LNK626 noise

4 posts / 0 new

We designed a 13 W dual output power supply with LNK 626 (flyback topology with zener clamp). Under light load conditions (below 30% of the maximum output current) we have a annoying noise coming from the smps. The frequency of the noise is about 8 to 20 kHz proportional to the output current.
To prevent this noise I dropped all EMC ceramic caps, replaced the bypass and snubber cap by foil types. I shorted all EMC inductors and tried varnished and potted transformers. But nothing helped.

The drain-source voltage looks good (small overshot, smooth settling withing 1µs). Eventhough I set a RC network to the feedback pin with no success. There are two to four consecutive switching cycles followed by a short pause. From out the datasheet of the LNK 626 this is a normal behaviour. Is it?
This pause is identical to the noise frequency. How can I adjust this "pulse frequency" to inaudible value?
Is there anything else I can do to prevent this noise?

Thanks for your help.

첨부 파일Size
smps.png35.67 KB


Can you share the PIExpert/PIxls transformer design dcoument?

Can you also share the PCB Layout and the switching waveform that you are describing.


I attached waveform plot and the pixls file.


I was not able to see complete pattern on the drain waveform that you have sent.

Can you also check to see if there are any oscillations on the feedback pin voltage?

If feedback pin waveform is clean then the other thing that you can try to minimize the audible noise is reducing the density in the transformer core.

Reducing the flux density to <1500 gauss can minimize the audible noise.

I did not see the PCB layout, would you mind sharing the PCB layout so that I can review.