Solution Guide

Products such as laptop and tablet PCs, smartphones, television sets, and automotive electronic devices sometimes make high-pitched sounds when they are running. This is a phenomenon known as "acoustic noise" and is sometimes caused by passive components including capacitors and inductors. The mechanics in acoustic noise are different between capacitors and inductors, but acoustic noise in inductors is particularly complex as it involves a mix of factors. This article introduces some causes of and effective measures against acoustic noise in power inductors, which are main components in power circuits of devices such as DC-DC converters.

Table of Contents

• Causes of acoustic noise in power inductors
• Mechanics of vibrations in the main bodies of power inductors and the amplification of sounds and noises
• Measures against acoustic noise in power inductors

Causes of acoustic noise in power inductors

Factors such as intermittent operations, frequency variable modes, and load changes generate vibrations of audible frequencies

Figure 1: The mechanics of the acoustic noise in power inductors

Power inductors of DC-DC converters are one of the causes of sounds and noises along with the increasing performance of electronic devices. DC-DC converters attain stable direct currents of fixed voltages by creating pulsed currents from ON/OFF statuses with switching elements and controlling the lengths (pulse widths) of the ON times. This is known as "PWM (pulse width modulation)" and is widely used as the mainstream method for DC-DC converters.

However, the switching frequencies of DC-DC converters are high ranging from several 100 kHz to several MHz, and the vibrations of these frequencies cannot be heard as sounds and noises exceed the human audible range. This leaves the question of why power inductors of DC-DC converters generate acoustic noises.

There are several possible causes, but one of the main causes could be DC-DC converters operating intermittently in order to save battery power, or DC-DC converter switching from the PWM method to the PFM (pulse frequency modulation) method and running on frequency variable mode. Figure 2 shows the basic principles of the PWM method and the PFM method.

Figure 2: The PWM (pulse width modulation) method and the PFM (pulse frequency modulation) method

Acoustic noise from intermittent operations of DC-DC converters such as during PWM dimming

Intermittent operations of DC-DC converters are incorporated in areas such as the automatic dimming functions of the backlight of liquid crystal displays in mobile devices for the purpose of, for example, saving energy. It is a system where battery life is extended by automatically dimming backlight brightness in accordance with the illuminance of the usage environment.
There are several methods of dimming, but the one that controls the length of period where an LED light is on and off is known as "PWM dimming". The dimming system with the PWM method is used for the backlight of devices such as laptop and tablet PCs because of their advantages including minimal changes in chromaticity due to dimming.

PWM dimming is a method of adjusting brightness by intermittently operating a DC-DC converter at a relatively low frequency of around 200Hz and repeating an on-off cycle. The brightness is increased if the lighting time is lengthened and decreased if it is shortened. There is very little flickering of the backlight that can be sensed by the eyes during intermittent operation at around 200Hz. However, it is an audible frequency so the main body of the power inductor mounted on a substrate can vibrate and generate an acoustic noise due to the effects of the flowing current from the intermittent operation.

Acoustic noise from DC-DC converters on frequency variable mode

DC-DC converters running with the PWM method feature the high efficiency rate of 80 - 90% or more during normal operation. However, efficiency drops significantly when load levels are low such as during standby. Switching loss is proportional to frequency. Therefore, efficiency drops because constant switching loss is produced even when load levels are low.

In order to fix this problem, a DC-DC converter, which automatically makes a switch from the PWM method to the PFM method when the load levels are low, is used. The PFM method is a method of controlling switching frequencies in accordance with the decreasing of the load with the ON time remaining constant. The switching frequency gradually decreases when the OFF time is lengthened because the ON time is constant. Efficiency at low load levels is increased when the frequency is decreased because switching loss is proportional to frequency. However, there may be acoustic noise in the power inductor if the lowered frequency reaches the range of about 20 to 20 kHz, which are audible frequencies.

Acoustic noise caused by load

Mobile devices such as laptop PCs incorporate various forms of power-saving technology in order to save battery power, but this is sometimes the cause of acoustic noise in the inductors. For example, CPUs of laptop PCs have a mode that periodically changes consumption currents in order to balance both low power consumption and processing capacity, but this can affect the power inductor and cause an acoustic noise, if this period is that of an audible frequency.

Mechanics of vibrations in the main bodies of power inductors and the amplification of sounds and noises

Vibrations that occur in the main bodies of power inductors generate acoustic noise through currents of frequencies in the audible range flowing in. Below are causes of the vibrations and the causes of amplified sounds and noises.

Causes of vibrations

1. (1) Magnetostriction (magnetic strain) of the magnetic core
2. (2) Attraction due to magnetization of the magnetic core
3. (3) Vibrations in the winding due to leakage flux

Causes of amplified sounds and noises

1. (1) Contact with other components
2. (2) Effects on surrounding magnetic bodies due to leakage flux
3. (3) Matching with natural vibration frequencies of entire sets including substrates

Figure 4 shows causes of vibrations that induce acoustic noise in power inductors and causes of amplified sounds and noises. Below is an explanation of the main causes.

Figure 4: Causes of vibrations that induce acoustic noise in power inductors and causes of amplifications

Causes that generate vibrations and their effects

Cause of vibrations (1): Magnetostriction (magnetic strain) of the magnetic core

There will be slight changes in the outer shape if a magnetic body is exposed to a magnetic field and is magnetized. This phenomenon is known as "magnetostriction" or "magnetic strain". Inductors with cores of magnetic bodies such as ferrites expand and contract due to the AC magnetic fields generated by the winding, and the resulting vibrations can sometimes be detected as sound.

Figure 5: Magnetostriction (magnetic strain) of magnetic bodies

Magnetic bodies are masses of small areas known as "magnetic domains" (Figure 5). The direction of the magnetic moments of the atoms inside magnetic domains is in unison so the magnetic domains are micro magnets in which spontaneous magnetizations are directed at constant, but magnetic bodies in their entirety do not show characteristics of magnets. This is because the many magnetic domains that compose the magnetic bodies are arranged so that the spontaneous magnetizations cancel each other and appear to be demagnetized.
The ranges of the magnetic domains change if these magnetic bodies in a demagnetized state are exposed to magnetic fields from outside, because each magnetic domain tries to be arranged in a way where the directions of the spontaneous magnetizations faces the same direction as the external magnetic fields. This is caused by displacement of the magnetic walls, which are the boundaries between the magnetic domains. The prevalent magnetic domains expand in area as magnetization progresses until there is a single magnetic domain at the end that faces the same direction as the external magnetic field (in a state of saturated magnetization). Minute changes of position occur on an atomic level during this process of magnetization so magnetostriction, that is to say, changes in the outer shapes of magnetic bodies occur on a macro level.
Changes in the outer shapes due to magnetostriction are very small at only about 1/10,000 to 1/1,000,000 of the original dimensions, but magnetic bodies repeatedly expand and contract while causing vibrations when they are wound with coils and alternating currents flow as shown in Figure 5. This is why the vibrations in magnetic cores caused by magnetostriction cannot be eliminated even in power inductors. Vibrations in single power inductors will be amplified and can be heard as acoustic noise even if they are at low levels, if they match the natural vibration frequencies of substrates when they are mounted.

Cause of vibrations (2): Mutual attraction due to magnetization of the magnetic core

Cause of vibrations (3): Vibrations in the winding due to leakage flux

The aforementioned acoustic noise from the mutual attraction between the drum core and the shielded core due to magnetization is a problem that does not arise with non-shielded type power inductors that do not have shielded cores. However, there is a different problem that arises with non-shielded types. The leakage flux acts on the winding because non-shielded types have open magnetic paths. Force acts on the winding in accordance with Fleming's left hand rule because electrical currents flow through it. Therefore, the winding itself may vibrate and cause acoustic noise if alternating currents flow through the winding (Figure 7).

Figure 7: Vibrations of the winding due to leakage flux

Causes of amplified sounds and noises

Causes of amplified sounds and noises (1) Contact with other components

Slight vibrations in inductors can be heard as acoustic noise if the inductors come in contact with other components in substrates of power circuits that have undergone high-density mounting in a large number of electronic components and devices.

Causes of amplified sounds and noises (2) Effects on surrounding magnetic bodies due to leakage flux

If there is a magnetic body such as a shield cover near an inductor, this may generate acoustic noise through vibrations caused by leakage flux of the inductor.

Causes of amplified sounds and noises (3) Matches with natural vibration frequencies of entire sets involving substrates

Vibrations in the air due to magnetostriction are normally not recognized as acoustic noise when it comes to the types of single compact magnetic cores used with inductors. However, multiple natural vibration frequencies that are audible will be generated and cause acoustic noise as the vibrations are amplified if an inductor is formed with a combination of multiple parts and mounted on a substrate. Furthermore, matching with the multiple natural vibration frequencies in the entire set can generate acoustic noise after embedding into the set.
Figure 8 shows examples of vibrations of substrates mounted on power inductors analyzed by computer simulation using the FEM (finite element method). An analysis model where the power inductor was placed at the center of the substrate (FR4) and two surfaces of the long sides of the substrates are fixed was used.
There are generally many characteristic values (natural vibration frequencies) at which structures resonate, and there are various vibration modes that are in accordance with them. Even with this [power inductor + substrate] analysis model, various vibration modes appear for each natural vibration frequency as the frequencies get higher. The power inductor is believed to be the source of the vibrations in the primary, secondary, quinary, and octodenary vibration modes shown in Figure 8. The vibration frequency of the primary mode is just about the same as the vibration frequency of a single power inductor. However, it is noteworthy that in the secondary mode where the vibration in the Z direction (height direction) is significant, frequencies appear extremely low when the power inductor is fixed to the substrate while they appear high with a single power inductor.

Figure 8: Examples of [power inductor + substrate] vibrations analyzed by computer simulation

Measures against acoustic noise in power inductors

Below are points for measures against acoustic noise in power inductors of DC-DC converters.

Point 1: Not letting currents of audible frequencies flow

Not letting currents of audible frequencies flow is the most basic measure.
However, please try the silencing measures such as the ones listed below if the conduction of audible frequencies cannot be avoided, such as in the case of intermittent operations or DC-DC converters on frequency variable mode for the purpose of saving energy.

Point 2: Not allocating magnetic bodies in surrounding areas

Do not allocate magnetic bodies that can be affected by leakage flux (such as shield covers) near inductors. If they must be allocated close to each other, please note the direction of their arrays while adopting shielded types of low leakage flux (with closed magnetic paths).

Point 3: Shifting natural vibration frequencies

Shifting or increasing natural vibration frequencies can sometimes reduce acoustic noise. For example, the natural vibration frequencies of entire sets that involve substrates can be changed by changing conditions such as the shapes, types, and layouts of the inductors as well as the fastening of substrates. Furthermore, the generation of acoustic noise can be found in relatively large power inductors of about 7mm or more in size. Adopting compact power inductors of 5mm or less in size raises natural vibration frequencies and can sometimes reduce acoustic noise.

Point 4: Making a replacement with a metallic integral molding type

As stated above, acoustic noise can be generated in gaps with full-shielded type power inductors due to the mutual magnetic attraction between the drum core and the shielded core. Vibrations of wires due to leakage flux can also produce acoustic noise in non-shielded type power inductors.
Replacements with metal casting types are an effective solution for such acoustic noise problems with power inductors. These are power inductors where air-cored coils have been buried and casted in soft magnetic metallic powder. The problem of vibrations in the winding due to flux can be avoided since there is no mutual attraction between the cores because there are no gaps and since the coils are integrated with the magnetic bodies and fixed. Furthermore, because TDK's products use metallic magnetic materials with low levels of magnetostriction, vibrations due to magnetostriction would be controlled and decreased acoustic noise could be expected by making replacements from non-shielded types and full-shielded types.

Comparisons of acoustic noise in full-shielded types and metallic integral molding types

We have conducted a study on the generation status of sounds and noises with full-shielded and semi-shielded type power inductors (TDK products of about 6mm in size) and full-shielded and metallic integral molding type power inductors (TDK products of about 12mm in size) as measurement samples. A microphone was placed in an anechoic chamber and the peak acoustic pressure was recorded with the sweeping of audible frequencies of 20Hz to 20kHz with power distributed for 60 seconds at 0A to rated current sine waves using the measurement samples mounted on the substrates (Figure 8).
As the graphs show, the levels of acoustic pressure differ according to frequency when comparing full-shielded types and semi-shielded types.
The differences are significant when full-shielded types and metallic integral molding types are compared. Sounds and noises at a level of around 30 to 50dB are generated over a wide frequency range with full-shielded types. On the other hand, with metal casting types they maintain the same low levels as background noise in wide frequency ranges, with the peak portions also being controlled at about 20dB when compared to full-shielded types. It can be seen that replacements with metallic integral molding types are effective as controlling of 20dB is on the level of 1/10.