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Original question:

UC3854A: Buck Converter using UC3854A

UC3854A: Buck PFC - Application note

Jeheu Jesse Dela Cruz
Prodigy 10 points
Part Number: UC3854A
Other Parts Discussed in Thread: UCC29910A

Tool/software:

I wanted to use UC3854A as a Buck PFC controller, utilizing its power limit feature. Upon following the application instructions DN-66, I can't seem to regulate the output on the desired voltage (400V) at the same time achieve accurate power limit (1.5kW)

  • 0
    TI__Mastermind 40580 points

    Hello Jeheu, 

    The UC3854A is designed to be a CCM boost-PFC controller.  The instructions in DN-66 apply to the traditional CCM boost-PFC configuration.  They do not apply to a buck-PFC implementation. 

    It is possible to adapt the UC3854A to function as a buck-PFC controller, but there are several special considerations to deal with:
    1.  Buck operation happens only when the input voltage is higher than the output voltage.  For 400V output, you will need an AC input of more than 300Vrms.
    2.  The boost-PFC design equations for inductance and other components do not apply to buck-PFC. 
    3.  "Soft-start" is prolonged because Vout starts at 0V.  Normal boost-PFC soft-start doesn't work as expected for buck-PFC.
    4.  If a high-side switch is used, you'll need a level-shifted driver.  If a low-side switch is used, you'll need level-shifted feedback.  

    These are the important issues that I can think of.  There are probably more.  I am not sure how to convert the UC3854A to control a buck-PFC configuration. 

    TI has a dedicated buck-PFC controller: UCC29910A, but it is optimized for 85Vac~265Vac inputs and ~80Vdc output. 
    However, its datasheet and other app-notes may have additional information about buck-PFC that may be useful to you for design of your application.
    Please see the documents available here:  
    TI_UCC29910A_Buck-PFC_Ctlr_TSSOP-14.pdf      App-Note_Buck-PFC_SEM-1900_slup264.pdf

    Regards,
    Ulrich   

  • 0 in reply to Ulrich Goerke
    Prodigy 10 points

    Hello Ulrich,

    Thank you for your suggestions. 

    As for the input voltage, we are using 380Vac or greater, and the gate drive is not concern as of the moment. As for the equations, I've already tried using buck applicable equations in the buck pfc design note. My main concern however is the power limiting factor. Is the equations for DN-66 in tuning the power limit applies with a buck pfc? i.e. Rmo, Rac, A, etc.

    Thanks again.

    Regards,

    Jeheu

  • 0 in reply to Jeheu Jesse Dela Cruz
    TI__Genius 12795 points

    Jeheu,

    We are not responsible to answer questions outside of the intended operation principles. If you decided to pursue that route, I'd recommend you to do some research online.

    Thanks,

    Ning