EE13 Flyback SMPS (TNY288DG) — Output drops from 12V to 8V under 32mA load, feedback loop investigated
Hi everyone,
I'm building a small flyback SMPS using a TNY288DG (TinySwitch-4) with an EE13 core transformer (126 primary / 16 secondary turns). The target output is 12V @ 500mA. The circuit topology is similar to the reference design in the TNY288DG datasheet, with a TL431CD + LTV-817D optocoupler feedback loop.
The problem: The output is a stable 12V at no load, but drops to ~8V under just 32mA of load.
What I've measured so far:
| Test Point | No Load | Under 32mA Load |
|---|---|---|
| Output voltage | 12V ✅ | 8V ❌ |
| BP/M pin | 5.9V ✅ | Not yet measured |
| TL431 REF pin | 2.5V ✅ | 1.37V ❌ |
| R7 top (8.2kΩ) | 12V ✅ | 8V (follows output) |
| R8 bottom (2.2kΩ) | 0V ✅ | 0V ✅ |
The feedback divider is R7=8.2kΩ, R8=2.2kΩ (targeting 12V output with TL431 2.495V reference). Calculated output: 2.495 × (1 + 8.2/2.2) = 11.79V ≈ 12V
At no load, TL431 REF sits at 2.5V and regulates correctly. Under load, the output collapses to 8V first, which then pulls the REF pin down to 1.37V — so TL431 appears to be a victim, not the cause.
What I've already tried:
- R5 (optocoupler pull-up): tried 100Ω, 200Ω, 1kΩ — no change
- Removed C8 (100nF reference bypass cap) — no change
My suspicion: The transformer may not be storing/transferring enough energy under load. The EE13 core with 126 primary turns might result in too high a primary inductance, limiting peak current and causing output collapse. However I'm not sure, and I haven't measured primary inductance yet.
Setup note: Currently prototyping on a breadboard, which I know is not ideal for SMPS. However the no-load regulation is perfect, so I believe the core issue is electrical rather than purely contact resistance.
Any guidance on what to check next would be greatly appreciated. Happy to share the full schematic or scope shots if needed.
Thanks!
Hello Elf,
Good day and thank you for choosing Power Integrations.
Unfortunately, we don't recommend testing our devices using a breadboard setup since it will affect the overall performance of the design due to many factors like the temperature rise of the device, unwanted parasitics and noise coupling that may prevent proper operation of the device like the one you're experiencing. Therefore, using a PCB board for actual testing and evaluation of the design is strongly recommended.
If you are interested in a ready-made evaluation board, we have the RDK-839, which is a 12 V, 1 A design (almost similar to your specifications) that operates in a universal input line. Below is the link to where you can purchase it, and I'm attaching its Gerber and design files in this thread if you wish to fabricate it yourself.
Additionally, I also highly recommend using our PI Expert online tool when creating a transformer design with our ICs.
Let me know if there's anything else that I can help you with or if you have any further questions or clarifications.
Thank you and I wish you a great day ahead!
Best regards,
Tommy