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UCC28700-Q1: valley switching operating

Dongbao Zhou
Expert 4170 points
Part Number: UCC28700-Q1

Hi Team,

May i know the reason why the mosef isn't constantly on at the first valley of Vds, sometimes it's second or third? It varies with input voltage and load current.

Thanks

Dongbao

  • 0
    TI__Expert 8210 points

    Dongbao

    The purpose of valley skipping it is that at certain operating points, such as high line at lighter load, having the turn on instance at the first valley causes the switching frequency, and corresponding switching losses, to be higher than what's optimal.  What valley-skipping mode enables is an effective reduction of the switching frequency, which reduces the switching losses and allows the converter to operate at a more optimal operating point, enabling higher efficiency.

    Best Regards,

    Eric

  • 0 in reply to Eric Faraci
    TI__Expert 4170 points

    Hi Eric,

    According to my understanding, if the input voltage and load are keep constant, the switching waveforms should be the same (first valley or second valley etc). However, customer see the valley skipping position is not the same even under stable condition (different input and load conditions are tested).

    May i know the reasons?

    Thanks.

    Dongbao

  • 0 in reply to Dongbao Zhou
    TI__Expert 8210 points

    Dongbao

    Which valley the FET turns on changing during a "steady state" operation can be expected operation.

    • UCC28700-Q1 has frequency jitter to reduce EMI, which modulates the switching frequency slightly at a steady state operating point.  This can cause the turn on instance to jump valleys
    • For AC input designs, while the input may be a fixed AC voltage the rectified DC that's applied to the UCC28700-Q1 design does have a significant ripple.  This ripple frequency is much lower than the switching frequency, so from the UCC28700-Q1 input perspective the input is actually a moving DC value.  This moving input voltage can cause the operating point to change over an entire AC line cycle, which can vary which valley UCC28700-Q1 turns on at.

    Best Regards,

    Eric