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.