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Original question:

UCC28070: Optimization for size and current harmonics requirements

UCC28070: UCC28070 input harmonic exceeds the standard

Gabriel Wang1
Genius 4795 points
Part Number: UCC28070


Dear Expert

We use UCC28070 for staggered PFC, but the positive degree of the input current waveform is not good. There is a platform in the zero-point attachment, which leads to the ITHD exceeding the standard. The measured current waveform and parameters of UCC28070 peripheral components are as follows, please check!

  • 0
    TI__Genius 11350 points

    Hi Gabriel,

    Please provide the peak power level for this design.

    There's some information missing from the schematic you provided. Please share the input stage and the rest of the schematic related to the controller.

    What is the value of R151? The  VINAC resistor divider should be the same as the VSENSE divider.  If not, making it so will improve the distortion.

    Where is C92 located?  It should be right at the VINAC pin.

    Regards,

    Ray

  • 0 in reply to Ray DiCecco
    Genius 4795 points

    Dear Ray

    Our charger is of 6Kw, and the PFC voltage increases to 630VDC for some reasons.

    The value of R151 is 19K1, and the position of C92 is shown in the figure below, which is close to VINAC

  • 0 in reply to Gabriel Wang1
    TI__Genius 11350 points

    Hi Gabriel,

    Please share the input stage schematic. The crossover distortion is likely due to too large of a Vin capacitor (the capacitor right after the bridge).

    Regards,

    Ray

  • 0 in reply to Ray DiCecco
    Genius 4795 points

    Hi Ray

    As shown below, there is a CBB capacitor of 0.47uF and 6 electrolytic capacitors of 820uF behind the bridge pile, please check!

  • 0 in reply to Gabriel Wang1
    TI__Genius 11350 points

    Hi Gabriel,

    Cvin = 0.47uF is about right for a 6kW design.

    However for the output capacitance, we usually recommend 1-2uF/W. This design has only 3 x 410uF = 1230uF/6000W = 0.205uF/W; this is way too low and consequently, the output ripple is likely very high.  My suggestion is to increase Cout to closer to 6000uF and see if the iTHD comes down. Assuming this improves iTHD, they can then adjust Cout to the optimal value.

    Regards,

    Ray