Ask Direct to Engineers
What capacitor and inductor are the best for a switching power supply?
Issues With Mounting －Audible Ringing－
Capacitor – Part 7 -
－Okay, now teach me about “audible ringing”, another issue that arises in mounting. It seems that audible ringing is the phenomenon that one hears a sound like “jeee” or “keen” from the board. For example, in an inexpensive charger for a cell phone, there is quite a sound.
That’s right. Depending on the device or the environment there may be audible ringing that one doesn’t notice, or that is not an annoyance. But, as you mention, in things such as cell phone chargers I think it is annoying when you hear it in the quiet environment of going to bed. And for things like audio equipment it is a big problem if you hear a sound other than the sound that is being replayed.
－Can you first explain the audible ringing mechanism?
A ceramic capacitor with a high dielectric constant has the property that, when a voltage is applied to the dielectric, the dielectric deforms (distortion). This phenomenon is the opposite of the piezoelectric effect, and is called the inverse piezoelectric effect. Having this type of characteristic may be called either piezoelectricity or inverse piezoelectricity. If a DC voltage is applied then the effect is just the associated distortion, but if the voltage has an amplitude then the MLCC periodically deforms and makes the PCB vibrate. And if the frequency is in the audible range of 20 Hz to 20 kHz then it is heard as a sound.
The figure above shows a little more specifically what I am talking about. The graph shows the relationship between the applied voltage and displacement of the MLCC, and in the case of a switching power supply the output voltage may be DC but it also includes a ripple voltage that originates from the switching frequency. This output ripple may induce a vibration of the MLCC that is used as the output capacitor.
On the PCB, electrodes across the MLCC are soldered, so displacement in the length direction between the electrodes (the double blue arrow in the figure) causes a displacement of the PCB in the direction of the surface (the double yellow-green arrow), and when this happens repeatedly it is a vibration. This vibration is amplified through the PCB, and when it reaches a sound pressure of an audible level, it is an audible ringing sound. Of course, it only happens under the condition that the vibration frequency is in the audible range.
－I think audible ringing, in a sense, is a classic phenomenon, so what kinds of measures are there for it?
Audible ringing is related not just to the dielectric material and the shape of the capacitor but also to things like the size of the PCB and how the components are mounted on it, so it may be necessary to investigate measures both for the capacitor itself and for the layout. In any case it is pretty difficult to stop the audible ringing completely, so the approach is to improve the allowed range. Here I will discuss four such measures.
① Improvement in the Material
MLCCs with a low inverse piezoelectric effect, that is, MLCCs that use dielectric materials with small displacement, have been developed. Basically, as shown in the graph at right, for the material with the lower dielectric constant, the distortion is less. For example, there is a product group that can reduce audible ringing, called the LD (Low Distortion) series.
②Improvement in PCB Design
This is an improvement in the PC board. For example, as shown in the figure, the same MLCCs are mounted on both sides on the same power supply line. The vibrations of the two MLCCs have opposite phases, so they cancel out and the vibration is mitigated.
③ Improvement in Structure: L/W (length/width)-Reversed Structure
In general, for an MLCC, the length between the electrodes is longer than the width. By shortening the length between the electrodes it is possible to reduce the displacement between the electrodes that is the cause of board vibration. As shown in the figure, a type of MLCC is provided where the width is larger than the distance between the electrodes. In the figure, “RGC” stands for “reversed-geometry capacitor”.
④Improvement in Structure: Metal Frame type
MLCCs of the metal frame type, which have been discussed as a measure against bending stress, contribute to audible ringing improvement. I think you can easily picture this from the structure, that the metal frame absorbs the vibration of the MLCC.
－What do you think about the effects of these various measures?
Of the four, the metal frame can be expected to have a great effect. This is obvious if you look at the test data. I think you can see that between the standard product and the metal frame type, the sound pressure is improved by as much as about 30 dB.
－Do you have concerns about measures for audible ringing?
As I initially said, audible ringing is related not just to the material and shape of the MLCC but also to the PCB and mounting, so depending on the situation it must be investigated in various ways. It’s not just the amount of the improvement effect, but in order to improve audible ringing, it may be necessary to change the board layout or the parts. These have practical restrictions, so trade-offs are sometimes necessary.
For example, the metal frame type gives a large improvement effect, but such parts have heights, and so this might not be usable when there are height restrictions. On the other hand, if one uses the L/W-reversed type because an effect can be verified and there is no height problem, then it is necessary to change the wiring pattern and layout. Please see the following chart that summarizes the relationships among the restrictions and measures.
Capacitor manufacturers can support comprehensive measures against audible ringing, including these, so I think it is a good idea to contact the manufacturer.
－By the way, does audible ringing have a bad effect such as on the reliability of the MLCC itself? For example, if the audible ringing doesn’t bother you, it is okay to keep using the device with the audible ringing?
I don’t think there is any effect on the MLCC itself. The vibration of the MLCC itself is extremely small, from picometers to nanometers. Compared to that, equipment that uses the piezoelectric effect, such as piezoelectric buzzers and ceramic resonators, aggressively use vibrations that are tens of times larger, and have sufficient reliability. From this I think you can also understand that the inverse piezoelectric effect of an MLCC has no particular effect on reliability.
－Finally, I have a question about the Japanese word for “audible ringing”. In the industry, many people pronounce it “otonaki”. But from the viewpoint of the Japanese language, and the etymology, I think “nenaki” would be correct, so what do you think? It’s not important, but I wonder about it.
Yes, they certainly say “otonaki”. Engineers are science people, so I don’t think they neglect language, but I don’t know.