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Original question:

UCC28750: Full load startup issues

UCC28750: ucc287506 startup at loaded condition

Adnan
Prodigy 45 points
Part Number: UCC28750

Tool/software:

Hi Himanshu,

I need some clarification on UCC287506(100kHz, OVP) controller. I am having an issue during its startup at loaded condition(tested at 10% of full load of 60W multi-output).

The observation is, the output voltage is not building up to desired level of 24V with loaded condition at the end of the soft start interval of 4.5ms. The feedback is clamped to around 5V and the frequency is around 200kHz which indicates OP condition.

After some 80ms the switching is stopped which I think is due to OPP protection, and it enters into hiccup mode and tries to turn ON again.

Thanks

  • 0
    TI__Guru* 85806 points

    Hello,

    Can you share a schematic and any waveforms that you might have?

    Regards,

  • 0 in reply to Mike O'
    Prodigy 45 points

    Yellow=gate drive, Blue=current sense, Green=FB to controller, Pink=output voltage

    As can been, voltage starts building up during soft start of 4.5ms and the feedback is clamped to 4.6V during this period. After that the controller takes in and it start normal switching and the output rises. After some time, there is again jump in feedback voltage to 4.6V and converter enters into OCP condition and output starts dropping, after some 80ms OCP fault triggers and switching is stopped.

  • 0 in reply to Adnan
    TI__Guru* 85806 points

    Hello,

    When I look at this waveform when the output voltage drops the VFB signal increases.  The CS signal also drops.  The controller should be asking for maximum duty cycle and maximum peak current.  There  most likely is something not setup correctly in your schematic.  

    There is a 60 W evaluation module for this device.  You might want to compare your schematic vs the schematic in your design.  Any differences that you find may be contributing to this issue.

    https://www.ti.com/lit/ug/sluucw5/sluucw5.pdf

    The other tool you have available to check your design is the UCC28750 excel design calculator that you can find at the following link.  This will help double check the components you have selected for your design.

    https://www.ti.com/tool/download/UCC28750-CALC

    Regards,