LM5161: Isolated fly-buck topology not working

Part Number: LM5161


I have an issue with LM5161 arranged in an isolated fly-buck topology (see the attached schematics).

My goal was to have a 3.5 V VMAIN (non-isolated) output and two 3.6 V isolated outputs, V_DMA and V_CSD.

The transformer model is T60403-F5046-X100 from Vacuumschmelze, with a turns ratio of 1:1.2:1.2. VIN_P is about 23 V.

What I get is 3.41 V on VMAIN (this could be acceptable), but 2.6 V on V_DMA and 8.4 V on V_CSD. The very strange thing is that I have two prototypes of the board, and on the other prototype the values on V_CSD and V_DMA are inverted (about 2.6 V on V_CSD and about 8.4 V on V_DMA). Moreover, the transformer gets hot on both boards.

The whole circuit (which has also one more LM5161, among other things, which provides the 10 V line) draws about 370 mA from the 24 V source; according to my design math this is too much, but I am unable to check currents on specific parts, at the moment, as I don't have "open circuits" tu use for the ammeter. Design math says that currents on V_DMA and V_CSD should be under 20 mA in idle conditions.

Do you have any suggestion on what to check or what to do in order to solve this issue?

Thanks in advance for your help.


P.S. Sorry for posting an "isolated" issue into a "non-isolated" forum, but when I wrote the part number this forum was automatically selected and I could not change it