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UCC28070: Questions regarding UCC28070 CS pin DC offset and PWM ramp

Wei-Hao Chen (TW-FAE)
Expert 3260 points
Part Number: UCC28070

Hi Support team champs:

My customer is having a question regarding adding a DC offset on CT to avoid the DCM ringing and noise. Suppose a DC offset is added on CS pin and PWM ramp, the current signal seen by the controller will always larger than the actual inductor current signal (say actual current signa + 120mV offset + PWM ramp).

Do we need to adjust the Rs value or the peak current limit threshold in order to compensate this offset? Otherwise the peak current level will always smaller than what it should be.

Also Why does add the PWM ramp can help to increase the signal noise immunity? Also what does this paragraph mean?

"To counter for the offset (VOFF) just requires adjusting resistors ROA and ROB to ensure that when the unit goes
discontinuous the current sense resistor is not seeing a positive current when it should be zero."

The offset is always there which the CS pin can always seen a positive current, how's adding ROA and ROB helps the controller not seeing this positive voltage?

 

Thank you.

Best Regards,

Wei-Hao

  • 0
    TI__Mastermind 34465 points
    Hello Wei-Hao,
    Most applications just add a small dc offset to the CSA and CSB signals.
    100mV to 150mV is the typical value.
    The effect of this offset is to add a "dead zone" around the zero crossing.
    The gate drive is off around the zero crossings because the output of the current amplifier is low due to the offset on the negative input of the current amp.
    You do not need to adjust anything because of this small offset.
    The sentence you quoted above is perhaps badly stated.
    It means that during the time when Rsa is below Voff (the DCM time mentioned in the text) the gate drive may go to full on because the sensed current is zero and the error amp has a finite output.
    This may saturate the current amp and cause large spikes of current around the zero crossings.
    Adding an offset on CSA/CSB prevents these large current spikes and generates a dead zone.

    Hope this helps
    Regards
    John
  • 0 in reply to John Griffin1
    TI__Expert 3260 points

    Hi John,

    Thanks for your quick reply. Are you suggesting adding the offset is sufficient and there is no need for adding the PWM ramp?

    Also I am a bit confused about this statement "The gate drive is off around the zero crossings because the output of the current amplifier is low due to the offset on the negative input of the current amp."  Does this mean the gate drive will turn off slightly earlier than the actual zero crossing point? I still don't get how does zero crossing is detected with the presence of this DC offset, could you please explain this in a bit detail?

    Thank you so much.

    Best Regards,

    Wei-Hao

  • 0 in reply to Wei-Hao Chen (TW-FAE)
    TI__Mastermind 34465 points
    Hello Wei-Hao,
    Answer to your first question is yes.
    Answer to your second question is yes.
    The controller knows where the zero crossing exist because of the multiplier output.
    The inputs to the current amp are CSA,CSB (negative input) and the multiplier output (positive input)
    With an offset on CSA,CSB at a time when the multiplier output is zero, the current amp is forced low and PWM is disabled.
    Sorry I cannot explain it any simpler.
    Regards
    John
  • 0 in reply to John Griffin1
    TI__Expert 3260 points
    Hi John,
    No worries I get want you meant. Thanks for your prompt reply and answering my question :)

    Best Regards,
    Wei-Hao