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UCC28051 soft start

CWilson
Expert 1920 points
Other Parts Discussed in Thread: UCC28051

Hi everybody,

I'm currently developing a PFC with the UCC28051. My problem is, that the core saturates during start-up (when the voltage boosts to 400V). It's not possible to use another magnetic core due to limitation of space on the PCB. My core saturates at about 7.5A and my maximum current is 6A at full load from 85VAC - so no problem at normal operation.

I'm trying to manipulate the COMP-pin and thereby make a soft-start procedure. When I connect about 8.8uF at COMP, the input current is low enough - but my regulation speed / settle time during step load is very slow (too slow).

Has anybody had luck with integration of soft-start on this IC?

  • 0
    Expert 1920 points

    Problem solved. 

    I've added a BAV199 diode with pin 1 on COMP and pin 3 on resistor network from Vcc (16V) to the resistor divider (100k and 47k) in the middle is 5V. Then ~26uF is added in parallel on the bottom resistor (47k).

    Voila.. the regulation during start up is slow, but still fast during transients/load step.