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Original question:

UCD3138: [PSFB]YN DPWM duty , auto mode switching threshold calculating

UCD3138: Delay when switching from mode 1(Phase-shift modulation) to mode 2( frequency modulation)

Marc Ferrer96
Prodigy 140 points
Part Number: UCD3138

Hi everyone,

I realized there is a delay in my controller when switching from mode 1(Phase-shift modulation) to mode 2( frequency modulation) in my LLC based DC/DC converter. If there is a transient being in phase-shift, the controller works properly in all range except from a zone between phase-shift and frecuency modulation). 

Is it possible there is an internal delay in the UCD when transitioning from one switching mode to anoher? 

If not, I'll assume that my phase-shift modulation is saturating before than expected.

Thanks,

Marc.F

  • 0
    TI__Expert 4070 points

    Hello Marc,

    An engineer is looking into this and will respond.

  • 0 in reply to Amiel Hussain
    TI__Mastermind 18370 points
    The mode switching happens in a single switching cycle from the time the filter output crosses the threshold level.
    How long is the delay that you are observing?
    Do you set the low and high threshold levels to the same value?
    Is your PID bandwidth fast enough?

    Regards,
  • 0 in reply to Yitzhak Bolurian
    Prodigy 140 points

    Dear Yitzhak,

    Thresholds are at the same level, so I expected to have no hysteresis, but  I think this delay is due to a  jump on Yn value which seems to be working like a hysteresis. The jump is at different Yn values depending on if Yn is decreasing or increasing.

    I am aware phase shift modulation is slower than freq modulation. Still, don't find the reason why there is this gap. 

    KCOMP ( freq. modulation) and Resonant duty ( phase shift) are different, which I think will induce in a jump on output value, but I am not sure if this explains the jump I have on Yn value.  With which filter output value is the threshold compared with?  The current filter output? or always the same one, for example filter duty?

    the delay is around 5 ms. I will confirm this value as soon as I can. 

    Thanks,

    Marc.F

  • 0 in reply to Marc Ferrer96
    TI__Mastermind 18370 points

    How do you know what the value of Yn is at the switching time?
    I would recommend that you use a spare DPWM output to serve as a DAC to monitor the value of Yn at all times by an oscilloscope.

    The threshold is compared to the filter duty that comes out of the filter that is connected to DPWM.
    No, not current filter output. That filter only functions at constant current mode. Always the voltage one and only the duty.
    Note that Yn gets multiplied by a big number (Period or KComp) to provide the filter duty output.

    Regards,

  • 0 in reply to Marc Ferrer96
    TI__Mastermind 18370 points
    Hi Marc,
    We have not heard back from you.
    Is the issue resolved?
    If not, please be so kind as to respond to the above suggestions.
    Regards,
  • 0 in reply to Yitzhak Bolurian
    Prodigy 140 points

    I know Yn as I have it monitored.

    So as I understand, thresholds are always compared to filter duty and never to filter period. In my case, I use Kcomp for filter period and resonant duty for filter duty output. If thresholds are always compared to filter duty, using different multipliers for filter duty and filter period shouldn't be the root cause of this hysteresis.

    Therefore, I think the issue  might be related to an overlap between phase shift working points and frequency modulation working points, which may be working as a hysteresis.

    Thanks, 

    Marc.F