LNK562 acoustic noise

Posted by: upda on 06/09/2016

Hi, I have a design with the LNK562 that is working according to my expectations apart from the fact that the acoustic noise from the transformer is strong when there is no/little load. Do you have any suggestions for this particular case or is it just a matter of transformer desgn, as per AN-24?

Thanks

Comments

Hello upda,

Did you varnish your transformer. In order to get the minimized audible noise, It is recommended that this value be below 1500 Gauss (150 mT) during normal operation.

Best Regards

Hello, thank you for reply.

Yes, the transformer was dip varnished. Flux was close to 200mT but later reduced to 150mT with only marginal difference in sound level.
The problem is most noticeable at zero load when the few amount of cycles enabled creates a lot of components in the audio range. Ideally one would want the current reduced in this mode as well, I guess.

Hello upda,

Thank you for the information. Are you able to measure the swicth frequecy when you have the worst audible noise point to see how much we get.

When you said a lot of components in the audio range, did you measure which component is the main source of the audio.

Best Regards

Well, the switch frequency is fixed for the LNK562, but the pulses come at a shorter spacing as a function of load.

At zero load they end up in the range of 1kHz.

At a little more load the pulses come often, and audible noise is present until loads take these components up to ~10kHz.

No pulse-grouping is observed.

Hello,

there are lots of components on the board will create the audible noise. I am not sure which one is the main cause.

In general, a surface mount capacitor can produce audio noise. For debugging, i suggest you can replace the primary clamp ceramic capacitor to polypropylene capacitor to see if you will get less noise. Another big noise source is from the bias winding ceramic capacitor, you can change to electrolytic cap to check if it helps or not.

Best Regards