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Part Number: UCC28881
Tool/software: WEBENCH® Design Tools
this is my first post in this forum and I just joined the TI support network.
I'm trying to build a 230 V AC / 16 DC converter and to understand the
necessaray design I started to follow the equations mentioned in the
datasheet in chapter 9.
I already get stuck on page 18 calculating the CBULK value. I tried to
insert the values seen in table 2 in formular (2) and I can't reach the
resulting value of 15.6 µF mentioned on page 19.
Furthermore I do not understand the sentence on page 19:
"By using a full-wave rectifier allows a smaller capacitor for C1 and
C2, almost 50 % smaller." Using a full-wave rectifier would mean to
se for RCT a value of 2 instead of 1. The resulting capacity is almost
wice as bis as with a value of 1.
Can somebody help me on this? Since I'm not familiar with formular (2)
I'm not sure if I can trust calculated results when starting my own design.
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In reply to DavitKhudaverdyan:
thanks for your reply and sorry for not getting back to this topic as fast as you did. The way you filled in the equation on wolfram alpha worked also for me. And you're
right there's a typo in the calculation. Luckily TI offers the WBENCH web utility to get an idea on how to select components for a certain input and output voltage.
Here you see the schematic which WBENCH produced for the following AC/DC conversion:
UIn: 230 V AC
UOut: 16 V DC
IOut: 150 mA
F: 50 Hz
As you see the capacitors Cin1 and Cin2 have been calculated to 1µ. Is that correct? The application is not far from that one shown in the UCC28881 datasheet in chapter 9. But there the calculated
value of CBULK is about 15.8 µF. Does it have to do with the AC input voltage range?
Furthermore I would like to know why D4 has to be able to conduct a current of 3 A?
Could you help me on this?
In reply to Markus Spiekermann:
I'm glad that the wolfram alpha calculation helped you out.
When we did that calculation, we did it with the idea that the Vin_min is 85V and the Vin_min_bulk used was 80V, and the switching frequency minimum was set to 57Hz. The calculator has been updated, and if you change the minimum and nominal values to suit your design, you'll get numbers that are close to the WEBENCH output.
As for the diode, I do not know the extent of the WEBENCH library, so I am assuming that the software chose the best fit from that library. I'll ask for some input on this and get back to you!
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