Howling is the term for the "creeping" sound that can occasionally be heard when using electronic devices such as laptops, tablets, cellphones, televisions, and in-car electronics.

Passive components such as capacitors and inductors could be the root of the "howling."

The howling of capacitors and inductors operates on separate principles, with inductors howling for a variety of intricate causes.

We shall discuss the root reasons for power inductors screaming, the essential elements of power circuits such as DC-DC converters, and practical remedies in this post.

Ⅰ. Power Inductor Howling Causes

1. Intermittent work, variable frequency mode, load changes, etc. may cause audible frequency vibration to the human ear

Human hearing is capable of picking up "sounds" in the 20 to 20 kHz frequency range. Sound waves are elastic waves that are traveling in the air. The main body of the inductor vibrates in power inductors used in DC-DC converters when alternating current and pulse waves of frequencies audible to human ears are present. The whistling phenomena can occasionally be confused with this occurrence, which is known as "coil noise" (Figure 1).

Figure. 1 Power Inductor Howling Mechanism

The power inductors of DC-DC converters have also evolved into one of the noise generators as the capabilities of electronic equipment keep growing. A pulse-like current is produced when switching devices ON/OFF the DC-DC converter. A constant voltage and a consistent DC current can be achieved by adjusting the ON time (pulse width). PWM (Pulse Amplitude Modulation) is the name of this technique, which is frequently employed as a standard technique for DC-DC converters.

However, the DC-DC converter's switching frequency is considerable, ranging from several hundred kHz to several MHz. No noise is felt since the vibration at this frequency is higher than what the human ear can hear. So why does the DC-DC converter's power inductor create a "creepy" whistle?

There are numerous potential causes. The first option is to switch the DC-DC converter from the PWM method to the PFM (pulse frequency modulation) method, or to allow the DC-DC converter to operate sporadically in order to conserve battery power. When using the variable frequency mode. Figure 2 illustrates the fundamental ideas behind PWM and PFM.

Figure. 2 PWM (pulse amplitude modulation) method and PFM (pulse frequency modulation) method

2. Howling caused by intermittent operation of DC-DC converters such as PWM dimming

The automated dimming feature of mobile devices' liquid crystal displays' backlights has been developed to work intermittently in order to save energy, among other things. This system increases battery life by automatically adjusting the backlight's brightness in accordance with the amount of ambient light.

There are several different dimming techniques, and one of them is PWM dimming, which regulates the LED's on and off times. The PWM dimming technology, which is mostly utilized in the backlight of laptop and tablet computers, has the advantage that the chromaticity variations brought on by dimming are smaller.

PWM dimming regulates the brightness by frequently turning on and off the DC-DC converter, which makes it operate at a lower frequency of about 200Hz. When the on time is modified to be longer, it will get brighter in a continuous cycle of on/off, and if it is shortened, it will become darker. The eyes essentially do not see the backlight stroboscopic scenario in the intermittent operation of 200Hz. However, because the frequency is within the range of human hearing, whistling will occur when the intermittent operating current passes through the power inductor installed on the substrate. This is because the inductor's main body will vibrate under the impact of frequency.

Note: Duty Cycle

In a DC-DC converter, the ratio of the ON time to the switching cycle (ON time + OFF time of the switching device) is called the duty ratio. When PWM dimming LEDs, the ON time/(ON time + OFF time) is called the duty cycle and represents the brightness.

3. Howling due to frequency variable mode DC-DC converters

The PWM mode DC-DC converter's unique feature is that, when used normally, its efficiency can reach as high as roughly 80%–90%. However, the efficiency will be significantly decreased in low load situations, such as standby time. Switching losses are inversely correlated with frequency. Due to frequent switching losses at light loads, efficiency is decreased.

In order to solve this issue, a DC-DC converter is employed under light load conditions that automatically switches from PWM to PFM (pulse frequency modulation). The PFM approach is a way of reducing the switching frequency while maintaining a constant ON time. Since the ON time is constant, the switching frequency will steadily decrease as the OFF time is lengthened. Lower frequency can produce improved efficiency at light loads since switching losses are proportional to frequency. However, the power inductor will whistle as the lowered frequency approaches the audible range of roughly 20–20 kHz.

4. Howling caused by load

Mobile electronics, such as laptop computers, use a variety of power-saving methods that could cause the inductor to whistle in order to preserve battery life. For instance, notebook computer CPUs offer a mode that periodically alters the current consumption to balance low power usage and processing power. The power inductor may experience this impact when the cycle is within the audible range of the human ear. There is howling produced.

Note: The role of power inductors in DC-DC converters

Direct current can flow smoothly through inductors, while alternating currents, which change through self-induction, can be stopped from changing by the generation of electromotive force, which then acts as resistance. The inductor now transforms electrical energy into magnetic energy, accumulates it, and then transforms it back into electrical energy. The value of the inductor's inductance directly correlates with the intensity of this energy.

The primary components in switching power supply circuits, like those found in DC-DC converters, are power inductors, also referred to as power coils and power choke coils. Pulses are more supple.

The power inductor of the power supply circuit has a high current flow, so the winding type is the most popular product. This is due to the fact that a high inductance value can be obtained with a minimal number of coils by employing a high permeability magnetic material (ferrite or soft magnetic metal) for the magnetic core, making the final product more compact. A power inductor is used in Figure 3 to depict the fundamental circuit of a DC-DC converter (non-isolated type and chopper type).

Figure. 3 Basic circuit of DC-DC converter (non-isolated type and chopper type)

Ⅱ. Mechanism of Vibration and Noise Amplification of the Power Inductor Body

Howling is a result of the power inductor's body vibrating when an audible current of frequencies passes through it. The tremor and noise could be caused by a number of different factors.

The cause of vibration

➀ Magnetic core magnetostriction (magnetic strain) effect

➁ Magnetic core magnetization leads to mutual attraction

➂ Leakage flux causes winding vibration

The cause of noise amplification

➀ Contact with other components

➁ Leakage magnetic flux causes effects on surrounding magnetic bodies

➂ Consistent with the natural vibration number of the entire assembly including the base plate

Various causes and effects of vibration

Cause of vibration➀: Magnetic core magnetostriction (magnetic strain)

A magnetic body's shape changes slightly as it is magnetized by the application of a magnetic field. Magnetic strain or magnetostriction are terms used to describe this phenomenon. The alternating magnetic field produced by the windings causes the magnetic core to expand and contract in inductors with magnetic cores, such as ferrites, and its vibrating sound can be heard.

Small-scale aggregates termed domains make up magnetic substances (Figure 5). The magnetic domain is a miniature magnet with a continuous spontaneous magnetization orientation because the magnetic moments of its constituent atoms are aligned in the same direction, but the magnetic body as a whole lacks the properties of a magnet. This is due to the arrangement of the several magnetic domains that make up the magnetic body, which causes the spontaneous magnetizations to cancel one another out and cause the magnetic body to look demagnetized from the surface.

The magnetic domain range gradually changes when a magnetic field is introduced to the magnetic material while it is in this demagnetized state because the spontaneous magnetization directions of the various magnetic domains are united in the direction of the external magnetic field. The magnetic wall, which forms the boundary between magnetic domains, is moving, which is what is causing this occurrence. Thus, as magnetization develops, the dominating magnetic domain steadily increases in size until it eventually unites into a single magnetic domain that faces the direction of the external magnetic field (saturated magnetization state). At the atomic level, this magnetization process results in minute positional changes; nevertheless, at the macroscopic level, it appears as magnetostriction or modifications to the magnetic body's shape.

In Fig. 1, the magnetostriction-induced shape change is depicted as being just 1/10,000 to 1/1,000,000 of the initial size. The magnetic body will oscillate and vibrate periodically when an alternating magnetic field is introduced. As a result, the power inductor cannot completely reduce the magnetic core's vibration caused by magnetostriction. The power inductor itself has a low vibration level, but when mounted on a substrate, if its vibration matches the natural vibration number of the substrate, the vibration is magnified and a whistling sound is produced.

Cause of vibration➁: Magnetic core magnetization causes mutual attraction

A magnetic body will exhibit the characteristics of a magnet and attract nearby magnetic bodies when it is magnetized by an external magnetic field. An illustration of a fully shielded power inductor is shown in Figure 6. There is a space between the drum core and the shielded magnetic core (ring core) in this power inductor, which has a closed magnetic circuit topology. Occasionally, noise is emitted from this gap. The drum core and the shield core, which are magnetized by the created magnetic field, will be drawn to one another by the magnetic force when an alternating current flows through the winding. Noise will be audible if the vibration falls within the human ear's audible frequency range. Although the gap between the drum core and the shield core is sealed with an adhesive, vibration brought on by mutual attraction cannot be totally eliminated since no hard material is utilized to avoid stress-related cracking.

Figure. 4 The drum core and the shielded core attract each other causing howling

Vibration cause➂: The leakage magnetic flux causes the winding to vibrate

The mutual attraction created by the magnetization of the drum core and the shielded core prevents whistling from happening in an unshielded power inductor without a shielded core. Unshielded products, however, experience additional issues. The winding thickness will be impacted by the leakage magnetic flux since the unshielded product has an open magnetic circuit configuration. According to Fleming's left-hand rule, a force operates on the windings since current flows in them. Because of this, the windings themselves tremble as an AC current passes through them, resulting in howling (Figure 7).

Figure. 5 Magnetic Flux Causes Winding Vibration

Ⅲ. Various causes of noise amplification

Causes of noise amplification➀ Contact with other components

If the inductor comes into contact with other components on a power supply circuit board where several electronic devices and components are arranged in a high density, the inductor's micro vibration will be magnified, and a whistling sound will be audible.

Causes of Noise Amplification➁ Leakage magnetic flux acts on surrounding magnetic bodies

When there is a magnetic body such as a shielding case near the inductor, the magnetic body vibrates due to the influence of the leakage magnetic flux of the inductor, and howling occurs.

Cause of noise amplification➂ Consistent with the natural vibration number of the entire assembly including the base plate

Normally, howling is not considered to be the air vibration brought on by the magnetostriction of a small magnetic core seen in devices like inductors. However, the inductor is made up of a number of parts, and when it is installed on the substrate, it will produce several naturally occurring vibrations at audible frequencies. When the vibration is increased, it will take the shape of howling. The likelihood of howling after installation in the module exists if the total amount of natural vibrations in the module corresponds.

Figure 8 depicts an illustration of how to use a computer simulator and FEM to analyze the vibration of a board attached with a power inductor (Finite Element Method). The power inductor is positioned in the middle of the substrate (FR4) in the analytical model that was utilized, and the substrate's long sides are fixed on two sides.

A structure typically resonates at a variety of natural values (natural vibration numbers), and as a result, there are numerous vibration modes. As the frequency rises, several vibration modes for each natural vibration number develop in this "power inductor + substrate" analytical model. The power inductor may serve as the vibration source in the first, second, fifth, and eighteenth vibration modes depicted in Figure 8. The first-order mode among them has a vibration frequency that is nearly identical to the inductor's own vibration frequency. It is important to keep in mind that the second-order mode, where the vibration in the height direction (Z direction) is more apparent, has a high frequency when the power inductor is acting alone but a very low frequency when the inductor is fixed to the substrate.

The power inductor is positioned in the substrate's middle using the "Analysis Model" (FR4).

Fix the substrate's long lengths on both sides to satisfy the boundary criteria.

1st mode: 2034Hz~

Secondary mode: 2262Hz~

5th mode: 4048Hz~

18th mode: 16226Hz~

Figure. 6 Example of analyzing the vibration of "Power Inductor + Substrate" with a computer simulator

Ⅳ. Power Inductor Howling Countermeasures

The following summarizes the key points of how to counteract the noise of power inductors in DC-DC converters.

Point 1: Avoid the flow of audible frequency currents to the human ear

The simplest countermeasure is to avoid current flow at frequencies audible to humans.

However, if the energy-saving intermittent operation or DC-DC converters operating in variable frequency mode are unable to prevent energization at frequencies audible to humans, please attempt the following noise-reduction strategies.

Point 2: Do not place magnetic objects around

Avoid putting magnetic items (shields, etc.) next to the inductor that could be impacted by leakage flux. When approaching is unavoidable, it is best to utilize a shielded (closed magnetic circuit construction) inductor with reduced flux leakage, and care should be taken when deciding where to install it.

Point 3: Stagger the number of natural vibrations

By increasing the frequency or spreading out the natural vibrations, howling can occasionally be diminished. The natural vibration frequency of the entire assembly, including the substrate, can be altered, for instance, by altering the shape, type, layout, and attaching of the inductor to the substrate. Howling is also frequent in huge power inductors larger than 7mm. The natural vibration frequency will be raised by employing a small power inductor of 5mm or less, which can lessen howling.

Point 4: Replacement with metal integral molding type

As previously mentioned, whistling happens in the gap where the protected magnetic core and drum core are magnetically coupled to one another in a fully shielded power inductor. Howling can be produced in unshielded power inductors by the wire vibration brought on by magnetic flux leakage.

It is a good idea to switch to a metal integral molding type for this kind of power inductor whistling issue. This power inductor was created by inserting an air-core coil in a finely ground, soft magnetic metal. The magnetic cores are not drawn to one another since there is no space between them.The issue of winding vibration brought on by magnetic flux can also be avoided since the coil is integrated with the magnetic body when it is fixed. Additionally, TDK's products use metal magnetic materials with low magnetostriction, which helps to reduce vibration brought on by magnetostriction. By replacing unshielded or completely shielded goods, howling is anticipated to be reduced.