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UCC28070: Current Sense Offset and PWM ramp circuit

Derek Chan1
Prodigy 190 points
Part Number: UCC28070

Hi,

We have prepared to add a PWM ramp and offset circuit to CSA and CSB as shown below. Before enable the PFC, we find the waveform at D2105 is look like ch2 waveform as shown below (Fig.1 single trigger). Is it normal waveform? Do you have the correct waveform to share with us? Thanks.

 Fig.1

Fig.2

  • 0
    TI__Mastermind 34465 points

    Hello Derek,

    Your waveforms were not attached to your posting but I will try to explain what you should expect with the help of SLUA479B application report.

    The ramp and offset adds a small DC error signal to the CSA and CSB signals. This is shown above as the "false current sense signal".

    It is just a DC voltage that stops gate drive that would otherwise would result due to an erroneous current transformer signal.

    Therefore you should be just adding a small DC signal to the current sense resistors.

    REgards

    John

  • 0 in reply to John Griffin1
    Prodigy 190 points

    John, FYI.

    Fig.1

    Fig.2

  • 0 in reply to Derek Chan1
    Prodigy 190 points

    Hi John, I attached the waveform again. Pls check.UCC28070 waveform.docx

  • 0 in reply to Derek Chan1
    TI__Mastermind 34465 points

    Hi Derek,

    The compensating waveform that you should expect to see is a small dc offset with a ramp on top. 
    I think that is what you have.
    You should run your pfc circuit so that you get the proper signals from the current transformers. Then perhaps add your DC offse and finally add the ramp.
    Check the effect of this compensating network.
    A lot of users will just used the small DC offset.
    This will create a small dead zone around the zero crossings of the AC waveform where there is no PWM switching

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 190 points

    Hi John,

    Will the dead zone around the zero crossing you mentioned will affect the power factor or not? If yes, how to improve the power factor. What I have done is we add the compensation circuit and found the power factor is lower than before. The circuit is in the attached file. Please help.

    Current sense compensation.docx

  • 0 in reply to Derek Chan1
    TI__Mastermind 34465 points
    Hello Derek,
    The compensation circuit is a tradeoff. If your current sensing results in ringing around the zero crossings (which may not be the case ) then you can add the compensation circuit and this will eliminate the zero crossing spikes on the AC current. Eliminating the current spikes will improve the harmonic response and slightly degrade the power factor. Most users of the UCC28070 add a small DC offset to the CSA, CSB inputs. This adds a small dead zone around the zero crossings and results in a slight decrease in power factor.
    As I explained in an earlier posting, you should adjust the amount of DC offset until you obtain the performance you are satisfied with.
    What does your line current look like with and without the compensation network ?
    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 190 points
    Hi John,

    I will try to double check the actual line current waveform before and after adding the compensation. What our interest is if anyone do not use the offset resistor and use the ramp from the PWM only? Is there any side effect?

    Thanks.
    Derek
  • 0 in reply to Derek Chan1
    TI__Mastermind 34465 points

    Hello Derek,

    I can think of no reason for doing this but you should try the experiment and check the results.
    Regards

    John

  • 0 in reply to John Griffin1
    TI__Mastermind 34465 points
    Hello Derek,
    I believe this post is solved. Please open a new thread if you need further assistance.
    Regards
    John