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UCC28740: flyback transformer choice process with height constraint

Nicolas Saugnier
Prodigy 80 points
Part Number: UCC28740
Other Parts Discussed in Thread: TIDA-00709, UCC24636
Hello,
I am designing an embedded AC/DC flyback power supply based around the UCC28740 for a consumer product, and the design of the product imposes a very strong limit on how thick the whole power board can be.
I found alternatives and options for all other tall components, only the transformer is a problem for me.
I read the TIDA-00709 design document and created a solution in WEBENCH, and these helped me with the selection of all the other components, but I do not get how to reconcile the design parameters for the transformer with my thickness constraint.
My design parameters:
Flyback SMPS with synchronous secondary rectifier, one output of 6V / 3.5A.
UCC28740 + UCC24636
VINmin = 90VAC
VINmax = 250VAC
Vout = 6V
Pout = 21W
Fsw_max = 100kHz
Vpp_max = 200mV
*** Max thickness for any component : 14mm if SMD, 12.5mm if THT
I have a reasonable surface available however, so a flat transformer covering even twice as much area as the default recommended models is OK, as long as at least one side is less than 25mm.
So far, I had our sourcing guy call his usual (chinese) vendors, the only options I found for the bobbin were around 15 to 18 mm thick (THT), depending on orientation.
Can you provide guidance on how I should proceed to solve this?
Thanks
  • 0
    TI__Mastermind 23290 points

    Hello Nicholas,

    I suggest that you look at the EFD20 ferrite core shape.  It looks to me like it should easily handle the ~21W  power level of your application, and is less than 12mm in height in both SMD and TH bobbin options.  The EFD15 core size might also work, but it may require an overly-large gap which may lead to excessive copper-loss and/or create EMI issues.

    I found it in the latest Ferroxcube catalog, although other manufacturers also carry such a core. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 80 points

    Thank you, it definitly looks like an interesting solution.

    We'll try to get samples made to validate the choice.

    Best regards,

    Nicolas