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UCC28070: question for the "Current Sense Offset and PWM Ramp for Improved Noise Immunity"

Part Number: UCC28070

Hi,  I have some questions for the "Current Sense Offset and PWM Ramp" which is mentioned in UCC28070 datasheet 8.2.2.8. Could you please help to answer them?

1, As the datasheet said, the purpose of this offset and ramp circuit is to add some offset at light load, when inductor current is in DCM. But i did not understand that, during PFC mosfet turned off period, UCC28070 will have current synthesizer to simulate the down slope current signal. That is say even there is current signal ringing when current is DCM, the chip should not be false triggered, because it is using the simulated current signal

2, could you please explain the figure 28 the false current ?  In this figure, Rsa voltage has some offest, so it look like the waveform of offset and ramp circuit impelemented. But why when Mosfet is off, Rsa has no offset voltage?  And if the ramp and offset circuit has already be implemented, why it is called "Discontinours current cause false current signal".

Thanks

Tao Wu 

  • Hi Tao,

    Thanks for your interest in UCC28070. We will look into your questions and get back to you.

    Best Regards,
    Ben Lough
  • Hi Tao,

    When the input voltage drops to very low values around the zero-crossings of the AC line cycle the inductor current will ring with the Coss of the boost mosfets. The effect of the ringing is to bring the starting point of the current waveform below zero at the start of the boost mosfet on-time. Because the current starts from a negative value this portion of the current will not be sensed by the CS transformer circuit as it only rectifies current which flows from drain to source in the boost fets, current in the positive direction, above 0. A small DC bias is added to ensure that the current sense signal is always above 0 so the true value of the current is sense by the CS transformer circuit.

    A ramp signal can be added to the CS signal during the on-time of the boost mosfets and is designed to increase the current sense signal magnitude to give a larger signal to noise ratio at light loads.

    Regards

    Peter