AN2755

Audible noise

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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 intermodulation 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 the vibration of wires or

ferrite which produces buzzing noise. Therefore to avoid this noise, 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 the bridge generate acoustic

noise because of the AC current that causes the electrodes to vibrate which produces

buzzing noise. Thus, in order to minimize the AC current, inserting a differential mode (pi-

filter between the bridge and the boost inductor) helps to reduce the acoustic noise and

additionally the EMI filter benefits. This type of filter decreases significantly the ripple current

that otherwise has to be filtered by the EMI filter. The capacitors to select are polypropylene,

preferably dipped type, because boxed ones are generally more at risk to generate acoustic

noise.

The grounding of the boost inductor may help, because 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.