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UCC28781-Q1: Transformer inductance calculation

Valentinas Pancerovas
Intellectual 410 points
Part Number: UCC28781-Q1
Other Parts Discussed in Thread: UCC28781EVM-053,

Hello TI,

We are doing calculations on a design that should have 650-900Vin and 68V 0.6A output. I have never seen transformers with inductance in 10mH range. Is our calculation correct here? What happens if the inductance is too high, will it still work?

We are using UCC28781EVM-053 as our reference. The transformer in the reference has much lower indutance.

  • 0
    TI__Mastermind 29720 points

    Hello Valentinas, 

    Thank you for your interest in the UCC28781-Q1 ZVSF controller.

    Yes, 10mH is an unusual value for magnetizing inductance, but it comes from the combination of high minimum input voltage and the specified high maximum duty cycle of 65%.  
    This calculates to a peak current of about 0.2A, so 1/2*10mH*0.2A^2*200kHz = 40W (for rough estimate).  
    Furthermore, the recommended turns ratio (as a consequence of the Dmax = 65% requirement) of 17.57:1 results in a reflected voltage of 1195V, on top of a max input of 900V requires a switch rating in excess of 2100V.

    I suggest that a lower maximum duty cycle will allow a higher peak current and lower reflected voltage.  I'm not sure why that parameter is an input rather than a result.  
    The problem with specifying a much smaller Dmax (such as 10%, for example) is that Dmax applies at full load lowest Vin and D will be even lower at the highest Vin, and even lower at light load at high Vin.  At some point, the leading-edge-blanking time (which essentially defines a minimum on-time) may result in a forced peak current that could require a fairly high minimum load to absorb to avoid output overvoltage.  

    If your output load range is fairly constant near max power, then tLEB should not be a problem and you can experiment to find a small enough Dmax to obtain a reasonable Lm and turns ratio.  If your load does go to zero, then it is likely to require a minimum preload to keep in regulation.

    For the UCC28781-Q1, I suggest to select Transition Mode calculation rather than DCM.

    Regards,
    Ulrich