Reducing MLCCs’ piezoelectric effects and audible noise
With the increasing popularity of MLCCs (or ceramic capacitors) in electronic circuits due to their low cost and low profile, 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 (Multi Layer Ceramic Capacitor) 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 aging 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 (20 Hz – 20 kHz), the capacitor produces noise, the so called audible noise.
A 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.
Experimental Environment and Setup
The measurement results have been acquired using a highly sensitive Se Electronics 1000A microphone and analyzed with the Spectro Frequency Analyzer 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 2.5 kHz to 3 kHz (Figure 2) and has relatively low response at low frequencies. In other words, for the same SPL, a sound with a frequency of 3 kHz will appear louder to our ear than a sound with a lower frequency like 50 Hz.
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.
Basically, the higher the absolute voltage, the more important the capacitor expansion will be. Therefore, the volume variation of the capacitor will also be more important as the signal amplitude (lVmax – Vminl) increases, leading to higher SPL (Figure 3). Furthermore, a signal with a duty cycle close to 10% or 90% will generate much less “noise” (12 dB lower) than a signal with 50% duty cycle.
Finally, a signal with steep rising/falling edges (like a square wave) will cause a quicker deformation of the capacitor, thus higher SPL than a slower changing voltage like a sine wave.
Component Characteristics Influence
As its name implies, a MLCC is made of multiple layers, and the characteristics of the capacitor will definitely have an impact on the noise generated. For instance, for the same physical size, a lower capacitance will require fewer layers and will thus create less deformation as shown in the following formula:
Δt = change in thickness (deformation) [m]
n = number of layers
V = applied voltage across the thickness t [V]
d33 = piezoelectric coefficient for thickness (“i=3” direction) change [m/V]
However, while a lower rated capacitor usually exhibits a higher capacitance at a given voltage than a higher rated one, the latter will tend to generate more noise.
Different contact area (width, length) will have little impact on the noise created but for the same capacitance a thick capacitor will generate a lower SPL than a thinner one. It can be noted that the thinner the capacitor the higher the electric field (as the layers are closer) and the higher the bias effect. For example, a 1 mm capacitor will produce 13 dB more than a 2.5 mm one.
PCB Board Influence
As the PCB board will act as the resonator of the described spring mass system, the combination capacitor / PCB board is critical in terms of size, placement and layout. A capacitor soldered away from the board – with no other contacts to the board than the soldering material on its terminals – will not produce any audible noise.
The self-resonating frequency of the board being as difficult to measure as to control, it can be ignored. Generally speaking the thicker the board, the less entitled to deformation it is, and thus the lower SPL it produces. At the same time, the more important the board surface surrounding the vibrating capacitor, the louder will be the noise. A capacitor placed at the edge of the PCB will be preferred. Measurements showed that a reduction from 2 mm to 1 mm thickness worsened 5 dB while a reduction from 14 cm² to 5 cm² improved 6 dB.
Placed next to each other, capacitors generate higher overall SPL (+14 dB between a single capacitor and three placed in parallel). On the contrary, when placed symmetrically on each side of the PCB board as shown on Figure 4, capacitors tend to annihilate each other’s vibrations.
Capacitors Soldered on Each Side of the PCB Board
Audible Noise in Electronic Circuitry
With electronic devices held close to the human ears such as note-PC, tablets, smartphones, etc… this noise generation can quickly become annoying and be seen as a critical buying factor.
The constraints of manufacturers concerning spacings and costs mainly are sometimes not allowing a lot of freedom when speaking about capacitor height, PCB board characteristics or capacitor placement. One parameter that can however be worked out beforehand for a converter is the load transient response to sudden load or line changes. As this parameter will have a direct impact on the voltage amplitude variation across the output capacitor(s), IC manufacturers will try to improve this transient response to help solving the audible noise concern.
External compensation provides better flexibility, but requires usually extra resistor and capacitor. As in any trade-off, the user will have to decide whether space or performance is more important.
Alternatives to MLCCs
Some passive components manufacturers have managed to develop MLCCs with low acoustic noise by adjusting the dielectrics. They help to reduce the audible noise, without being able to suppress it completely, similar comments apply to all the applicative solutions such as modifying the layout, or selecting MLCCs with different characteristics.
Since the deformation of the capacitors’ layers is at the origin of this noise problem, Tantalum type capacitors can be preferred to MLCCs. Nevertheless, the chemical composition of these capacitors (MnO2) makes them unsafe as they can easily catch fire: a no-go for most of the applications. Other types such as aluminum electrolytic capacitors are usually not providing good enough electrical performances to be widely used.
Another alternative to these are the so-called POSCAPs. These onerous polymer / tantalum capacitors are getting popular in note-PC applications as their structure makes them noise free, and they outperform the electrical performances of all other types, especially when speaking about the bias effect. Fewer capacitors can then be used to get the same capacitance as several MLCCs placed in parallel for example.
Summary
There are various ways to reduce the audible noise generated by MLCC capacitors. Working on the PCB layout, the PCB board specification or the capacitor selection will help reducing the SPL level, without eliminating it. It might however be enough to reach the desired level. If the noise is persistent, it might be a smart move to change the capacitor type to a more suitable one. The solution might become a bit more expensive, but one gets what one pays for.
About the author
Nicolas Guibourg joined Texas Instruments Germany as systems engineer in 2006, where he works on application support and new product definition for the Display Power Converters group. He has an electrical engineering degree from ISEN – Institut Supérieur de l’Électronique et du Numérique (France).
References and acknowledgements
1. Andreas Maier for his time spent on the setup and measurements
2. T. Noji, K. Kawasaki, H. Sano, N. Inoue, K. Malhotra “Development of Multilayer Ceramic Capacitors with Low Microphonics”, Murata Electronics, April 2006
3. “SANYO Capacitor” presentation, October 2009
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