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UCC28730: Increasing output Hold-time

tc
Prodigy 140 points
Part Number: UCC28730
Other Parts Discussed in Thread: TIDA-01560,

Hi,

I'm interested in using the UCC28730 in a new design.  The TIDA-01560 reference design fits my requirements, however, I would like to increase the output hold time ie time from AC input power being disconnected to the output voltage going out of spec.  I'd like to have a minimum hold time of 100ms (and more if possible).  I think it would be just a matter of increasing Cbulk, but in the UCC28730 calc tool spreadsheet its says that Cbulk should be set within 10% of the recommended value, 22uF.  So is there an issue with increasing Cbulk?  If not, how do I calculate parameters for a 100ms holdtime.

Thanks

Tony

  • 0
    TI__Guru 66115 points

    Hello,

    You should be able to use a larger bulk capacitor without an issue. You are correct increase Cbulk will do that for you.

    Then you should be able to use charge control law to size Cbulk

    Cbulk = Iin*dt/dV

    I believe the tool generally size the bulk capacitor and the flyback converter to operate down to a minimum of 70V.

    With the slight change in the control law equation you can account for the minimum input voltage.

    Cbulk = Iin*dt/(Vinmin_peak - 70V)

    Regards,

  • 0 in reply to Mike O'
    Prodigy 140 points

    Thanks for the reply Mike.

    I've used eq 9 in the UCC28730 datasheet to calc Cbulk for a given hold time (specified as half cycles in this eq).  

    Thanks