I am worried that the SMPS I bought might be faulty. I got a MeanWell 75W SMPS and when I connect only AC to the SMPS it has a high frequency hum sound.
However if I start drawing some power (5W) from it then the hum goes away. Is this normal?
I'm concerned because my application varies from 0 - 30W, depending on configuration and I certainly don't want a hum at low power consumption times.
Many SMPSs change frequency with light load.
The supply might run at 100khz above 3 watts. (above what you can hear) Below 3 watts it does 'cycle skipping'. So it might run at 50khz, 33khz, 25khz, 20khz.......... At no load it needs only one cycle out of 100 keep the voltage up. That drops the 'frequency' to 1000. That might be what you are hearing.