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MLCC solutions for suppressing acoustic noise in the battery lines of laptop computers


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1. Introduction

2. Mechanisms of acoustic noise

3. Methods of evaluating acoustic noise

4. Evaluation of a laptop computer

5. Examples of replacement solutions for suppressing acoustic noise in a laptop computer

6. Summary

7. Introduction of low acoustic noise capacitors

8. Mechanism of acoustic noise (supplement)


SubTitleIconNo01 Introduction


Large numbers of tantalum capacitors and aluminum electrolytic capacitors have conventionally been used in electronic equipment, but are now being replaced with ceramic capacitors for reasons such as product downsizing and reliability issues.
As electronic equipment becomes more multifunctional and quiet, the “acoustic noise” caused by ceramic capacitors, which was not formerly noticeable, has become a major design issue in the power circuits of various devices, such as laptop computers, mobile phones (smart phones), DSCs, and flat-screen TVs.

Acoustic noise caused by the capacitors used in the battery line can be an issue for a laptop computer.
When the operating mode is changed, such as sleep mode or camera start-up, the internal operation of the laptop computer changes, so the level of the acoustic noise and how it is heard also changes, based on the operating mode.

In this report, we introduce the mechanisms by which capacitors produce acoustic noise and methods for evaluating and suppressing acoustic noise in the battery line of a laptop computer.


Laptop computer operating modes in which acoustic noise easily occurs
  • Sleep mode
  • PWM dimming of the LCD backlight
  • Camera start-up
  • Video playback


SubTitleIconNo02 Mechanisms of acoustic noise


Why do ceramic capacitors cause acoustic noise?

The mechanisms through which acoustic noise occurs and the methods used by Murata to evaluate acoustic noise are described below.


SubTitleIconNo03 Methods of evaluating acoustic noise


1) Measurement of sound pressure level

Acoustic noise is quantified by measuring the sound pressure level.
The measured object is set to the operating state inside an anechoic box, and a noise meter is used to measure the sound pressure level via a microphone.
In addition, the sound pressure level frequency characteristics are checked using a FFT analyzer to facilitate evaluation and solutions.



2) Measurement of circuit board displacement

The degree to which the capacitors vibrate the circuit board can be checked by measuring the circuit board displacement.

The circuit board displacement is measured by irradiating the circuit board with a laser while the measured object is in the operating state, and detecting the Doppler shift of the reflected light.


3) Measurement of voltage fluctuation

Whether the target capacitor may be a cause of acoustic noise can be investigated by measuring the “voltage fluctuation” at the capacitor end.

This investigation checks whether a ripple voltage in the audible frequency range (20 Hz to 20 kHz) is applied to the capacitor while the measured object is in the operating state.



4) Relationship between sound pressure level and circuit board displacement

When a capacitor is a cause of acoustic noise, the circuit board displacement increases at the same frequencies as the sound pressure level (the range inside the red dotted line).



5) Relationship between sound pressure level and voltage fluctuation

When the voltage fluctuation spectrum at the capacitor end increases at the same frequencies as the sound pressure level spectrum (the range inside the red dotted line), that capacitor is determined to be a cause of acoustic noise.


SubTitleIconNo04 Evaluation of a laptop computer


1) Difference in sound pressure level due to operating mode

When the operating mode of a laptop computer is changed, such as to sleep mode or camera start-up, the internal operation of the laptop computer changes, so the sound pressure level, circuit board displacement, and voltage fluctuation also change.
This means that evaluation should be performed in operating modes in which acoustic noise easily occurs.



2) What types of capacitors easily cause acoustic noise in a laptop computer?

2)-1 Schematic of laptop computer battery line

Large numbers of capacitors are used in the battery line (DC-DC converter primary side) of a laptop computer, and the use of ceramic capacitors in this battery line may produce acoustic noise.



2)-2 Reasons why capacitors in the battery line easily cause acoustic noise

  • Use of capacitors with large capacitance
    ⇒ Dielectrics with high dielectric constants undergo large expansion and contraction due to electric fields.
  • Power supply to CPU, camera, RF module and other circuits
    ⇒ Voltage fluctuation easily occurs.
  • High battery line voltage (10 to 20 V)
    ⇒ Reverse piezoelectric strain occurs easily in proportion to the electric field.


3) Circuit block diagram of laptop computer battery line



SubTitleIconNo05 Examples of replacement solutions for suppressing acoustic noise in a laptop computer


When acoustic noise is a problem in the battery line
Battery line capacitors (12 V power supply line)

1) Capacitor mounting locations

These capacitors are mounted in five different circuits (circuits A to E), such as the CPU circuit.


2) Circuit block diagram of battery line

We evaluated acoustic noise in the capacitors in the battery line indicated in pink, and took measures against it.





3) Replacement evaluation results

The data shown below are reference values obtained through our own evaluations. The sound pressure level reduction effects differ depending on the circuit board shape, parts mounting state, and other conditions. Note that effects are not guaranteed for all cases.

The sound pressure level differs depending on the operating mode, so acoustic noise was evaluated and solutions were proposed for four operating modes that have a high sound pressure level: camera start-up, sleep mode, video playback (LCD screen maximum luminance), and standby screen.

※All evaluations were performed in the fully charged state with the AC adapter connected.



The sound pressure level could be reduced as shown below by replacing all of the battery line capacitors with low acoustic noise “KRM” capacitors.


The detailed results for each operating mode are shown below.

<Detailed results of replacement (1/4) > Camera start-up




<Detailed results of replacement (2/4) > Video playback



<Detailed results of replacement (3/4) > Sleep mode



<Detailed results of replacement (4/4) > Standby screen



Change in sound pressure level due to replacement with low acoustic noise capacitors (KRM) circuit by circuit (1/2)

The battery line capacitors (area enclosed in the pink frame) are located in the same power line before splitting into each circuit at the DC-DC converter, so they all experience approximately the same voltage fluctuation.
The capacitors in this battery line were replaced with low acoustic noise capacitors circuit by circuit, and the effects were evaluated.



Change in sound pressure level due to replacement with low acoustic noise capacitors (KRM) circuit by circuit (2/2)

The battery line capacitors shown in the figure above were replaced from normal capacitors (GRM) to low acoustic noise capacitors (KRM) in order, from circuit [A] to circuit [E].
The sound pressure level decreased gradually as the number of capacitors replaced with KRM series capacitors increased.

The sound pressure level can be reduced even further by replacing not just some, but all of the capacitors with low acoustic noise capacitors as a solution to suppress acoustic noise in the battery line.



SubTitleIconNo06 Summary


1)Mechanism of acoustic noise

Acoustic noise is produced by capacitors as follows. When an AC electric field is applied to a capacitor made of material with a high dielectric constant, the capacitor deforms and vibrates. This vibrates the circuit board, which results in the issue of “unpleasant noise” when the vibration cycle is in the audible frequency range (20 Hz to 20 kHz).

2) Method of evaluating acoustic noise

The sound pressure level is measured, and the acoustic noise is quantified.
The voltage fluctuation and the circuit board displacement are measured and evaluated, and the causes of the acoustic noise are investigated.
The current capacitors are then replaced with low acoustic noise capacitors, and the sound pressure level is measured and evaluated to confirm the effects.

3) Capacitors and operating modes that easily produce acoustic noise in a laptop computer

Capacitors are used in the battery line (DC-DC converter primary side) of a laptop computer.
This battery line is typically high voltage and supplies power to circuits that have high power consumption. This means that voltage fluctuation easily occurs, which results in operating conditions in which capacitors easily produce acoustic noise.
In addition, operating modes in which acoustic noise easily occurs include sleep mode, camera start-up, PWM dimming of the LCD backlight, and video playback.

4) Evaluation of replacement in a laptop computer

When the operating mode is changed, the internal operation of the laptop computer also changes, so it is necessary to perform evaluation in each operation mode in which acoustic noise easily occurs.
When multiple ceramic capacitors are used in a battery line (DC-DC converter primary side), the sound pressure level can be suppressed even further by using low acoustic noise capacitors for not just some, but all of the battery line capacitors.

SubTitleIconNo07 Introduction of low acoustic noise capacitors


<Factors influencing acoustic noise, and solutions>

When acoustic noise due to the influence of ceramic capacitors becomes an issue, Murata proposes solutions to suppress this acoustic noise, such as the use of low acoustic noise capacitors or modification of the component layout, depending on the factors influencing the acoustic noise.



 



 


SubTitleIconNo08 Mechanism of acoustic noise (supplement)


The ferroelectrics used in multilayer ceramic capacitors always have piezoelectric properties.
When an electric field is applied, deflection occurs and the chip expands and contracts, so acoustic noise is produced.