LNK623 Offline Flyback output voltage drift problem

Posted by: atanyeli on 12/20/2015

I have designed an 220V AC to 12V DC/0.5A offline flyback converter with LNK623 using PI expert tool.

Regulation is being done by an auxiliary winding and no optocoupler stuff is used. Transformer have

also been produced exactly the tool's suggestions.

While testing the first five prototype I have measured initial output voltages from 11.5 to 11.8V at half load.

So far so good. The behaviour I see from each of the prototypes; the output voltage start drifting in positive

direction gradually from it's initial value. It reaches the values above 14.5V after an hour. It drops around

1.5 to 2V when I apply airflow via a small fan and start increasing after the fan stops. Since it's operating at

half load, no significant heat is produced.

Have anybody ever experienced this problem before?

Thanks and regards

Atilla Tanyeli

Comments

How did you do the lay out of your board? Since it's not direct feedback and an auxiliary winding feedback , selection of feedback components makes a big difference and lay out of feedback components makes a difference. Send us the schematic.

Please go through below Design Guide:

https://ac-dc.power.com/sites/default/files/product-docs/an45.pdf

Please find attached the document contains the schematic and PCB photos.

Componet selection is done according to the PIExpert tool's suggestions.

Thanks

Attachment	Size
atanyeli flyback.pdf (227.5 KB)	227.5 KB

In the Schematic i see an RC circuit which is connected to the FB pin , however i didn't see it on the lay out , there is no Ref. Designators.

Please go through the page No :14 in the attached design guide , lay out recommendation for the design. Feedback circuit is important in the lay out and which causes the output voltage to drift.