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Troubleshooting voltage regulation with the UCC28711EVM-160

Colin Bussiere
Prodigy 130 points
Other Parts Discussed in Thread: UCC28711EVM-160, UCC28710

Hello,

I'm hoping you can please help me with a voltage regulation stability issue I am having.  This issue is occurring on both my board and the TI evaluation board that I have, UCC28711EVM-160 .  The output voltage can decay due to the controller suddenly reducing switching on-time (current) and frequency of operation.

While using the EVM, I have a fixed resistor load of 100 Ohms attached to the output and have a 115VAC input.  The output voltage will intermittently decay as follows;

The auxiliary winding voltage shows that just prior to the issue (-5ms on the timeline) the controller ramped up the on-time as follows;

During the issue the on-time and operating frequency are near the minimum for the controller, here is an example of the resultant auxiliary winding voltage;

At the end of the issue the controller aggressively ramps up the on-time and frequency to restore the output voltage as follows;

I'm hoping to understand these fluctuations in output voltage using the EVM and then carry this knowledge back to my board to resolve the same issue.  Please let me know if there are circuit alterations that I could make to improve the voltage regulation stability of the EVM.  Currently I am collecting measurements using an unaltered UCC28711EVM-160 with lot number 3382943WBG.  This issue seems to have similarities to the posting entitled "UCC28710 capacity on VS - voltage sense".  Any help would be greatly appreciated, thank you.

  • 0
    TI__Genius 9130 points
    Colin,

    It looks like maybe noise on the CS and/or VS pins that is affecting the loop stability.

    If you vary the load up or down, does the regulation improve? Or is it always misbehaving? Is the misbehaviour confined to a certain load range?


    I have ordered one of the EVM's to check this issue, but it will take a few days before it arrives in my lab.


    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    Prodigy 130 points

    Hello Bernard,

    Thank you for the reply.  Yes I do find that if I reduce or increase the load the probability of triggering on the issue is reduced.  What I have observed so far is that regulation stability becomes a challenge if operating in (or near) the first FM section of the control law profile when in constant current mode.

    The probability of occurrence also seems to be impacted by the input voltage, at times increasing the input near 240VAC will induce the regulation dips.  As an experiment on my board I have increased the current sense resistor to force the controller to move into the AM region and second FM region of the control law profile.  This helped with stability but limited the operating range of the supply.  I haven't tried any changes on the EVM yet but suspect this same change could help if my thoughts are correct.

  • 0 in reply to Colin Bussiere
    TI__Guru 65805 points

    Hello Collin,

    I managed to get a hold of an EVM and verified the disturbance that you observed at the 100 ohm, 5% load.  CH1 = Vout, CH2 =Vaux. The output droops about 70 mV when this occurs.

    I zoomed on the auxiliary winding and it is operating at 33 kHz before this occurrence.  This appears to agree with previous post that the converter is jumping in and out of the AM band.  This when the voltage amplifier is sitting at 2.2V.  This is a known behavior with the PSR controller family.

    The solution has been to increase the output capacitance so you pass output ripple voltage specifications. The other possibility is to change your maximum  switching frequency so this event occurs at lighter load.  This requires redesigning your transformer magnetizing inductance.

    Regards,

    Mike