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UCC28780: min VIN for start up

Garret Holt
Intellectual 1050 points
Part Number: UCC28780

Hi Team,

We are working with a 50W, 16Vout application utilizing UCC28780 in a design closely resembling SLUUBV6.This design uses a 170-200uH magnetizing inductance and a 0.125 Ohm sense resistor.  This works out to ~190.4Vin per the equation 1 in "Debugging UCC28780 ACF Converter Start-up Issues" with 170uH, but the converter is able to start around ~110Vin Is there anything else impacting the 0.28V threshold? I notice that this equation does not take into account Rcomp but do not think that this is related. Let me know if you have any other suggestions as to why you think this design is starting up at this low of a voltage.


Thank you!


Garret

  • 0
    TI__Guru* 86266 points

    Hello,

    In regards to the 0.28V in equation 1. Section 7.4.11.1 discusses this as a CS fault shutdown.   If VCS does not reach VCS(SM1) = 0.28V in 2 us (tscf1) in 2 us the controller initiates a fault shut down, due to possible CS pin shorted to ground.  You can find tcsf1 specifications in specification table of page 8 of the data sheet.Equation 1 is not checking for input under voltage.  It is checking to make sure the VCS reaches 0.28V in less then 2 us. V = L*di/dt

    Your converter should try to turn based on the IVSL(run) and IVSL(stop) current.

     IVSL(run) = 365 ua, VS Line-sense run current

    IVSL(stop) = 305 uA, VS line-sense stop current

    IVSL is the current coming out of the VS pin when the aux voltage is negative.  please note that there is an internal clamp when VAUX is negative and RS2 is shorted to ground.

    Vin(on) > or =IVSL(run)*Rs1*Np/Na, 4 for consecutive switching cycle  while main FET is on

    Vin(off) < IVSL(stop)*Rs1*Np/Na, for 4 consecutive switching cycles

    First I would suggest double checking the transformer calculations and components using the excel design tool.  You can find it at the following link.

    Regards,