Hello.
We have made two power supply designs, using the TI UCC29910A-730 and have experienced the same type of problems in both designs. Our designs are very similar to the design in the users guide for the UCC29910A-730 ( http://www.ti.com/lit/ug/sluu505a/sluu505a.pdf ). On page four there is a schematic, which i will use the components names from in the following description.
Our design is a 230Vin-84V out AC/DC PSU. To minimize the ripple, we have chosen a rather large primary inductor (L4). Our inductance is 180 uH, instead of 88 uH in the schematic.
Our problem is that the varistor VR3 tends to explode, when we apply voltage to the system. After removing it and measuring the voltage across the pads on startup, we can see that the voltage climbs to more than 500V for 25 ms. There is no transient etc. coming from mains, so it seems it is a design "feature". The capacitors C5 and C6 also seems to fail catastrophically from time to time (after we removed the varistor). The problem seems to be solved after installing a larger varistor V250LA20A that can clamp the large voltage.
Questions:
-Is the varistor VR3's primary function to handle the voltage overshoot? We initially though it was for transient protection.
-Is our large voltage overshoot caused by the increased inductance in L4?
-Is the correct solution to our problem simply to use a larger varistor and handle the voltage overshoot by clamping it with a varistor?
Thank you for your help.
Best regards
Peter Selmer Gade
I-NO