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UCC28700 start up problem solved with 10pF on VS pin

Other Parts Discussed in Thread: UCC28700

Hello TI team,

I have building a fly back regulator with the UCC28700 few months ago.  With our first samples I had no problems. Now we have produced 60 items and now we have startup problems (starting again and again) with about 10 % of all. A small cap (10pF) at the VS pin solved this problem. My question is now: can I get any trouble with this small cab in the feature in relation to the  control behavior of the fly back?

The VS input resistor divider is 82.4k / 27k. I used the schematic of design example “PMP10344 - 85VAC-480VAC Input, 12V/25W Flyback”.


  • Hi Oliver. In general we recommend not putting capacitance on the Vs pin for a PSR controller as it adds a phase delay to the controller and can affect the sample voltage. However, as you have seen, noise on the pin can also create problems, so sometimes a little capacitance is required, the direction is to keep it as small as possible. We have used values like 10pF before to filter this noise, even on some of our EVMs.

    In short, it should be fine, but I would try and avoid increasing it too much. 



  • Hi Billy, thanks for the fast response. I think so too. The 10pF cap isn't too much and I have made some load tests and I could not see any impacts. I ask me only, why have some devices this behavior and other not. Maybe tolerances of transformer or so...
  • Oliver,
    How is your design progressing?
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