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UCC2818: Oscillation in input current waveform

John11513
Expert 5720 points
Part Number: UCC2818

Hi Team,

My customer is designing with our UCC2818. They observed an issue with input current waveform input oscillation. We observed significant oscillation in input current waveform, the frequency is around 10kHz, related to input X capacitor, the higher the capacitance the lower the frequency. The oscillation only occurred when we use adjustable transformer as input source. The issue disappeared once AC source was used. Bellow is the input current waveform:

We suspected it was due to the resonance between the leakage inductance of the adjustable transformer and X capacitor. We did a test: inserting a serial resistor (10ohm) on X cap (1uF), then the oscillation disappeared. 

My question is:

1. Do you think this test can prove the root cause of the oscillation is resonance between X cap and transformer leakage?

2. If yes, how to resolve this issue? (serial resistance is not acceptable)

Thank you

John

  • 0
    TI__Mastermind 25570 points

    Hello John

    This seems to be a classic input filter oscillation - please see the attached document for some additional information.

    /cfs-file/__key/communityserver-discussions-components-files/196/4062.input-Filter-Oscillations.docx

    The problem you have is that the output impedance of the input filter - (including the source impedance of the adjustable transformer) is too high. You could reduce this impedance by adding some capacitance at the output of the transformer.

    It is also possible to damp the resonance by adding a parallel RC filter across the resonant capacitance (the X-Cap in your case).

    You should also consider whether this oscillation could occur when the PSU is connected to a normal AC line socket.

    Please let me know how you get on.

    Also - please let me know if the link above does not work -

    Regards

    Colin