• State TI Thinks Resolved
  • Locked Locked
  • Replies 8 replies
  • Answers 2 answers
  • Subscribers 38 subscribers
  • Views 542 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28950: IC seemingly not regulating.

Boaz De-Medonsa
Prodigy 10 points

Part Number: UCC28950

Tool/software:

I'm trying to set the converter as 28V to 180V, and it doesn't seem to want to regulate to the correct voltage.

Pin 2 EA+ is set to 2.5V, and pin 3 EA- is settling at 1.9V. The duty cycle under this condition is ~75%.

  • 0
    Prodigy 10 points

    Some more info: COMP pin is steady at 4.25V, SS_EN at 5V, and CS never exceeds 2V.

  • 0 in reply to Boaz De-Medonsa
    TI__Guru* 86286 points

    Hello,

     

    Your inquiry has been received and is under review.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    Hello,

     

    6.5 to 1 is bit aggressive for an input design range.  A typical design is 3 to 1.    I have seen some designs go; as high as, 5 to 1.  This is because you can run into issues with duty limitation.  You should reconsider your input range and design for less than 5 to 1.

     

    Your COMP pin is saturated high and SS_EN is at 5 V in your design.  This indicates that the controller is trying to demand 100% duty cycle.  More than likely your transformer turns ratio cannot handle the input voltage range of your design.

     

    The following link will bring you to an application note that goes through the step by step design process of PSFB using the UCC28950.  There is a link inside the application note that will bring you to an excel design tool.  It uses the same calculations that are present in the application note.  These design tools will help you in the design process; as well as, checking your existing design.

    https://www.ti.com/lit/an/slua560d/slua560d.pdf

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

     

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Hi, and than you for the response.

    Sorry for the misunderstanding - the converter input is 28V and output is 180V. Not the range on the input.

    It would appear that the problem with the simulation tool is the transformer. If I replace the transformer with a set of 3 coupled inductors from the ANALOG directory, the regulator operates fine.

  • 0 in reply to Boaz De-Medonsa
    TI__Guru* 86286 points

    Hello,

     

    Thanks for letting me know and I happy that you have your design working now.

     

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Another question: why don't the delays between QA_G and QB_G match the calculations on the datasheet.

    I'm setting VADEL to 1.8V and RDELAB /RDELCD to 19.4k. According to the application note calculations this should result in a deadtime between QA_G and QB_G of 40ns, but I'm getting 371ns

    As another datapoint, setting RDELAB/RDELCD to 36.1k should yield a deadtime of 70ns, but I'm getting 685ns.

  • 0 in reply to Boaz De-Medonsa
    Prodigy 10 points

    For reference, setting VADEL to 0.2V and RDELAB/RDELCD to 30.1k (per the application note example) the deadtime is 580ns vs calculated 346ns - much closer.

  • 0 in reply to Boaz De-Medonsa
    TI__Guru* 86286 points

    Hello,

     

    The UCC28950 data sheet was updated on 10/24/98 and the timing equations were updated based on statistical data.  The turn-on delay equations in the application note were based on 2011 information given from the systems engineer.

    I would use the delay equations in the data sheet for setting the delay timing.  I will notify the documentation that the application note needs to be update timing equations on the next revision.

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,