• State
  • +1 person also asked this people also asked this
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 146 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.

Original question:

UCC28730: WEBENCH®︎ Design: Flyback DC DC 120V-150V to 175.00V @ 0.17A with UCC24650 as wakeup

UCC28730: Startup problems with the flyback.

Claas Hartmann
Prodigy 20 points

Part Number: UCC28730

Tool/software:

Hi,

I redesigned my Webench design and created a corresponding PCB.
For the pcb layout, I followed the recommendation from the datasheet.

Currently, I have the problem that the circuit does not start up.
VDD is charged via the internal current source from the HV pin until it reaches about 21V.
Then the IC tries to drive the MOSFET. That looks fine so far.
But afterwards, it goes into a fault condition and starts again from the beginning.

Attached is an oscilloscope screenshot:

  • green shows the voltage at the auxiliary winding,
  • purple shows the voltage at the shunt resistor (0.36Ω),
  • turquoise shows the voltage at the FET’s DRV pin.

I’ve also attached the schematic. The only modification I made was removing the connection from HV-in to VDD, since that proposal doesn’t make sense without a series resistor.
It’s also unnecessary. I think there may be an error in the Webench design suggestion here as well.

I’d really appreciate any feedback and help.

Best regards

  • 0
    TI__Guru* 86286 points

    Hello,

     

    I believe that you are shutting down due to fault and more than likely it is a false fault.

     

    I reviewed the schematic and would repopulated the RCD clamp resistor and capacitor that you have Xed out.  This helps dampens out ringing on the switch node that can trigger a false OVP fault.

     

    I also would add a capacitor to the CS pin for filtering to remove noise from triggering false OCP fault.

     

    The following link will bring you to an application note that discuss how the UCC28730 primary fault sensing works.  The issues with it and how to resolve them with snubbing, filtering and the use of an RCD clamp.

    https://www.ti.com/lit/pdf/sluaac5

     

    Regards,