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Minimise MLCCs' piezoelectric effects, audible noise

Posted: 31 May 2012     Print Version  Bookmark and Share

Keywords:multi layer ceramic capacitors  Tantalum Electrolytic  Sound Pressure Level 

Multi layer ceramic capacitors (MLCCs) in electronic circuits are gaining popularity due to their low cost and low profile. However, their inherent piezoelectric effect exhibiting audible noise can become a problem as more and more electronic devices tend to be handheld.

Objectives and background
MLCCs show numerous advantages compared to commonly used Tantalum Electrolytic Capacitors which include:

 • A very low Equivalent Series Resistance (ESR)
 • A very low Equivalent Series Inductance (ESL), a small size
 • Lower ageing and a high reliability of its dielectric
However like all ferroelectric dielectrics, it is affected by the piezoelectric effect: certain materials generate an electric potential or electric field on the surface by mechanical deformation. If the dielectric is now subjected to a varying electric field intensity that operates at a frequency which is located in the audible frequency range of the human ear (20Hz – 20kHz), the capacitor produces noise, the so called audible noise.

An MLCC alone is in most cases not sufficient to generate a problematic or disruptive Sound Pressure Level (SPL). But soldered on a PCB board the MLCC generates a spring mass system, which increases or dampens the oscillations depending on the frequencies (figure 1). Influences, possible causes and solutions to reduce audible noise in ceramic capacitors are investigated and discussed in this article.

Figure 1: MLCC deformation under electric field influence.

Experimental environment, setup
The measurement results have been acquired using a highly sensitive Se Electronics 1000A microphone and analysed with the Spectro Frequency Analyser 2.0 software. All the following numerical results are not meant to provide absolute data but rather to be used for relative comparison to each other in order to understand the different influencing factors of the audible noise in MLCCs. All reasons for such "noise" are not entirely understood yet and this article mostly exposes facts while not trying to explain why such a parameter produces such an action. In most cases, common sense can explain the reasons of the "noise". In others, the reader's skills will be roped in.

Frequency influence
The response of the ear to sound is dependent on the frequency of the sound. The human ear can peak response around 2.5kHz to 3kHz (figure 2) and has relatively low response at low frequencies. In other words, for the same SPL, a sound with a frequency of 3kHz will appear louder to our ear than a sound with a lower frequency like 50Hz.

Figure 2: Human ear audible range.

The influence of the frequency will not be taken into account in the rest of this article.

Signal characteristics influence
If an alternative voltage is applied across a MLCC's terminals, the capacitor will contract and expand at the frequency of the signal, with a deformation amplitude depending on several factors.

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