UCC28700-Q1: Need help reviewing the design including transformer

Natthapol,

Thanks for your interest in TI here.

To better understand what's going on could you provide transformer inductance(primary magnetizing), and waveforms showing Vcc, DRV, and the aux winding waveform at the transformer? (Preferabley together on the same scope shot if possible). If we get all those signals we should be able to understand how the controller IC itself is behaving on your board.

Additionally can you share the full Webench design document that prints out all specs and values?

Webench doesn't support multiple outputs yet so if you did not do already I suggest to be sure you add up the total output power and use that to calculate as if it was a single output design with the total output power of your full design. That will ensure the peak current and magnetizing inductance are more realistic.

The diode differences you mentioned may actually impact this as the turns ratio and regulation point do need to take into consideration the diode drop.

If you want even more flexibility to look at the details of the design procedure beyond Webench you can also run your design through our excel design calculator here:

That will allow you to see more on the impacts of each component selection. You would still need to add the total power and use that in the calculations as the calculator is focused on a single output design.

Additionally, since it is the weekend and most of our applications engineers will not be working again until Monday, you may consider to review the following debug resources we have available:

I hope we can resolve this with you very soon. Let me know if you have any questions on what I've suggested.

Regards

John

Ok; thanks. Let us know.

Hi Eric,

this is actually seems like good trick, I have never thought of doing that before. Let me think this through first, it seems like a good idea.

I understand that the auxiliary voltage is taking from the secondary winding during MOSFET off phase.

The information in the datasheet, also show that the auxiliary winding voltage sometime goes below 0V, during the energy transfer state. I think the LDO on auxiliary winding method will have a problem here.

Yes, but you use a diode and capacitor to rectify the voltage before the LDO. Just like D3 and Cdd in your schematic creates Vdd for the IC.

Hello John,

i have replace the diode on the secondary side with low Vf Schottky and done some measurements but the isolation transformer that i was using, decided to stop working. So i will give you what i have from the last measurement session until the replacement transformer arrive.

I still see no voltage on auxiliary winding, not even a slightly move. On the secondary side however, does rise to about 0.5V then drop back to 0V.

The measurement was done with no load on the secondary side, but there is a 154Ohm resistor across each secondary. I also could not get all the measurements on the scope shot.

VDD

VDD ramp to 21V and restart.

DRV

DRV sends 3 pulses

I have also attach the WEBENCH design report.webench_design_4899890_28_767448723.pdf

here are some more measurements

CS pin

VS pin

Auxiliary winding at transformer pin (1 switch pulse)

Primary winding = ~340V

Isolated secondary winding at transformer pin

non-isolated secondary winding at transformer pin