UCC256403: output overvoltage because of frequency drop

Hello,

It sounds like your transformer turns ratio and your Lr/Lm ratio may not be correct in your design.

There is an excel tool that you can use to double check your design that you can find at the following link.

https://www.ti.com/tool/download/SLUC675

Regards,

Hello,
Thanks for the quick reply.
I have calculated the data of the transformer myself and the supplier of the transformer independently.
When entering the data into the above pecified tool, everything seems to be ok.

However, I do not understand why the UCC256403 neither increases the switching frequency nor switch to burst mode
even at low supply voltage when output power decreases.
(According to information from the transformer manufacturer, burst mode should not even be necessary.)

Which reasons would prevent an increase in switching frequency, especially as the primary current is visibly inductive?

Hello,

Do you have a schematic that I could review?

Regards,

Hello,

in the meantime, I have reduced the lower resistance RLL/SS from 56k to 39 k and can now regulate down to almost minimum power. (RLL/SS_Upper=100k)
Only at the highest input voltage the output voltage increases slightly at minimum load, although the switching frequency can still be higher according to the transformer manufacturer.

Confidence is rising :-)

Do you perhaps know a description for the optimum design of the BMTL/BMTH ratio?
I can't really make heads or tails of the data sheet ... :-(

Hello,

There is an application note sluaal2 that is a trouble shooting common issues with this device.  There is a section on the information you have requested.

This information should help improve your designs behavior.

https://www.ti.com/lit/an/sluaal2/sluaal2.pdf

Regards,