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UCC28711 not powering up properly

Raja Saha94
Intellectual 575 points
Other Parts Discussed in Thread: UCC28711, UCC28710

Dear All,

I am using UCC28711 IC for my flyback SMPS design having primary voltage range 400-800V DC and output voltage 110V,1.25A ( single secondary).

I am following tidu412a.pdf file as my reference design.

while giving the DC bus voltage near about 430V, I am observing the VDD of the UCC28711 swings between 21V to 14V. And also HV_start pin swings between 370V to 25V. It seems to me that PWM controller IC UCC28711 is not enabling up properly. Also the IC is also releasing only one Gate pulse, however in the UCC28711 datasheet and also in TI design document the IC releases three pulse at start up. The IC is entering in START up mode again & again and hence the VDD varies between 21V to 14V. 

Can it be a problem of IC or xmer design.

Please help

Regards

  • 0
    Prodigy 10 points
    Hi,
    I have the same problem with the UCC28710 the Vdd is bouncing between 21v and 14v, but the HV pin is constant because it is connected directely to the Input power
    (my design is for 365 vdc - 800 vdc input to 15v and 5v 0.5amp output)
    warm regards .
  • 0 in reply to Dimitri Al Khoury
    Intellectual 575 points
    Hi Dimitri,
    just check whether your MOSFET is getting turned on properly or not.
    In my case ( after replacing the IC), i found three pulse ( width of 3-4 usec, as my switching freq is 100KHZ and in my design Dmax is chosen as 0.335), but still the VDD comes down to 8V.
    Then I observed at VDS of MOSFET remainging high always means my MOSFET is not switched on. And hence
    no transformer switching is taking place and no voltage at auxiliary / bias winding is generated. Therefore Vdd comes down to 8V.
    My assumption behind this phenomena (i..e MOSFET not switched on) is the unability of UCC28711 IC to provide that amount of drive current, as I used a MOSFET with 2400 PF of input capacitance).
    Please put any other comments if anyone have any other.
    Regards
  • 0
    TI__Expert 5900 points
    It sounds that the driving capability is not enough for you design. You design is more than 100W, you may need a lower Rdson Moset which may have a larger Ciss.
    If it is right you may need to use external drving circuit for it.
  • 0 in reply to Kening Gao
    Intellectual 575 points
    My problem seems to be solved. It was a problem of MOSFET with high input Capacitance & also in xmer. Due to wrong xmer, current protection was getting enable which was restarting the IC.
    However I am not getting the desired output voltage , getting 86V instead of 110V.
    It seems to me the turn in auxilliary winding is more than required, but I followed the design rule of tidu412a.
    Now I reduced the Rs2 resistance which is giving me 106V at secondary.