Useful Tips | Why does MLCC capacitor make noise? How to make it shut up!

Latest update time：2022-01-18

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With the popularity of laptops, mobile phones and other devices, the "howling" problem caused by capacitor vibration has attracted more and more attention. How to optimize the capacitor howling of each power supply architecture and make the capacitor shut up is an interesting question.

The whistling of MLCC capacitors is mainly caused by the piezoelectric effect of ceramics. Due to its special structure, when the electric field applied to both ends of the MLCC capacitor changes, it can cause proportional changes in mechanical stress. This is the inverse piezoelectric effect. When the vibration frequency falls within the range of human hearing, noise will be generated, which is the so-called "whistling". The direct piezoelectric effect is the opposite, and it is a process in which an electric field is generated under the action of force.

Whether it is a laptop or a mobile phone, the requirements for power supply are getting higher and higher. Usually, a large number of MLCC capacitors are connected in parallel on the power supply network. For example, in power supplies with BUCK and BOOST architectures, when the design is abnormal or the load working mode is abnormal, "howling" is easily generated.

In a laptop computer, howling is likely to occur when the computer is in sleep mode or the camera is turned on.

In mobile phones, the most typical example is the PA power supply used in GSM. The characteristics of this power line are large power fluctuations and a typical fluctuation frequency of 217Hz, which falls within the hearing range of the human ear (20Hz~20Khz). When a GSM call is made, it is easy to hear a "hissing" howling sound by listening to the capacitor on this power line with a special stethoscope.

How to suppress?

1. Buck power supplies usually have two working modes: PWM and PFM. The ripple is small in PWM working mode and is used under the condition of relatively high load power consumption. In order to avoid the switching frequency of charging the capacitor in the PWM working mode of the buck causing howling within the range of human ears, some power supplies will deliberately avoid the switching frequency of 20hz~20Khz.

2. When the power supply is in light load mode, it will work intermittently and output several pulses intermittently. The frequency of this intermittent pulse may also be heard by human ears. Therefore, it is also necessary to optimize the working frequency of the intermittent pulse when PFM is working from the perspective of power supply or load to avoid howling.

3. The other is an implicit state. In the early stage of the project, the system is often unstable. The load switches repeatedly between normal and low power modes. The power supply is also prone to switch repeatedly between PWM and PFM modes. This switching time slot may also cause howling. Software optimization of the system stability is required to avoid abnormal switching of load working modes to avoid howling.

4. If the saturation current of the BUCK inductor is not selected properly, the output current may increase, which may mistakenly trigger the power supply to enter overcurrent protection. The power supply repeatedly switches between normal working mode and overcurrent protection mode, which is called hiccup mode. There is also a certain possibility of causing howling. The inductor must be selected appropriately.

5. The switching power supply itself has large ripple. The multi-phase switching power supply has the advantages of small ripple and large current. By staggering the phases, the ripple of the power supply can be effectively reduced and the howling can be suppressed.

6. To suppress howling, in addition to the above-mentioned modifications to software, parameters, and architecture, a typical solution is to use anti-howling capacitors, such as Murata's KRM series and ZRB series.

Its special structure can reduce the capacitor's howling phenomenon, absorb stress caused by heat and mechanical shock, and achieve high reliability. Compared with Ta capacitors, the voltage fluctuation ⊿V of anti-howling MLCC is 7 to 22% smaller than the initial stage.

7. When laying out the circuit, you can also optimize the layout by staggering the capacitors to suppress vibration.

8. Some people even proposed digging grooves next to the capacitor to alleviate the howling.

The above is the principle of capacitor howling and suggestions for avoiding it.

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