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TPS53515: Schematic review and unstable current

Hirokazu Takahashi
Expert 8420 points
Part Number: TPS53515


Dear Support team,

Could you please review below?

TPS53515.pptx

Best Regards,

Hirokazu Takahashi

  • 0
    TI__Mastermind 24325 points

    The current is measured incorrectly with current probe between two output capacitors.   

    Move all output capacitors to inductor side of loop (where ferrite bead was) to measure output current.

    The output  current should be DC. 

    To measure inductor current move all output capacitor to load side of loop (where ferrite bead was).

    The inductor current will be triangular when configured for FCCM operation. 

    Measured output voltage with ferrite bead installed to check voltage spike. 

  • 0 in reply to David_Daniels
    TI__Expert 8420 points

    Hello David-san

    Thanks for your support.

    We measured the output current at the load side after FB.

    But we couldn't find any improvement.

    Could you let us know about any idea to improve it?

    Best Regards,

    Hirokazu Takahashi

  • 0 in reply to Hirokazu Takahashi
    TI__Mastermind 24325 points

    There is no reason for the load current to oscillate.  

    The output voltage is steady and the sw pulses are stable.

    The scope has the Ch1 set on AC coupling.   

    Change DC coupling to measurement the dc load current 

    and measure current from vout tp. 

    can you send a scope plot with dc coupling 

  • 0 in reply to David_Daniels
    TI__Expert 8420 points

    Hello David-san

    Thanks for your support.
    It is difficult to measure the output current at point after all capacitors with Ferrite bead.
    We measured the waveform of output current and voltage at same point with DC coupling.
    We found some improvement with larger inductance 6.8uH.
    Do you think that larger inductance will be effective for improving this issue?
    If you have any other suggestion, please let us know.

    TPS53515 (2021 04 29).pptx

    Best Regards,
    Hirokazu Takahashi

  • 0 in reply to Hirokazu Takahashi
    TI__Mastermind 24325 points

    Takahashi-san,

    This measurement is better with dc coupling.    The output voltage is stable, so the output current should be stable. 

    Another measurement to consider, measure the inductor current by adding a wire in series  to measure the inductor current with probe. 

    The inductor current should be a triangle with a dc offset.   The peak current should be the same over the switching cycles.

    See figure 2 in https://www.ti.com/seclit/ml/slup206/slup206.pdf#page=4

    The average of the inductor current is the output current. 

  • 0 in reply to David_Daniels
    TI__Expert 8420 points

    Hello David-san

    Thanks for support.

    Did you mean that both waveform at 4.7uH and 6.8uH are proper?

    Or we need more take another actions?

    Best Regards,

    Hirokazu Takahashi

  • 0 in reply to Hirokazu Takahashi
    TI__Mastermind 24325 points

    I  have never measured current in that location to assess if proper.   But I can see the output voltage is stable

    and the dc current level is ~2.5A and the current ripple is periodic and stable.

    Measuring the inductor current (dc coupled) will show the stability of the inductor current and the dc/dc switching.   

    Removing ferrite bead and using an external load on the capacitor near the inductor is another method to measure output current. 

    The ripple current in .ppt  shows the output current + current charging the capacitor near load.