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UCC28700: need confirmation on T_DMAG(Min), transformer design guidance, and SLUC408B question

Kevin Lai
Intellectual 2916 points
Part Number: UCC28700

Hi team,

the customer is trying to solve for transformer DMAG(Min) from equation 10 in the datasheet. they are getting ~14us because the input voltage is so high and the NPS can't go higher since they still want to support lower input voltages (110-480VIN). 

Is the target supposed to be 1.1us or is that just the absolute minimum you can have and 14us is ok?

additionally, in SLUC408B (excel design tool), line 113 shows 600V as the max drain voltage of the FET. my customer wants to use 1000V rated FET but the line is not a user-input cell. is this a bug?

Finally, apart from the SLUC408B and the datasheet, is there any other documentation we can provide to help with transformer design?

Thanks,

Kevin

  • 0
    TI__Mastermind 31580 points

    Hi Kevin,

    The minimum DMAG time is 1.1us.

    If you go to the TI refrence designs you will find some examples of wide input range designs,

    The tools are intended to guide customers through the design process for universal mains input, 85Vac to 265Vac.

    There are a number of papers on transformer design in the TI power supply design seminars,

    Regards

    Peter

  • 0 in reply to Peter Meaney
    TI__Intellectual 2916 points

    Hi Peter,

    apologies, i meant to ask if 14us is fine? 

    thanks for the links.

    Thanks,

    Kevin

  • 0 in reply to Kevin Lai
    TI__Guru 66065 points

    Hello,

    The excel design tool calculates a minimum Vds voltage. Using more than the tool calculates is acceptable.  So 1000V is fine to use in the design.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 66065 points

    Hello,

    If Tdmag is 14 us when and you are using dmag of 0.425 this will put the switching period at 32.9 us and your maximum switching frequency will be 30.4 kHz roughly.  This will have the converter operate in the lower FM band only.  Please refer to figure 15 in the data sheet that shows the control law of the FM/AM/FM bands only.  I have seen designs were customer use only the lower AM band and this may require a little more pre-loading at no load to keep the design in regulation.

    When it comes to doing wide input designs you just need to make sure the on time at the maximum load is 4X TCSLEB of 235 ns if you want to use the upper FM band.  The following application note discusses how to design PSR flybacks for a wide input range.  You may find this helpful also.

    Regards,