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UCC25640EVM-020: Does not work properly with open loop control by using application note (sluual2)

Shinichi Inoue
Guru 21656 points
Part Number: UCC25640EVM-020

Dear Specialists,

My customer is evaluating UCC25640EVM-020 and has a question.

I would be grateful if you could advise.

---

Open loop control using UCC25640EVM‐020 does not work correctly.
Open loop is described in application note (sluual2)

https://www.ti.com/lit/pdf/sluaal2

I changed the circuit to the 100kHz circuit (described in 1.7 How to Operate the UCC25640x Controller in the Open Loop? on page 10), but it does not work properly.
The top VCR capacitance (C13) is removed, the lower VCR capacitance (C21) is installed with 10nF and the Rfb is implemented 78kΩ.

Connecting both the top VCR capacitance (C13) and lower VCR capacitance (C21) seems to work correctly.

Could you please advise what the problem is.

In the circuit of UCC25640EVM
C13 (Top VCR capacitance) is removed.
C21 (VCR_lower) is connected 10nF.
C24 is removed.
Then RFB(78kohm) is implemented.
U1 is removed.

Please see the attached file.

Question about UCC25640EVM.pdf

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Genius 16155 points

    Hi Shinichi,

    Could you share the waveforms of VFB, VCR, LO, ISNS signals.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Guru 21656 points

    Hi Manikanta,

    Thank you for your reply.

    I ask the customer to measure the waveform.

    He will get the data next week, because he is busy with other duties. 

    I will send the data as soon as I get it.

    Could you please wait a couple of days.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Genius 16155 points

    Sure. No problem.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Guru 21656 points

    Hi Manikanta,

    I could obtain the data.

    Could you please see attached files.

    Actually, while checking the waveform data, I found out that it works with just C21.
    It doesn't work with no load, but it seems to work if you pull a load of 13W or more.
    The waveforms in the attached document are for no-load operation.
    The waveform with both C18 and C13 connected is also attached.

    ICR measures the resonant capacitor current directly with a current probe.

    UCC25640EVM_test_result .xlsx

    In addition, there are fluctuations in frequency and duty when the device enters steady operation.
    This was also the case with C13 and C21. A stable switching state cannot be achieved.
    I took a video of this symptom, so could you please see it?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21656 points

    Hi Manikanta,

    The attached movie file is not supported.

    Could you please advise without movie file.

    Shinichi

  • +1 in reply to Shinichi Inoue
    TI__Genius 16155 points

    Hi Shinichi,

    The VFB voltage is loosely regulated at 5.6V. Due to that, current through the external resistor at the FB pin varies. So you would see the changes in the frequency. If you want to have a constant frequency, instead of using a resistor, you need to use a current mirror that would draw the constant current.

    For open loop operation, C13 needs to be depopulated. 

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Guru 21656 points

    Hi Manikanta,

    Thank you for your reply.

    I'll share your answer with the customer.

    When the customer has an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21656 points

    Hi Manikanta,

    The customer could understand your answer.

    They have an additional question, could you please advise.

    ---

    I understand the answer.

    The open loop control described in application note (sluual2)

    ・Does soft start work properly?
    ・Do protection operations such as short circuit protection work properly?

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • +1 in reply to Shinichi Inoue
    TI__Genius 16155 points

    Hi Shinichi,

    Both soft start and protections should work when you operate the controller in the open loop.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Guru 21656 points

    Hi Manikanta,

    Thank you for your reply.

    I understand soft start and protection should work at operating in the open loop.

    I'll share this with the customer.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi