|"While switch-mode power supplies operate at frequencies well above the range of human hearing, they can produce audible noise under certain load conditions. Potential sources of audible noise can vary widely. Noise can be the result of design flaws such as an oscillating output voltage, or it can be generated by noisy components such as capacitors or transformers. In some cases, the high-pitched whine or hissing noise you hear can be something as simple as a cooling fan oscillating at an unusual frequency or the supply 19s proximity to an external electromagnetic interference (EMI) source such as a fluorescent light or another converter causing a beat frequency due to the interaction of the two. With today 19s need for reduced standby power, converters often employ lower average switching frequency modes, which also tend to increase audible noise.|
Here is an approach to troubleshooting the most common sources of audible noise in flyback power supplies, along with potential solutions to the problem. All of these procedures can be conducted with an electronic load and a programmable ac source or Variac. Keep in mind that in some cases the audible noise your supply is generating may be so low that it will not present an issue if the power supply is intended for use in a sealed enclosure."