Cap across TL431 in UCC28740 design spreadsheet

Peter Millett

Other Parts Discussed in Thread: UCC28740, TL431, UCC24610

(Hopefully) a simple question:

In the UCC28740 design calculation spreadsheet, and in one (but not most) of the reference designs, there is a 1uF cap across the TL431 ("CSS431" in the spreadsheet).

I don't see this mentioned in the datasheet nor explained elsewhere.

What is it's purpose?

Thanks, Pete

over 10 years ago

0 Lisa Dinwoodie over 10 years ago

TI__Genius 15910 points

This capacitor adds a soft start to the secondary side.

0 Peter Millett over 10 years ago in reply to Lisa Dinwoodie

Prodigy 80 points

Thanks, I suspected so...

Pete

0 Lisa Dinwoodie over 10 years ago in reply to Peter Millett

TI__Genius 15910 points

I'm actually in the process of revising this particular design calculator (I found a "bug" in the Recommended Nps vs the Max Nps turns ratio) so if you also end up with the recommended Nps exceeding the Max Nps...use the max Nps value. I hope to get the corrected version uploaded this week.

I also want to add the secondary side winding voltage to help determine if the UCC24610 can be used on the secondary side (it has a VD max voltage of 50V so having this calculator tell you that directly would be nice).

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Cap across TL431 in UCC28740 design spreadsheet