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UCC28061: Soft Start Question

Chuchen Wang
Expert 3470 points
Part Number: UCC28061
Other Parts Discussed in Thread: UCC28063A, UCC28063

Hi,

My customer has the following question:

Can you provide me with any tech notes or TI white papers regarding the SoftStart feature of the PFC controllers UCC2806x?

We are now using the UCC28061, but having trouble after a momentary Vac dropout lasting longer than the 440mSec brown out timer.  We put in a soft start circuit that limits the rise slope of the Comp pin and then disconnects after a short time.  This circuit gets reset when the auxiliary bias dies but that time is much longer than the few seconds of Vac dropout.

I found app note SLUA587 which tries to do the same thing as we are doing by dragging down the Comp pin for a while but it too suffers from an intelligent way to reset the softstart at a momentary dropout.

The UCC28063A shows some internal chip connections that affect the Comp pin.  Do these help the situation we have?

Is the UCC28063A pin-pin  compatible to replace the UCC28061 without redoing a major amount of validation testing or pcb layout changes?

Thanks,

Chuchen

  • 0
    TI__Prodigy 110 points

    UCC28063A is pin to pin compatible with UCC28061. With UCC28063A you do not need external components to manage soft start.

    Soft start feature is integrate and properly reset by the IC itself.

    Anyway there are some differences.

    Mainly you need to review the circuit connected to COMP pin.

    With reference to the figure 1 of application note SLUA587 that you mentioned, you should decrease R2 from 66k to around 12k.

    Regards

    Davide 

  • 0 in reply to Rosario Davide Stracquadaini
    Prodigy 90 points

    Rosario Davide Stracquadaini said:
    With reference to the figure 1 of application note SLUA587 that you mentioned, you should decrease R2 from 66k to around 12k

    It seems that SLUA587 was written in 2011 to address soft-start for the UCC28061.  This was before the UCC28063 was developed and included its own internal circuits to handle soft-start, which should by my thinking, eliminate the need for and external soft start circuit such as SLUA587.  Correct?   If so then what is the reason for changing the R2 value of the compensation network?  

  • 0 in reply to R Wagner
    TI__Prodigy 110 points
    In UCC28063 soft start is obtained by the slow charging of capacitance Cz.
    If R2 is high the drop on this resistance will rise comp voltage partially eliminating the soft start.
    You can use higher R2 Voltage. Higher is R2 higher would be phase margin but 12k up 18k are generally enough.