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UCC28730: Realizing a Two Ouptut Converter with UCC28730

Dario Moscardi
Prodigy 115 points
Part Number: UCC28730
Other Parts Discussed in Thread: TIDA-01560

Good Morning,

I am trying to design an off-line two output converter with UCC28730 and an Off-The-Shelf transformer supplied by WE : 750811613. 

The transformer reasonably matches all the design procedure from the IC's datasheet except for the auxiliary to secondary turns ratio.

What could happen if the auxiliary turns ratio is much bigger than the ideal Nas from the datasheet procedure? From the AN "TIDA-01560" i can guess that it could result in an output lower voltage in some conditions but I can't figure out what they are. Unfortunately it is not much treated in the UCC28730 datasheet and i could not find an AN  that can help me on this topic.

Can you please explain what could happen in this situation?

Thanks in advance,

Dario Moscardi

  • 0
    TI__Mastermind 26840 points

    Hello Dario, 

    Thank you for your interest int he UCC28730 PSR flyback controller. 

    In the design procedure the transformer turns are selected before the VS resistor-divider values are determined. Therefore, one may in principle achieve desired output voltage regulation for any Nas ratio.  In reality, this ratio is limited by the VDD rating of the UCC28730.

    For the target Vout, an aux winding with higher turns than ideal will present a higher voltage to the VDD input.  The UCC28730 can tolerate up to ~35V on VDD, so if the Nas is not too high, the transformer can be used directly.  But excessive VDD voltage will increase standby power, so it cannot be as low as it could be with ideal turns ratio.  You may also need a series linear regulator to drop some volts before feeding it to the VDD.  

    The UCC28730 Design Calculator Tool (obtained here: 

    )  can determine the correct resistor values to scale Vout to Vaux for PSR.  It will provide the "ideal" turns ratios, but since you already have a chosen transformer, put the actual ratios used into the calculator and it will proved the resistances needed for those ratios.  

    Essentially, Vout will not be lower than spec; VDD (and Pstby) will be higher than ideal.

    Regards,
    Ulrich