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UCC28742: Please confirm the significance of Dmagcc = 0.475

Jeff Melvin
Prodigy 180 points
Part Number: UCC28742

I have a 420VDC source and want to use a 700V switch with an 80% voltage derating during secondary conduction, so there will be a 140V flyback voltage.

Now, as I understand it, the chip will limit switching frequency to maintain a maximum 47.5% secondary conduction duty cycle.  So, with 420V on the primary during the on-time (t1) and 140V during the off-time (t2) the t2 to t1 ratio is 3:1.  Now, to operate near 80kHz in these conditions means the transformer design must provide the output power limit with t1=2.00µs, t2=6.00µs and t3=4.63µs.  And to operate at any other maximum frequency, with these parameters, the t1, t2, t3 duty cycles must be still be 15.83%, 47.5%, 36.67%.  Is this correct?

  • 0
    TI__Mastermind 27105 points
    Hi Dox,

    Thanks for your interest in UCC28742. Yes, the controller will limit the maximum secondary conduction duty cycle to be 47.5%. 3:1 is an appropriate turns ratio for the transformer. I'm not sure what you mean by t3. Are you referring to the 1/2 DCM resonant ring period?

    I would refrain from assuming the duty cycles will remain constant at different maximum frequencies as this is assuming the efficiency remains constant. I would suggest using equations 10 through 14 of the datasheet to select proper transformer parameters for a desired max switching frequency.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 180 points

    Thank you for the prompt response.  Yes, t1 is the primary switch on-time, t2 is the secondary switch on-time and t3 is the dead-time (with resonant ringing for valley switching).

    My assumptions regarding duty cycles (and everything) are assuming ideal conditions.  The subtle point buried in that comment is that higher power can be transferred at a lower frequency with higher current and the same inductance.

    And I'm sorry, but equations 10-14 and the text surrounding them caused the confusion that generated the initial questions... following these led to DMAX=46.9% and NPS(max)=31.3 and crazy amounts of voltage on the primary switch!