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UCC28780: UVLO: how can I adjust it?

Wilhelm Reinstadler
Prodigy 10 points
Part Number: UCC28780

Hello,

I have the 45Watt demoboard - UCC28780EVM-021, and I want understand how I can move down the start up undervoltage from 107 VDC to 95V DC or 90VDC. Can you please help me?

Wilhelm

  • 0
    TI__Mastermind 40400 points

    Hello Wilhelm, 

    Thank you for your interest in the UCC28780 ACF controller.

    The start-up input voltage is set by choosing the appropriate value for Rvs1 after the primary-to-auxiliary turns ratio is known.  Rvs1 is the upper resistor of the resistor-divider network connected to the VS pin.  It corresponds to R29 in the schematic diagram of the EVM.   Equation 13 (page 30 of the UCC28780 datasheet) describes how to calculate the value of Rvs1 after selecting the target start-up voltage (Vrms). 

    On the UCC28780EVM-021 board, Rvs1 = R29 = 36.5kR which already programs the start-up voltage to ~93.3Vdc on the bulk voltage rail, which corresponds to ~66Vrms at the AC input. 

    Changing the EVM start-up voltage involves recalculating the value for R29 based on the new target, and altering R29 to the new value.  Because the VS input also monitors VOut for overvoltage protection, the value of Rvs2 = R14 must also change proportionate to any change in R29.  The current ratio of R29/R14 = 12.7k/36.5k = 0.348, so the value of R14 must = 0.348 x value of R29, whatever R29 is changed to.  This will keep the OVP setting the same. 

    Note: a few consequences of reducing the start-up voltage must be considered: 

    1.  Start-up at a lower input voltage allows steady-state operation at a lower voltage.  At full load, the input current will be higher and primary-side rms losses will be higher than designed for.  Appropriate cooling (airflow from a fan, for example) should be applied if you intend to operate the EVM at input voltages below the minimum-rated 85Vrms level. 
    2.  Reducing Rvs1 too much can allow the current out of VS pin to exceed the 2mA absolute maximum limit when input voltage is at the highest line (264Vac).   Based on the EVM's turns-ratio do not use a value less than 26.8kR for R29.  
    3.  If you change the value of R29 without also proportionally changing the value of R14, the OVP threshold may go up or down, depending on how R29 changes.  This could either allow a higher than expected OV condition to exist or could trigger OVP shutdown at lower output voltage. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 10 points 
    Hello Sir,

I am William's father. We are in the process of expanding our product range of DC/DC converters (see www.elektra.it) and would like to use the UCC28780 controller.
 We should realize a minimum input voltage of 75V DC. Following your instructions, we calculated R29 to be 30.7K and R14 to be 10.6K. However, the starting threshold remains the same at around 105V DC. (original EVM)
What are we doing wrong. Please give us the correct values ​​for R29 and R14.
Thank you very much
Egon Reinstadler.
  • 0 in reply to Wilhelm Reinstadler
    TI__Mastermind 40400 points

    Hello Egon, 

    The UCC28780EVM-021 transformer has a turns ratio of P:S:A:SA = 1 : 0.19 : 0.14 : 0.1 which can easily be iterated to 21T : 4T : 3T : 2T. 

    Equation 13 is based on using an AC input voltage.  But after rectification, the peak of the line voltage is stored on the bulk capacitor and since there is no load before start-up, it is effectively a DC voltage.  Restating equation 13 for DC:  Rvs1 = 75Vdc * (3T / 21T) / 365uA = 29.35kR (for R29).  I cannot recreate your calculated value of 30.7k (which is off by a +4.6% factor somewhere). 

    The nearest 1% value (in E96 series) is 29.4kR.  R14 would then be 29.4k * 0.348 = 10.23k, or 10.2kR, 1% standard value.
    The EVM should start up with DC input voltage near 75V (not exact because of variances) when R29 = 29.4kR. 

    You report that your EVM starts at 105Vdc, although that voltage does not calculate from the value of 36.5kR at R29 in the EVM's schematic diagram.  It should start at ~93Vdc, and I don't understand why it does not.  I don't have an EVM here locally to test.  

    If the 36.5kR had been removed, then reinstalled, perhaps its value has increased due to excess heat from solder/desolder or some damage to the resistive element.  I'm just trying to think of some reason to account for the higher start-up voltage, assuming that the Ivsl(run) current of the UCC28780 did not increase by 13%.  

    I hope the value of 29.4kR (along with10.2kR) works for you. 

    Regards,
    Ulrich