Flyback converter fault

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Dear admin
I designed a fly-back circuit with TNY280 and I got wrong answers. I actually want 14.5 volt and 1 A in the out put but I after a while (about three seconds) the voltage drops down and it will goes up again. The other problems is that the output voltage varies with the load. Fore example, if I use 18 ohm as the out put load I get 14.5 volt in the out put and also have the above problem, but If I use 33 ohm in the out put I get 22 volt in the output and also face with the above problem, as well. I should notice that: as I rise the out put ohm, the out put voltage drops down sooner.
The schematic, further information and picture of the circuit is attached to this massage.


EE22 trans3.92 KB
schematic33.16 KB
implemented circuit in the lab2.82 MB

Hi Hamedn,

Thank you for using power integrations product. Can you please answer the questions below:

Can you please confirm the device you are using? TNY280P or TNY280G?
What type of operation are you using CV or CV/CC?
Can you send the full details of the transfomer? What inductance?
Are you not planning to use a bias winding?

Can you please get the waveforms on the Voltage and Drain current on the Primary on the
different scenarios that you are describing?

Please also probe the Voltage at Bypass pin and Output voltage.
It is reccommneded to use ceramic type on the bypass pin capacitor.

Based on the image you have sent, we usually recommend good layout practices to
ensure proper operation of the device.

To help you more on your design, Please use our online tool PI Expert online

This will provide you recommendations for schematic and layout and BOM.
It will also take care of the transformer calculations and help you choose the transformer properly.

you can also refer to the TinySwitch3 Design guide below

Thank you