• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 155 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.

UCC256404: Board startup issue with full load condition

Namita Pawar
Intellectual 680 points
Part Number: UCC256404

Tool/software:

Hi,

We are using UCC256404DDBR controller for our application. The converter is 12V and 4.166A (50W).

We have taken the reference of  UCC25640EVM schematic for the feedback loop compensation.

Currently the value of LR is 24uH, CR is 66nF and the VCR cap values are 180pF (Upper) and 10nF (lower). The RSNS resistor is 360Ohm and the series capacitor is 150pF. Soft start capacitor is set to 180nF.

With these values we can get the output 12V with no load condition. When we setup our electronic load directly at full load 4.16A, we are not getting output. The observation is the gate pulses are not generating properly, and the VCC pin of  UCC256404DDBR is showing voltage around 25V. At no load condition when we start the board we are getting 12V output and when we slowly increase the load we getting the expected result and VCC pin is showing desired supply voltage provided by bias winding that is set to 15V.

When we use RSNS value very low around 75 ohm then we could not able to provide a short circuit protection/over current but the board can startup with full load. On the other hand we tried to increase the VCR capacitor from 180pF value to a value around 680p or 1n, it is helping a little bit but as per the design calculator the recommended value for the upper VCR sense capacitor is around 350pF and I am not sure but increasing that value may create an unknown issues or not.

For time being I am not using the bias winding sensing feature that is BW pin, currently I used the upper resistor of BW divider as 66.5K and lower resistor is 5.76K ohm. Expected bias winding voltage is 15V as per turns ratio.

Can you please help with this startup issue occurring at full load test. Please suggest what value of VCR setting, FB compensation circuit or RSNS would help.

Thanks,

Namita

  • 0
    TI__Genius 16155 points

    Hi Namita,

    The VCR capacitors are needs to be tuned such that at full load, at min input voltage, the VCR pk-pk should be close to 5.5V.

    You will have to use the design calculator for initial values for ISNS and VCR and Compensation and finalize the values based on the evaluation results.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Intellectual 680 points

    Hello Manikantan,

    By tunning the VCR caps, I could be able to resolve this startup issue. But now another problem we are facing is about the burst mode.

    At 80% load the controller is entering into burst mode around, when the input AC 127V. 

    Currently the value of LR is 24uH, CR is 66nF and the VCR cap values are 280pF (Upper) and 10nF (lower). The RSNS resistor is 360Ohm and the series capacitor is 150pF. Soft start capacitor is set to 180nF. The Lm value is 192uH.

    Will you please help with the issue and let us know what component we need to tune to avoid this burst mode.

    Thanks,

    Namita

  • 0 in reply to Namita Pawar
    TI__Genius 16155 points

    Hi Namita,

    could you try removing upper LL pin resistor. That would make BMTL and BMTH as 0.2 which is lowest burst thresholds. 

    Regards

    Manikanta P