UCC28631: UCC28631 PSR Design: No output on Aux winding (15V) and Secondary (19V) when powering MCU from Aux

Part Number: UCC28631

Hello TI team,

I am using UCC28631 in a PSR flyback power supply design.
In my transformer design:

• The auxiliary winding output is 15 V
• The secondary winding output is 19 V

I encountered the following issue during hardware testing:

1. When there is no external power load connected to the 15 V auxiliary winding, the 19 V secondary output is stable and normal.
2. When I add an external load of about 2–3 W to the 15 V auxiliary winding (for example, powering an MCU), all outputs collapse, including:
   • the 15 V auxiliary winding output
   • the 19 V secondary winding output

So the current experimental phenomenon is:

• No load on auxiliary winding (15 V) → secondary output (19 V) works normally
• Add 2–3 W load on auxiliary winding (15 V) → all outputs become 0 V

I would like to ask:

1. In a UCC28631 PSR flyback design, is it acceptable to use the auxiliary winding to supply an external power load of 2–3 W, such as an MCU?
2. Could this issue be related to:
   • VDD UVLO / restart
   • insufficient auxiliary winding load capability
   • poor cross-regulation between auxiliary winding and secondary winding
   • VSENSE sampling disturbance caused by loading the auxiliary winding
3. Does TI recommend that the auxiliary winding in this kind of design be used only for controller bias supply, instead of directly powering external loads?
4. Which signals should I check first to identify the root cause?