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TNY278NP low output voltage (12V -> 11.5V)

Posted by: Raul Diez on Mon, 07/19/2021


I have built a Flyback converter with a TNY278PN and the PI Expert Suite
with a little modifications.

- The primary inductance of the transformer (hand-made) is 835uH.
- Add a 150ohms resistor in the coupler (AN-14 recommendation).
- C5 replaced by two 2.2nF in series.
- R3,R4 deleted.
- R1 replaced by two 100Kohm in series.
- Coupler LTV817A -> PC821.
- Diode replaced by similar.

When I turn on power (AC100V 50Hz) the output not rise to the 12V and keep stable at 11.5V with not load.

Where is the problem?.

I attach the design sheets of the PI expert and one more with the modifications (8 pages in total)


PD. Excuse me for my ugly english
Raul Diez.

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Submitted by PI-Star Lord on Fri, 07/23/2021

Hi Raul.

Thank you for getting in touch.

I cannot see the attachments you are reffering to.

May you please re-post them?


Thank you very much.

Best regards

PI-Star Lord

Submitted by Raul Diez on Sun, 07/25/2021

Hi PI-Star Lord.


Thanks for your attention.


I re-post the attachment (1 PDF, 8 pages in total).


Please check it.



Attachment Size
TNY278PN 210719_0.pdf 1.18 MB
Submitted by PI-Star Lord on Tue, 07/27/2021

Hi Raul.

You can start by fine-tuning the values of resistor R6 and the 150 Ohms resistor you added (let’s call it R7)

In particular, R6 should be selected in such a way that VR2 is biased at 0.5mA (for an Iz = 5mA) when it goes on reverse avalanche condition. This makes sure that VR2 works near its knee voltage.

R7 limits the maximum feedback current during a possible load transient and it is used to adjust the output voltage.

In your case, I would start by modifying the value of R7 to 220 Ohm and repeat the measurements. If you see any improvements, you can fine tune R6 and R7 further.

If, after the fine-tuning process, the output voltage regulation does not satisfy your requirement, I would suggest opting for a feedback circuitry which uses a reference IC (TL431). PI Expert offers full support for this type of network and you can swiftly modify your design.

You can find further info on AN-90.

Hope this helps

Best regards.

PI-Start Lord