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Start-up diode in boost PFC circuit using UCC28180.

Xiaoming Kong
Prodigy 90 points
Other Parts Discussed in Thread: UCC28180, UCC27531

Hello Sir

I am designing a 1.5kW PFC boost circuit using UCC28180. There is a dedicated small size start diode across the pfc inductor and boost diode in the recommended circuit, what is the function of this diode, and will the PFC circuit work if this diode is deleted?

The FET used is FCP104N60 from Fairchild. is the gate drive of ucc28180 capable for this FET drive without using a FET driver such as ucc27531 ?

best regards

kong

  • 0
    TI__Intellectual 2230 points
    Hi Xiaoming,

    I have asked one our application engineers to look at this question for you. Thanks

    BR
    Oliver
  • 0
    TI__Genius 9790 points

    Hi, Xiaoming

    The start diode provides a current path for input inrush current. If there is inrush current going into PFC, the start diode bypass the current to protect PFC MOSFET and SIC diode. At stable status, the start diode does not work, it only works when input voltage is higher than output voltage. If AC input voltage is quite stable, the PFC works without this diode. But I recommend to keep this start diode.

    UCC28180 has 1.5A peak current capability, whether UCC28180 can drive FCP104N60 directly or not, it depends on the switch speed requirement and thermal of UCC28180.

    Regards,

    Yunsheng 

  • 0
    Prodigy 90 points
    Hi, Yunsheng

    Thank you very much for your prompt reply.

    The inductance of my pfc inductor is 0.6mH. The switching frequency is recommended to 92kHz according to the excel calculation sheet. Is there any problem if I set the sw frequency to 65kHz ?

    best regards
    xiaoming
  • 0 in reply to Xiaoming Kong
    TI__Genius 9790 points
    hi, Xiaoming

    If you select sw frequency as 65kHz, the ripple current of inductor would be increase with 0.6mH, which will affect iTHD and thermal of MOSFET and Sic diode.

    Yunsheng
  • 0 in reply to Xiaoming Kong
    Prodigy 90 points
    Hi, Yunsheng

    Thanks for your kind reply.

    An additional issue concerns with pfc output vdc voltage. I set RFB1=1.5Mohm, RFB2=20.8kohm, which leads to a desired Vout=365V. But the actual output voltage is 371V @Il=0.43A, and 379V@Il=0.63A, and 406V @Il=1.02A, why the output voltage unstable ?

    Switching frequency 95kHz, VCC is 15V, L=0.6mH, DC-link CAP is with 1120uF, input ac voltage is 220V.

    best regards
    xiaoming
  • 0 in reply to Xiaoming Kong
    TI__Genius 9790 points
    Hi, Xiaoming

    did you design this PFC based on the calculator TI provide? please refer to below link.
    www.ti.com.cn/.../getliterature.tsp

    is there any other circuit connect to VSENSE pin of UCC28180?
    another point is to check the ground of PCB, is the power ground affect signal ground of UCC28180.

    Regards,
    Yunsheng
  • 0 in reply to Xiaoming Kong
    Prodigy 90 points
    hi Yunsheng

    Yes, I intend to build the pfc board according to the design data with the excel tool that ti provided, and to seperate the power GND and the signal GND. I would like you can have a review of my pcb layout, how can i send you the files or pictures ?

    best regards
    xiaoming