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UCC28950: Converter Stops Regulating When Heavy Load is present on its Output at Turn ON

Amir Bigdeli
Prodigy 10 points
Part Number: UCC28950

Tool/software:

Converter regulates to a bit over 28V (set output voltage) at no load with no issues.

Converter regulates to 28V when load increases while it's already powered ON. So when I add a load to output while the converter is running, the regulation is good.

Converter does not regulate (shows 0.5V-1V output) when the load is connected to the output prior to power ON and it makes noise.

  • 0
    TI__Guru* 86266 points

    Hello,

    This may have something to do with your load.  What kind of load are you using?  

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Just a resistor. 8ohm to be exact. I have a status LED on the output that turns ON when 28V is present. It turns ON at no load but it start flickering when I connect the resistor and and turn ON the converter. I suspect there's an issue with the startup sequence/transient that causes the controller to get stuck in the hiccup/burst mode

  • 0 in reply to Mike O'
    Prodigy 10 points

    one other thing to note is the exact setup works when there a e-load connected but it does not with a resistor only

  • 0 in reply to Amir Bigdeli
    TI__Guru* 86266 points

    Hello,

     

    Thanks for that information.  I thought you might be trying to startup into a current load and that is why you are having an issue.

    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 to an excel design tool inside the application note that uses the same calculations as the design tool.  This application note was used to design the 600 W UCC28950EVM-442 reference design.  You can use these tools to check your schematic and design.  I would recommend that you use this tool to check your design.

    https://www.ti.com/lit/pdf/slua560

     

    I do agree that your issue could be with the startup sequence.  You might want to probe VDD during power up just to make sure the UCC28950 is properly biased during powerup.

    If you do have VDD then I would move on to probing SS, voltage amplifier output, CS and the output voltage at the same time to see why the device is not starting up.  When probing the voltage amplifier output you cannot probe it directly.  You will need to put 1 k ohm resistor in series with the scope probe.  This is covered in the data sheet.

    Before troubleshooting I would recommend reading up on the cycle by cycle current limit and the hiccup mode over current protection.  This information will help you in the trouble shooting process.

     

    Regards,