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UCC28C40: ucc28c40 issue

Jingguo Wang
Intellectual 1730 points
Part Number: UCC28C40
Other Parts Discussed in Thread: UCC28C41,

Hi team, there is an urgent issue from our customer anker,  Could you give some suggestions?

the SCH is as the follow picture

...

Key issue : 1.12V input and 34 V output, the system fails to start up due to the Vcs voltage exceeding 1V and triggering the overcurrent protection when powered under no-load conditions. However, the output voltage remains normal. and with an output load of up to 0.1A, the output voltage will can not be sustained. 

could you help to analyze the reason of startup failure or overcurrent during the startup? What are the possible factors, and how can they be avoided?

the another question is that is there  a calculation formula corresponding to the figure 7.1 from datasheet? it seems that it is not the same as UCC28C41, Customer want to know the parameters design for a 105kHz fs.

  • 0
    TI__Guru 61785 points

    Hi,

    No schematics can be found. Please check and send.

    I am not understanding how 1.12V can make UCC28C40 to start.

    Figure 7.1 has no change - please provide what you see with what RT and CT in use so we can understand why you say " it seems...".

  • 0 in reply to Hong Huang
    TI__Intellectual 1730 points

    sorry for type error,  It is 12V input,  I will send the SCH by e-mail, So is there  a calculation formula corresponding to the figure 7.1 from datasheet?

  • 0 in reply to Jingguo Wang
    TI__Guru 61785 points

    Hi,

    No calculation formula for figure 7.1 - the design needs to pick up the values for RT and CT for desired frequency. The variation from part to part is within 10% based on the design and the test we made to validate the devices. 

  • 0 in reply to Hong Huang
    TI__Intellectual 1730 points

    The SCH has been sent to you by email, and could you give some suggestions on the key issue? 

  • 0 in reply to Jingguo Wang
    TI__Guru 61785 points

    Hi,

    There are several things to check:

    1. Check the transformer polarities - your description is at no load, seems working but CS reaches 1V while at a bit load, Vout cannot stay, that is a typical behavior that the transformer polarities have some wrong, so your intended flyback now is forward. But as no output inductor, then the current can increase fast but at no load, the duty is small, then the feedback loop still can make regulation, but with even small load, the duty cycle increase cannot be maintained due to the over current detected.

    2. Is it possible the transformer Lp too small? If Lp is too small, then the current increase can be really fast so your converter cannot get correct operation as well.

    But I think the first needs to check and confirm. You need to take the waveforms to decide if the transformer polarities correct or not as it is possible some windings wound in wrong direction by mistake.

  • 0 in reply to Hong Huang
    TI__Intellectual 1730 points

    thanks a lot for your reply, and I will let customer check the Lp, and the customer reflects that the transformer polarities is correct. and it will trigger the overcurrent protection when startup before add load. 

  • 0 in reply to Jingguo Wang
    TI__Guru 61785 points

    Hi,

    You need to show the waveforms to support your statement, Vds, AUX, secondary side these three waveforms.

    As you mentioned CS over 1V you would need to find why CS can reach that value so fast, L dI/dt = V, is L too small? 

    These are fundamental in electrical circuit you need to find where your circuit to get wrong so causing current increase so quickly and so high.

    Also I see you have a slope compensation is it due to the slope compensation too strong? Can you remove that slope compensation to see if the issue can be resolved?