,

    I don't think anything else could be putting the PWMs in a high impeadence state(unless they were reconfigured as an input, but i doubt that) so I would guess that this noise is some coupling... Maybe backing feeding from the output?

    Could you remove or at least turn off the FET drivers to try an isolate the C2000 from the other circuits? I am hoping that we can isolate the issue to one part of your design.

    Regards,
    Cody 

  • 0 in reply to Cody Watkins
    Intellectual 770 points

    Thank you for your response.

    I tried removing the FET drivers but I really don't know what to expect because I am using high side FET drivers; thus, without the drivers, there is nothing to see (power mosfets won't turn on properly). The only noticeable things are the ordinary PWM inputs of the microcontroller which were just as expected normal square waves. 

    Which test do you suggest I carry out?

    Hope to hear from you soon.

    Kind regards.

    David.

  • 0 in reply to David Bamgboje
    TI__Mastermind 42195 points

    David,
    Well, thats good. We can at least say that the C2000 is driving the buffers correctly.

    This sounds like a system level issue, Which will require debugging on your end.

    I am not an expert in this area, and will likely be unable to help, but you should probably start checking the basics:

    Do you have any filtering? Are your power rails clean and supplying enough power? Are all of your FETs and FET drivers functioning correctly? Are you using good isolation techniques?

    Do you see this issue on one board or many? Is this a new design or an existing one?(you may have answered these questions before)

    Regards,
    Cody