Audible noise

AN2485

20/29

4

Audible noise

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

(like in a PFC) may produce audible noise due to inter-modulation between operating

frequency and mains line frequency.

The phenomenon is produced because of mechanical vibration of reactive components like

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

ferrite which produces buzzing noise, therefore, to avoid this boost and filter inductors have

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

Frequently, X-Capacitors and Filter Capacitors after bridge generate acoustic noise because

of AC current that causes the electrodes to vibrate and produce buzzing (noise). Thus,

minimize AC current inserting a differential mode, Pi-filter between the bridge and the boost

inductor helps to reducing the acoustic noise and additionally the EMI filter will benefit too.

Such actions decrease significantly the ripple current that otherwise has to be filtered by the

EMI filter. The capacitors selected are polypropylene, preferably dipped type, because

boxed ones are generally more at risk to generate acoustic noise.

The acoustic noise can also be due to an incorrect circuit behavior for an abnormal current

modulation. It can be due to the Dynamic OVP intervention that stops the IC switching

cycles which consequently creates a discontinuity of the input line current shape that can

produce vibrations as described above, occurring typically at light load. In this case, only a

board re-layout can help, taking into consideration the points outlined in Section 3: Layout

hints. In addition, grounding the boost inductor may help, because in this case we decrease

the emissions from the most efficient "antenna" of our circuit. In fact, in case of improper

layout some picofarads of layout parasitic capacitance together with the very high dV/dt of

the MOSFET Drain voltage may inject noise somewhere in the circuit.

5

Thermal measures

In order to check the design reliability, a thermal mapping by means of an IR Camera was

done. Figure 35. and Figure 36. show thermal measures on board component side at

nominal input voltages and full load. Captions visible on the pictures placed across key

components show the relevant temperature. Table 1. provides the correlation between

measured points and components for both thermal maps. The ambient temperature during

both measurements was 27°C. According to these measurement results all components of

the board are working within their temperature limits.