This application note explains the major sources of audible noise and offers useful tips to engineers to solve the audible noise problem in their Fairchild Power Switch (FPS) applications.
Switched mode power supplies using Fairchild Power Switches (FPS) can generate audible noise during burst switching operation at light load conditions. This document identifies the primary sources of this noise as mechanical vibration in transformers and piezoelectric effects in ceramic capacitors within snubber networks.
The note offers several practical techniques to mitigate these issues. These include varnishing or gluing transformer elements, implementing slope compensation circuits to reduce current peaks, replacing ceramic capacitors with film types, adjusting RCD snubber values, and modifying the feedback network to shift the fundamental frequency of burst switching away from the human ear's most sensitive range (2-4kHz).
Audible noise is primarily caused by mechanical vibration in transformers and piezoelectric characteristics in ceramic capacitors during burst switching operation.
The most effective method is to varnish or use adhesive material on the transformer to prevent physical movement of elements like coils and isolation tapes.
Slope compensation reduces the current peak in burst switching, which lowers the flux swing and consequently reduces mechanical vibration.
Film capacitors should be used instead of ceramic capacitors because they do not exhibit the piezoelectric characteristics that cause audible noise.