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Variable Audible noise in flyback TNY278

Posted by: RK4872 on 일, 06/13/2021

Hi

i have designed a flyback converter using TNY278 controller. input voltage 230v and output spec is 12v / 1A. in no load condition there is a little audible noise that can be neglected but when the load is added to the output the frequency and volume of the noise changes. in different load conditions i hear different frequencies. i suspected that there is something wrong with my feedback loop. would you please help me
i have attached the schematics of my converter. 

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TNY278.pdf 298 KB

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Submitted by RK4872 on 일, 06/13/2021

schematics

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Schematics 298 KB
Submitted by PI-Wrench on Wed, 06/16/2021

If your transformer is not varnished, it can cause audible noise due to the core magnetostriction. You might want to try dip-varnishing your transformer. Normally, you would use a transformer varnish formulated for the purpose, but even polyurethane varnish from the hardware store can be used for a simple test.

Submitted by PI-Wrench on Mon, 06/28/2021

TinySwitch regulates by skipping cycles, so there are patterns of cycle skipping that can excite resonant modes in the transformer core due to magnetostriction. In most cases, varnishing the transformer is sufficient to clear up the audible noise issue.