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UCC28700 valley switching

Peter Han1
Intellectual 1020 points
Other Parts Discussed in Thread: UCC28700

Fig.18 of  UCC28700 datasheet.

why the first turn on is in second valley, but the second turn on is in first valley ?

what happen when the first turn on ?

and what is the meaning of zero-crossing timeout delay?  

  • 0
    TI__Expert 5900 points
    Hi Peter,
    As you saw, it is called valley skipping.
    First, the DRV supposed to output high at the valley of Vds. To reduced EMI, the IC would choose different valley to turn on as you see. it could be the first, second or third valley. Of course, the diffrent valley means different turn off time, so the turn on time would also be changing to keep a constant output voltage.

    The tZTO is the time that the IC will wait for a zero crossing for valley switching after the control dictates to start the next switching cycle.
    IC indicated to start the next switching cycle supposed to be after the end of the secondary current conducting.
    and zero crossing detection is used to decide the valley of Vds.

    Br