Hello.
I designed it as shown in the attached circuit diagram.
There is audible noise from the transformer at no load and full load.
When I turn off the dithering function, the noise disappears.
Are there any other solutions?
IC_MP8017 + MP6951
OUTPUT_5V/2.4A
Switching frequency_500khz
Hi sw-yun,
I could not find the part number online for the transformer you are using. What is the transformer that you are using ? Is it EP7 rated? Our datasheet and evaluation board highly recommends to use an EP7 transformer for this flyback converter. Something like this AC1584-AL
Also, the inductor is a wire wound type, which can be a problem as well, molded inductors are much better because they offer soft saturation and also decreases the audible noise generated from AC. We do have the MPL-AT series inductors which can be good for your design. A 6.8uH or 10uH should be good to choose.
From your schematic, I see you have a DNS resistor at M/D pin for dithering function which is given for PSR mode but you are implementing a Secondary Side regulation via optocoupler.
I would recommend adding a 15 ohm resistor between M/D pin and AGND and to reduce any transient noise further, adding a capacitor will be helpful if you are not using dither function, else leave it floating Please refer below to the snapshot from the datasheet
If this does not work out, I will suggest adding a bleeding resistor in parallel with the output capacitor which should discharge any leakage output with no load that can reduce noise to some level.
Let me know if you have any more questions on this.
Regards,
Adhish
