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.
Sound waves are elastic waves that pass through air and a human hears the frequency domain of about 20 to 20 kHz. The main bodies of power inductors of DC-DC converters vibrate when alternating currents and pulse waves of frequencies in the audible range flow, and this results in acoustic noise which is sometimes called "coil whine" (Figure 1).
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
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.
With DC-DC converters, the ratio of the switching period (ON time + OFF time of switching elements) to the ON time is called the "duty ratio". In the case of PWM dimming with LED lights, ON time of the light / (ON time + OFF time of the light) is the duty ratio, and it indicates the degree of brightness.
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.
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.
Inductors let direct currents flow smoothly, but they generate electromotive force in the direction for deterring changes from self-induced effects and behave like resistance when it comes to currents that change such as alternating currents. This is when inductors change electrical energy into magnetic energy and store it, or change it into electrical energy and emit it. The size of this energy is proportional to the inductance values of inductors.
Power inductors, also known as power coils or power chokes, are major components used in power circuits running on a switching method in devices such as DC-DC converters, and they play the role of smoothing the high-frequency pulses created by the ON/OFF statuses of the switching elements through the coordination with capacitors.
High currents flow in the power inductors of power circuits so winding types are mainstream. High inductance values and smaller sizes can be attained with a lower numbers of coils by using high-permeability magnetic bodies (ferrites and soft magnetic metals) at the cores. Figure 3 shows the basic circuitry of DC-DC converters (non-isolated type / chopper method) using power inductors.
Figure 3: Basic circuitry of DC-DC converters (non-isolated type / chopper method)
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
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
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.
Figure 6: Acoustic noise due to mutual attraction between
a drum core and a shielded core
Magnetic bodies show the characteristics of magnets and become mutually attracted with surrounding magnetic bodies when they are magnetized due to external magnetic fields. Figure 6 shows an example of a full-shielded type power inductor. This is a power inductor with a closed magnetic path and it has a gap between the drum core and shielded core (ring core), where sounds and noises are generated. This is a result of magnetized drum core and shielded cores attracting each other through magnetic force due to magnetic fields generated by the flowing of alternating currents in the winding, and sounds and noises can be heard if the vibrations are in the range of audible frequencies.
The gaps between drum core and shielded cores are closed with adhesives, but vibrations arising from mutual attraction cannot be completely suppressed because materials that are too solid may create cracks due to stress and therefore cannot be used for this purpose.
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
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.
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.
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
< Analysis model > The power inductor is allocated in the center of the substrate (FR4).
Boundary condition: The two surfaces of the long sides of the substrates are fixed.
Below are points for measures against acoustic noise in power inductors of DC-DC converters.
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.
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).
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.
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.
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.
Figure 9: Evaluations of noises and sounds with each type of power inductor
|Sweep frequency||20Hz to 20kHz|
|Sweep time||60 seconds|
Metallic integral molding type power inductors by TDK are effective in reducing acoustic noise, and are optimal in situations such as when they must be allocated near signal lines as they have extremely low leakage flux. Please see the application note "Selection Guide for Power Inductors in Consideration of Leakage Flux" for details.
The types of power inductors by TDK that use ferrite cores come in a wide variation of inductance and are characterized by their ability to support high inductance values. They are also excellent when it comes to mass productivity so they are adopted in various kinds of devices.
The various types of power inductors each have their own characteristics and benefits in use. Please find the right type for the right situation and let it help with your manufacturing.
The entire product series of optimal inductors for power supplies can be viewed by shape and type. Furthermore, detailed information can be seen by clicking the product series.