Whistling refers to the phenomenon of hearing a similar "chirp" or "squeak" sound from a PCB board. For example, I have heard that some portable devices use cheap chargers that emit a considerable whistle.
There may be some devices or environments that do not notice or do not care even if they produce a whistle, but if you are in a quiet environment when you go to bed, you should be able to notice the whistle of portable device chargers and the like. In addition, audio equipment and other sounds that are heard outside of the played sound are more serious problems.
First, explain the mechanism of whistle.
Ceramic capacitors with high dielectric constants have the characteristics of dielectric deformation (distortion) when a voltage is applied to a dielectric. This is the opposite of the piezoelectric effect, known as the "inverse piezoelectric effect". In addition, this property is sometimes expressed as "piezoelectric" or "inverse piezoelectric". If a DC voltage is applied, only the corresponding distortion is generated, while if there is an amplitude voltage, the MLCC is periodically deformed and causes the PCB board to vibrate. If its frequency is 20Hz to 20kHz in the audible band, the sound can be heard.
The figure above is a more specific diagram showing the applied voltage in relation to the MLCC deformation. From the perspective of switching power supply, the output voltage is DC, including the ripple voltage caused by the switching frequency. The output ripple induces vibration of the MLCCs used as output capacitors.
In the PCB board, since the electrodes at both ends of the MLCC are welded, the deformation of the length direction between the electrodes (the blue double arrow in the figure) deforms the surface of the PCB board (the yellow-green double arrow in the figure), and so on repeatedly leads to vibration. The vibration is amplified through the conduction of the PCB board, and the sound pressure that becomes the degree that the human ear can hear is a whistle. Of course, the condition is that the frequency of vibration is audible.
Whistles are a typical phenomenon, what is the countermeasure?
The acoustic noise is not only related to the dielectric material and the shape of the capacitor, but also to the size and installation status of the PCB board, etc., and it is actually necessary to discuss the countermeasures and layout of the capacitor itself. In any case, it is quite difficult to make the scream disappear completely, and the method of improving to the tolerance can be used. Here are four countermeasures.
Improvement through materials
The LMCC was developed using a dielectric material with a low inverse piezoelectric effect, that is, a small deformation. Basically as shown in the figure on the right, the distortion of the low dielectric constant material is lower. For example, there are product groups such as the LD (Low Distortion) series that can reduce acoustic noise.
Improvements are made through board design
This is an improvement in the PCB board. For example, for the same power cord, as shown in the figure, the same MLCC is installed on both sides. The vibrations of the two MLCCs cancel each other out in opposite directions, and the vibrations are relieved.
Improvement by structure: LW (length-width) reverses the structure
The length between the electrodes of an MLCC is usually greater than the width. By shortening the length between the electrodes, the deformation between the electrodes that cause the PCB board to vibrate can be mitigated. As shown, a type of MLCC with a wider width than the electrodes is prepared. The figure called "RGC" is a reverse structure.
Improvement by construction: metal frame type
The metal-framed MLCC mentioned in countermeasures against bending stress also helps to improve acoustic noise. As immediately conceivable from the structure, the metal frame absorbs the vibrations of the MLCC.
What is the effect of the various countermeasures?
Among these four countermeasures, the metal frame can be expected to have a good effect. Looking at the test data, it is clear at a glance. As can be seen from the following data, the metal frame type can improve the sound pressure by about 30 dB compared with the standard product.
Regarding the countermeasures of the whistle, pay attention to the place
As mentioned earlier, the whistle is not only related to the material and shape of the MLCC but also related to the PCB board and installation, so some cases need to be studied from different angles. Not only the size of the improvement effect, but also the PCB layout and components may be changed in order to improve the acoustic noise. In reality, these can be limiting matters that sometimes require trade-offs.
For example, although the improvement of the metal frame type is good, the height of the component is limited and may not be used. Conversely, if you use the LW reversal type because the improvement is good and the height problem can be solved, you need to change the routing mode and layout. The following table summarizes the relationship between these restrictions and countermeasures for reference only.
Capacitor manufacturers can take comprehensive acoustic countermeasures including these methods, so it is also a good idea to consult the manufacturer.
In addition, does the acoustic noise adversely affect the reliability of the laminated ceramic capacitors themselves? For example, if the sound is not a problem, can you ignore the sound and continue to use it?
The whistle is thought to have no effect on mlcc itself. The vibration of the MLCC itself is very small, only in the micron to nanometer level. In contrast, piezo buzzers and ceramic oscillators that utilize the piezoelectric effect are products that actively utilize vibrations of up to tens of times and have full reliability. From this point of view, it can also be understood that the inverse piezoelectric effect of MLCC has no particular impact on reliability.
Source: techclass