Hissing sound

[tin1078]

I am troubleshooting a switchmode flyback topology power supply with three outputs. When operating the power supply at maximum load and minimum input voltage (around 80Vac), a hissing sound can be heard. Increasing further the input eliminates the sound. What could have been causing this problem? Furthermore, disabling one output and once again operating the supply at full load the hissing sound can not be heard.

 

[Lewish]

I would say the hissing sound is voltage arcing across a very small gap. I would look ( or listen ) closely at the snubber circuit. I suspect you will find a component breaking down. At low input voltage, the snubber is dealing with a much higher kickback voltage than when you are at a higher line voltage. Continued operation this way in either kill your switch transistor or ruin you transformer. The transistor will probably go first.

Lewis