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UCC2818A: the output voltage is rising rapidly

Yuta Nishie
Genius 3310 points
Part Number: UCC2818A

Hi,

I have a question about startup behavior. Because the output voltage is rising rapidly, the output voltage has reached the specified value (380 V) before the end of soft start. I suspect that this is because the duty at startup is large, and it is rising rapidly. Can the rise of the output voltage be relaxed by changing the PKLMT setting? Also, does the PKLMT limit work only at startup? Or does it always work?

UCC2818A.pdf

I compared my circuit with the Typical Application Circuit on the datasheet, but I couldn't tell the difference. Therefore, I would like you to take a look at the circuit diagram. Is it possible to communicate with each other by private message?

Best Regards,

Nishie

  • 0
    TI__Mastermind 30670 points

    Hello Nishie-san, 

    Thank you for your question about start-up behavior with the UCC2818A PFC controller.

    The soft-start description (Section 9.3.5.11, page 11 in the datasheet) indicates that the SS voltage controls the VAOUT error voltage during start-up. 
    So Vvaout rises as Vss rises.  Since Vout rises too quickly to avoid overshoot, I think the SS capacitance can be increased to slow down the rise of VAOUT and keep the duty-cycle smaller.  

    The PKLMT function is always working, so it is not suitable to use to limit current during start-up.  

    I sent a private message for you to share your circuit diagram.  

    Regards,
    Ulrich