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ucc28070

ron f
Prodigy 130 points
Other Parts Discussed in Thread: UCC28070

using ucc28070 for 8kW pfc. two parts synced  so there are 4 boost converters interleaved, each processing approx 2kw each. My input current waveform is distorted. Checked layout and it seems fine. Just wondering is there were any other things that could cause this problem. The input inductors are not saturating, I am operating each phase at approx 90khz , I have attached input current waveform (green) and one phase current (blue)

Thanks

  • 0
    TI__Mastermind 34465 points

    Can you please attach the waveforms and schematic.

    Thanks

    John

  • 0 in reply to John Griffin1
    Prodigy 130 points






    line current is green and one phase current is light blue shown below

  • 0 in reply to John Griffin1
    Prodigy 130 points

    see attach word doc, pasting did not seem to work

    pfc_ti.docx
  • 0 in reply to ron f
    TI__Mastermind 34465 points

    The waveforms look like the inductor is too small and the current goes discontinuous add/or the current synthesis resistor is not setup correctly. You can check the current sunthesis resistor Rsynth with the Excel design tool based at

    http://www.ti.com/lit/zip/sluc114

     

  • 0 in reply to John Griffin1
    Prodigy 130 points

    Inductor values are those that are recommended in design tool. The Rsynth resistor was a little higher than recommended so I will give that a try and let you know how it works out

    The turns ratio of the CT that the design tool comes up with seem way out (700), not sure how this is arrived at

  • 0 in reply to ron f
    TI__Mastermind 34465 points

    The calculator sets the CT turns ratio (Ns/Np) =10 times the peak diode/FET current. So in your case it appears the peak current is 70A in the power stage. Does this sound correct?

  • 0 in reply to John Griffin1
    Prodigy 130 points

    I am now having a problem with what I thought was current limit ie, once I get to a certain power level the output voltage of the pfc starts to droop. I checked the current sense settings and everything seems fine so now I am not so sure what the cause is. Any suggestions would be appreciated. I am thinking that perhaps it might be a duty cycle limit issue with the Vo setting being too low (360V) at 230Vac input but it even occurs at a lower line voltage. Perhaps it is that energy storage in the inductors is too low?? It starts to happen at around 6kW (depending on Vbus setting and line voltage)

    Ron

  • 0 in reply to ron f
    TI__Mastermind 34465 points
    The calculator will show you what the max duty cycle at peak of low line you can expect.
    Is this a reasonable value?
    The calculator will also give you a value for inductance and current rating. The inductor you select should be rated for both of these parameters.
    It may be that your input supply is going into current limit. With a 6kW load, the input supply may need to be able to supply up to 40A load.
    Perhaps even a voltage drop in the supply cables.
    If the issue is not a pfc current limit then it is mostly likely the above scenario.

    John
  • 0 in reply to John Griffin1
    Prodigy 130 points

    John

    I check the inductor value, I limit and duty ratio setting and nothing seems to be improving the situation. I did hear from a co worker that that in the past, he used the part and had a problem with the current synthesis programming value and that this caused a similar problem. Do you know anything about this possibility?

    Thanks

  • 0 in reply to ron f
    Prodigy 130 points

    John

    Changed the Rsyn resistor from 100k to 150k and the problem got worse which seems to indicate there is some issue with the synthesizer. According to the data sheet, the value for the power level I am at, should be in the range of 150k so I am not sure why the problem is worse with a value that is closer to the recommended one. Any suggestion would be appreciated

    Ron

  • 0 in reply to ron f
    Prodigy 30 points
    Hello,
    I'm using the UCC28070 400V / 2KW and I have some issues about this.
    When I try to increase the output Power, the PFC stops working.
    I checked both the current and voltage limits, the controller don't reach these points.
    I tried to change the CAOA components, the VAO filter, The Rimo resistors... . And I checked Vsense, VAO, Vinac, Vss it seems to be fine.
    I don't understand why it stops.
    Do you have any idea ?

    Thank you,