With the popularity of devices such as laptops and mobile phones, the acoustic noise caused by capacitor vibration is receiving more and more attention. It is an interesting problem to optimize capacitor whining and make capacitors shut up.
The acoustic noise of MLCC capacitors is mainly caused by the piezoelectric effect of ceramics. As shown in Figure 1-37, due to the special characteristics of the ceramic material, when the electric field applied to the two ends of the capacitor changes, the mechanical stress of the ceramic material can change accordingly. This is called the inverse piezoelectric effect. When the vibration frequency falls within the audible range of the human ear, noise is generated, which is called "whining." The opposite effect is the direct piezoelectric effect, which is the process of a ceramic capacitor generating an electric field under the action of force. Simply put, capacitor whining occurs when the voltage at both ends of the ceramic capacitor changes, causing it to vibrate. When the capacitor is welded to the PCB circuit board, it will vibrate along with the board, and this vibration will produce sound that can be heard by people.
 |
| Figure 1-37 Piezoelectric Effect of Ceramic Capacitor |
Both laptops and mobile phones have increasingly demanding power requirements. Usually, a large number of MLCC capacitors are connected in parallel on the power network, such as BUCK and BOOST topology structures. When there is an abnormal design or load working mode, it is easy to produce "whining." In laptops, whining is easily produced when the computer is in sleep mode or when the camera is activated. In mobile phones, the most typical case is the PA power used by GSM, as shown in Figure 1-38. The characteristics of this power line are large power fluctuations, with a typical fluctuation frequency of 217Hz, falling within the audible range of the human ear (20Hz-20KHz). When making a GSM call, a special fault stethoscope can be used to listen to the capacitor on this power line (note that the vibration probe of the stethoscope should be in contact with the capacitor body, not the metal pin of the capacitor. The electronic stethoscope converts the vibration into sound and outputs it through the connected headphones. This is a common method for locating device sound sources). It is easy to hear a "sizzling" whine, and even when the GSM call signal is poor at night, people with sensitive ears may hear the phone "sizzling."
 |
| Figure 1-38 Numerous Ceramic Capacitors on Mobile GSM Circuit |
How to suppress capacitor whining?
👉1. Switching power supplies usually have two working modes: PWM and PFM. The PWM working mode has small ripple and high efficiency and is used in conditions where the load power consumption is relatively high. In order to avoid the switch frequency of the BUCK entering the audible range of the human ear and causing whining during PWM working mode, some power supplies deliberately avoid the switch frequency of 20Hz-20KHz. When the power supply is in light load mode, it will work intermittently and output several pulses. The frequency of this intermittent pulse may also be audible to the human ear. Therefore, the working frequency of the power supply can be optimized from the perspective of the power supply or the load to avoid whining. Figure 1-39 shows the waveform of the switch node VSW in the PFM mode of a switching power supply.
 |
| Figure 1-39 PFM Mode Switching Waveform of Switching Power Supply |
👉2. Another implicit state is that in the early stages of the project, the system is often unstable, and the load switches back and forth between normal and low power modes, which makes the power supply easily switch between the PWM and PFM modes. This switching gap (sometimes PWM and sometimes PFM) may also cause whistling and requires software optimization to improve system stability and prevent abnormal load switching from causing whistling.
👉3. When the saturation current of the BUCK inductor is improperly selected, the output current may increase, as shown in Figure 1-40, which may trigger overcurrent protection of the power supply. After a period of time, the voltage returns to normal operation, and the power supply switches back and forth between normal operation mode and overcurrent protection mode, which is commonly known as "hiccup mode" and may also cause whistling. The inductor selection must be appropriate.
 |
| Figure 1-40 Current waveform of switch mode power supply |
👉4. The switch mode power supply itself has a large ripple, and multi-phase switch mode power supplies have the advantages of small ripple and large current. By staggered phase, the ripple of the power supply can be effectively reduced and whistling can be suppressed, as shown in Figure 1-41.
 |
| Figure 1-41 Current waveform with phase stagger |
👉5. In addition to the above software, parameter, and architecture modifications, a typical solution to suppress whistling is to use anti-whistling capacitors, such as Murata's KRM and ZRB series and Samsung's noise reduction capacitors, as shown in Figure 1-42. Its special structure can reduce the whistling phenomenon of capacitors. The cushioning pad between the capacitor and PCB can absorb stress caused by heat and mechanical impact, relieve the phenomenon of capacitor vibration causing circuit board vibration and sound due to reverse voltage effect, but this type of capacitor is usually very expensive.
 |
| Figure 1-42 Anti-noise capacitor |
👉6. When laying out the circuit, the layout can also be optimized by arranging the capacitors in an interleaved pattern to offset each other's vibration.
👉7. Some people have even proposed a solution of digging grooves next to the capacitors on the PCB to alleviate whistling, as shown in Figure 1-43.
 |
| Figure 1-43 PCB with grooves |
Comments
Post a Comment