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Original question:

UCC28070: UCC28070 out-of-sync bug

UCC28070: UCC28070 Abnormal PFC waveform

Gabriel Wang1
Genius 4425 points
Part Number: UCC28070

Dear Expert

With full load, voltage from high voltage to low voltage test input undervoltage protection, voltage at about 180Vac, power factor drops to about 0.86, input current distortion. As shown below: (1# waveform is DS waveform, 2# is input current waveform, and 3# is GS drive waveform)

  • 0
    TI__Mastermind 24365 points

    Hi Gabriel,

    Thank you for the query on UCC28070.

    What I think could be probably happening from the waveforms is that when the voltage 180Vac (254Vpk), there is a discontinuity at this point due to Kvff (feedforward gain update). So when you decrease from a higher level to lower voltage, the gain increases and this probably increases the input current seen as sudden spikes. This is probably leading to overcharging output capacitor leading to OVP situation where the converter stops switching as seen in the blank periods of the current waveform.

    One thing which you can try is increase the value of output capacitor so that the rise is voltage increase is minimum during the Kvff update and see if this improves the situation. There could also be a possibility that there is voltaage drop at source at rated load causing toggling of multiplier gain which can lead to disturbances. So you might consider relocating the VINAC sense before the input capacitor where the AC signal is clean (before diode rectifier and input filter) and see if there is any improvement. Please share a schematic/UCC28070 calculator so that we can verify it as well.

    Regards,

    Harish