Hi
I used the WEBench designer for an AC/DC design using the UCC28C54, and got the design in the attached PDF.
I built it according to the design, but the UCC28C54 does not start.
Digging into the details I notice two things.
First of all, the voltage level at pin 7, VDD is around 3 V. And that's not strange, as the DC level from the diode bridge is about 320V, and the Voltage divider made up of the Rstart resistors and the Rfbt+Rfbb resistors yields about 3V at pin 7 VDD.
The UCC28C54 needs 14.5V to startup.
Secondly, the UCC28C54 datasheet says that the capacitor Cvdd (or C12 in the datasheet) needs to be 22uF and that the IC not starting up is a common error caused by this capacitor being too small.
In the WeBenchdesign it is only 3.3uF.
So my question is - How is the WeBenchdesign supposed to work, and how can I get it to work?
Please note that I replaced the 3.3uF Cvdd with a 22uF capacatior, but it did not make any difference.
I also tried lowering the Rstartup resistors to give about 18V, but for some reason it seems not to go higher than 12.6V.