Has the transformer been varnished? It is relatively common for an unvarnished transformer to make audible noise since the core faces on the outer legs will rub together in response to the varying AC flux.

I have changed clamp circuit RCD with "Diode - Diode" (TVS + fast diode combination) circuit and also increased the value of filter cap C5 to 330uF. The auidible noise decreased significantly. It is negligible now. 

I'm not sure that  changing the primary snubber would have made much of a difference for the audible noise, but increasing the bulk cap value would definitely change the peak to peak B+ ripple voltage and perhaps move the average operating frequency away from the resonant frequency of the transformer core. As I previously pointed out, varnishing the finished transformer also helps to quiet audible noise.

Thank you for your comment. I will also apply varnish to the transformer.

Does the audible noise point to improper design and can ruin functionality of the circuit or it is general issue when designing AC/DC converter?

Regards,

Ahmet

The audible noise can be a consequence of the on-off control scheme used by the TinySwitch family for regulation. Audible noise can be experienced if the TinySwitch operates in pulse-bunching mode. Please post the circuit that you are using to regulate the battery charger - it may provide some insight to your problem. As I have mentioned several times before, varnish impregnating the transformer can go a long way toward quieting a noisy transformer, as it locks the core halves together and prevents them from vibrating against each other. It also helps prevent vibration in the transformer windings.

Thank you for your further explanation. I have attached the circuit to this comment.  In the circuit, I am using R15&R16 to limit current to about 600mA and using TL431 + Optocoupler diode Vf to set battery charging voltage to about 13.6V. 

Your current limit circuit may not be slow enough. If the current limit circuit tries to interact with the output ripple when it is active, it can cause pulse bunching and audible noise. You may find it beneficial to shift the position of the electrolytic compensation capacitor as shown in the attached markup and make the value of R14 somewhat larger. You may need to try several values of compensation capacitor, as 10uF might be a little too big for the job - maybe try starting with 1-4.7uF.

Hi,

I am sorry for my late reply. I redesigned the PCB and produced it, so it took time.

I modified my circuit according to your comments. I decreased electrolyte compensation cap. value to 2.2uF, increased base resistor value to 100 ohm. I also increase output filter capacitor value from 220uF to 470 uF . To decrease TNY268PN DC bus ripple, I also increased the capacitor value in the output of the full bridge rectifier circuit from 2.2uF to 22uF. Lastly, I changed clamp circuit from RCD to TVS + Fast diode pair. Right now, I cannot hear any audible noise under no load and also load conditions.

Thank you very much for your help.