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UCC28070

Steven MacDonald
Intellectual 450 points
Other Parts Discussed in Thread: UCC28070

Has anyone using the UCC28070 at a power level of 1kW-2kW been able to achieve a THD of 5% at Vin = 115Vac? It is required and if it is not possible to achieve I would like to know sooner rather than later.

 Thanks

  • 0
    TI__Guru* 86166 points

     The 300W EVM has less than 5% THD at 115V, so it should be possible at 230 to 265V it is not.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 450 points

    Currently we have an input current THD of 8.9%. There is about a 1ms flat spot in the current waveshape at the zero crossings. Is 1ms what is expected or can a lower time be achieved?

  • 0 in reply to Steven MacDonald
    TI__Guru* 86166 points

    The flat spot is most likely from the offset and PWM ramp you added to deal with the discontinuous inductor current.  This is covered in page 8 through 11 in the application note.  You can reduce this by decreasing the amount of PWM ramp that you have.  However, if you see spikes at crossover you have gone too far.  Also you might need more boost inductance to reduce the amount of discontinues inductor current the design could see near crossover.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 450 points

    By changing the offset to a lower value we decreased the THD to ~6%. It looks like the current waveform is now following the input voltage very good when the input voltage is decreasing towards zero. However, when the input voltage is increasing from zero the input current does not follow very well unti Vacin reaches ~30 volts. The drives look like the are on about 70% duty then they increase to 97%, I would have thought they would be at the maximum we had set, 97%? Any thoughts on what part of the control to consider?

  • 0 in reply to Steven MacDonald
    Prodigy 210 points
    I have gotten less than 3% THD also You may need to feed VinAC from before the rectifier and you will need two additional diodes
    for this the offset may be coming from any residual capacitance voltage you have after the bridge rectifier until it is discharged low enough to give a correct input signal which could account for this issue. I also had to increase the dead time slightly to .95 due to slow slewing rate of the inductor due to leakage L. This is what I did in my PF-DC/DC design at 2KW.