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Original question:

UC3854: looking for PFC controller

UC3854: output current limit

Gabriel Wang1
Genius 4385 points
Part Number: UC3854

Dear Expert

Customer input 192V-288V Vdc ，and output 370 Vdc 1200W . And they found some of our IC could be output current limit when doing test in functional test ,and if they change another IC ,and could be work well .

They try to ruduce the IAC input resistor to expand current limit thread ,but still have some IC could be current limit when doing test in functional test . Could u give some suggestion how to inprove these application. 

  • 0
    TI__Mastermind 32500 points

    Hello Gabriel, 

    Thank you for your interest in the UC3854 PFC controller. 

    Can your customer please provide details about the test conditions and how the output current is being limited? 
    The UC3854 does not have an output current limit function, but it does have an input power limit (through choice of IAC and Rmultout) and it has input PKLMT which limits the inductor peak current.  PKLMT will indirectly affect the output power which can reduce output current if a resistive load is used. 
    PKLMT will limit inductor current at the same level no matter what the IAC resistance is.

    There is +/-10mV tolerance around the PKLMT threshold (0V typical) and a bias current coming out of the PKLMT pin (also with some tolerance).  
    Maybe some ICs have the combined tolerances in a favorable direction, and other ICs have them in an unfavorable direction. 
    The unfavorable PKLMT tolerances can affect the threshold to limit input current too soon and limit output power.
    Replacing the IC with a part that has favorable tolerances can allow full power through. 

    Please ask your customer to check if PKLMT is being activated. 

    Regards,
    Ulrich