AN2994 datasheet(28/38 Pages) STMICROELECTRONICS

28 / 38 page

Audible noise

AN2994

28/38

Doc ID 15796 Rev 2

Audible noise

Differential mode currents in a circuit with high- and low-frequency components (like in a

PFC) may produce audible noise due to inter-modulation between the operating frequency

and the mains line frequency.

This phenomenon occurs because of the mechanical vibration of reactive components like

capacitors and inductors. Current flowing in the winding can cause the vibration of wires or

ferrite, and as such produce buzzing noise. To avoid this, the boost and filter inductors have

to be wound with a correct wire tension and the component has to be varnished or dipped.

Frequently, X-capacitors and filter capacitors after the bridge generate acoustic noise

because of AC current that causes the electrodes to vibrate, producing buzzing noise. Thus,

minimizing the AC current by inserting a differential-mode, Pi filter between the bridge and

the boost inductor helps to reduce the acoustic noise. The EMI filter will also benefit from

this. In fact, such a filter significantly decreases the ripple current that otherwise has to be

filtered by the EMI filter. The capacitors to be selected are of a polypropylene (preferably

dipped) type because boxed ones are generally more at risk of generating acoustic noise.

AN2994 Datasheet(PDF) 28 Page - STMicroelectronics