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UCC25660: ISNS Pin Behavior in UCC256601 LLC Converter Design

Conor
Guru 12115 points
Part Number: UCC25660

Tool/software:

Hi,

My customer is currently designing an LLC converter using the UCC256601, and I would like to consult with you regarding an issue related to the behavior of the ISNS pin.

Issue Description:

  • The ISNS pin voltage exceeds the OCP threshold, triggering OCP.
  • The startup waveform is shown below. The ISNS terminal rating has been exceeded, causing the device to shut down due to OCP.

The differentiator circuit for ISNS is currently configured as follows:

Questions:

  1. The datasheet specifies CISNS = 150pF. Are there any potential issues with the placement or capacitance of the capacitors in the circuit above?

  2. We observed fluctuations in the ISNS signal during turn-off. Would adjusting RISNS (e.g., increasing from 180Ω to 200Ω or 220Ω) be an effective way to suppress this fluctuation? Alternatively, should we consider adding a ferrite bead to the ISNS line for noise filtering?

I would appreciate your insights and recommendations on this issue.

Thank you for your time and support.

Best regards,
Conor

  • 0
    TI__Genius 16155 points

    Hi Conor,

    Looks like controller is operating is operating in the burst mode. Do you see controller is restarting every 1sec? Its not clear from the waveform.

    Please confirm.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Guru 12115 points

    Hi Manikanta,

    Thanks for the reply. Before I request additional waveform measurements for the customer, I would like to understand the background of your comments in more detail.

    Q1. Fault State Transition and Restart Behavior Upon OCP
    According to the datasheet, when OCP is detected for 7 consecutive switching cycles, the device moves to the fault state. I understand that the fault state means the controller stops normal switching operation and enters a protection mode. In this state, does the controller remain in the fault state for a certain fault idle time before attempting to restart? In other words, does your comment about "restarting every second" refer to this fault idle time? Additionally, to confirm whether the controller is indeed restarting every second, should we check the VCC voltage? Or would it be more appropriate to monitor the gate drive switching waveform?

    Q2. Impact of Burst Mode on ISNS Pin Behavior
    As you mentioned, given that this is a no-load condition, it is highly likely that the controller has entered "burst mode". Since burst mode involves alternating ON and OFF switching periods, is there a possibility that the ISNS pin experiences significant intermittent fluctuations (spikes or noise)?  I would also like to confirm whether the current ISNS peripheral circuit (RC values, etc.) is correctly configured.
    Specifically, I would appreciate your comments on the following:
    ・Are the recommended CISNS and RISNS values appropriate?
    ・Is the placement of the filtering capacitor(110pF) correct?

    Additional Note
    While it would be best to analyze this after checking the actual waveform, I will have a discussion with the customer tomorrow, so even a preliminary hypothesis or insight would be very helpful.

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Genius 16155 points

    Hi Conor,

    I have responded to the customer issue in different thread. Looks like, the \wrong connections are done in the schematic. Please see my comments:

    (3) UCC25660: ISNS has an indefinite value when turned on - Power management forum - Power management - TI E2E support forums

    Regards

    Manikanta P