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UCC28070: Question on 8.2.2.8 Current Sense Offset and PWM Ramp for Improved Noise Immunity

Masazumi Ishii
Genius 3945 points
Part Number: UCC28070

Hello.

I have a question about UCC28070.

Data Sheet P35 8.2.2.8 Current Sense Offset and PWM Ramp for Improved Noise Immunity
, Recommended circuit for improving operation at light load (discontinuous operation) is described.

In addition to DC offset, triangle wave offset is added to this circuit.

Currently, we are checking the operation.
However, when operating with only DC offset, operation at light load is improved, but adding triangular wave offset lowers power factor and operation becomes unstable.
(In order to prevent erroneous current detection due to ringing when switching FET is OFF, I think whether only DC offset is sufficient.)

It is a question.
Adding a triangular wave offset lowers the power factor and makes the operation unstable.
What are the possible causes?

Is there a problem with handling only DC offset?
What is the reason for adding triangular wave offset?

Thank you.

  • 0
    TI__Mastermind 34465 points
    Hello Masazumi Ishii,
    You are correct and many users of the UCC28070 just add a small DC offset to CSA,CSB in order to switch off the gate drive signal around the zero crossings.
    What happens here is that around the zero crossing of the AC sine wave, the output of the current amplifier saturates because the current sensed by CSA/B falls below the set value. The output of the current transformers are simply not giving an accurate measurement of the current .
    This creates a very high current demand signal and once the zero crossing is finished the result will be a a large current spike.
    You can eliminate the current spike by adding a combination of DC offset and a ramp offset. It was found that the ramp offset may not have had the benefits that were theorized and by the results of many applications it was found that adding a small adjustable DC offset to CSA/B the current waveform can be maximized for good power factor and THD.
    I recommend that you use a small DC offset only on CSA/B and adjust this offset to minimize pf and THD.
    The amount of offset required is based on how the current transformer behaves around the zero crossings of the AC waveform.
    Regards
    John