“When running laptops, tablets, smartphones, televisions, and in-vehicle Electronic devices, you may sometimes hear a “squeaky” noise. This phenomenon is called “howling” and may be caused by passive components such as capacitors and inductors. The principle of howling of capacitors and inductors is different, especially the howling of inductors, the reasons are various and very complicated. In this article, we will introduce the causes of howling of power inductors, the main components of power circuits such as DC-DC converters, and effective countermeasures.
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When running laptops, tablets, smartphones, televisions, and in-vehicle electronic devices, you may sometimes hear a “squeaky” noise. This phenomenon is called “howling” and may be caused by passive components such as capacitors and inductors. The principle of howling of capacitors and inductors is different, especially the howling of inductors, the reasons are various and very complicated. In this article, we will introduce the causes of howling of power inductors, the main components of power circuits such as DC-DC converters, and effective countermeasures.
Power Inductor Howling Causes
Intermittent work, variable frequency mode, load changes, etc. may cause human audible frequency vibration and sound waves are elastic waves propagating in the air, and human hearing can hear “sounds” in the frequency range of about 20 to 20 kHz. In power inductors of DC-DC converters, when alternating current and pulse waves of frequencies in the audible range of the human ear flow, the main body of the inductor vibrates. This phenomenon is called “coil noise” and is sometimes heard into the whistling phenomenon (Figure 1).
Figure 1: Power Inductor Howling Mechanism
As the functions of electronic equipment continue to increase, the power inductors of DC-DC converters have also become one of the sources of noise. The DC-DC converter is turned ON/OFF by switching devices, thereby generating a pulse-like current. By controlling the ON time length (pulse width), a stable DC current with a constant voltage can be obtained. This method is called PWM (Pulse Amplitude Modulation), and it is widely used as a mainstream method of DC-DC converters.
However, the switching frequency of the DC-DC converter is high, reaching several 100 kHz to several MHz. Since the vibration of this frequency is beyond the audible range of the human ear, no noise is felt. So, why does the power inductor of the DC-DC converter make a “creepy” whistle?
There are several possible reasons. The first possibility is to allow the DC-DC converter to work intermittently for the purpose of saving battery power, or to switch the DC-DC converter from the PWM method to the PFM (pulse frequency modulation) method. In case of operation in variable frequency mode. Figure 2 shows the basic principles of PWM and PFM.
Figure 2: PWM (pulse amplitude modulation) method and PFM (pulse frequency modulation) method
Howling caused by intermittent operation of DC-DC converters such as PWM dimming
For the purpose of energy saving, etc., the automatic dimming function of the backlight of the liquid crystal Display of mobile devices has introduced the DC-DC converter to work intermittently. This is a system that automatically adjusts the brightness of the backlight according to the ambient illuminance, thereby extending battery life.
There are various methods of dimming, among which, the method of controlling the on-time and off-time of the LED is called PWM dimming. The advantage of the PWM dimming system is that the chromaticity changes caused by dimming are less, and it is mainly used in the backlight of notebook computers and tablet computers.
PWM dimming makes the DC-DC converter work intermittently at a lower frequency of around 200Hz, and adjusts the brightness by repeatedly turning on/off. In a constant cycle of on/off, when the on time is adjusted longer, it will become brighter, and if it is shortened, it will become darker. In the intermittent operation of about 200Hz, the eyes basically do not perceive the backlight stroboscopic situation. However, since it is at the audible frequency of the human ear, when the intermittently operating current flows through the power inductor mounted on the substrate, the main body of the inductor will vibrate due to the influence of frequency, resulting in whistling.
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.
Howling Caused by Frequency Variable Mode DC-DC Converters
The characteristic of the PWM mode DC-DC converter is that its efficiency can be as high as about 80~90% in normal operation. However, under light load conditions such as standby time, the efficiency will be severely reduced. The losses due to switching are proportional to frequency. For this reason, constant switching losses occur at light loads, thus reducing efficiency.
Therefore, in order to improve this problem, a DC-DC converter that automatically replaces the PWM method with the PFM (pulse frequency modulation) method is used under light load conditions. The PFM method is a method of controlling the switching frequency with a fixed ON time in accordance with load reduction. Since the ON time is constant, by increasing the OFF time, the switching frequency will gradually decrease. Since switching losses are proportional to frequency, lower frequency can achieve higher efficiency at light loads. But the reduced frequency will enter the audible range of about 20~20kHz, and the power inductor will whistle.
howling caused by load
In order to conserve battery power, various power-saving technologies are used in mobile devices such as notebook computers, which may cause the inductor to whistle. For example, in order to balance low power consumption and processing power, the CPU of a notebook computer has a mode that periodically changes the current consumption. When the cycle is in the audible frequency range of the human ear, the power inductor may be affected by this effect. Howling is generated.
Note: The role of power inductors in DC-DC converters
Inductors allow direct current to flow smoothly, and for currents that change, such as alternating currents, through self-induction, electromotive force is generated in the direction of preventing the change, and it acts as a resistance. At this point, the inductor converts electrical energy into magnetic energy, accumulates it, and releases it after converting it into electrical energy. The magnitude of this energy is proportional to the inductor inductance value.
Power inductors, also known as power coils and power choke coils, are the main components used in switching power supply circuits such as DC-DC converters. Pulses are smoother.
Since a large current flows in the power inductor of the power supply circuit, the winding type is the mainstream product. This is because by using a magnetic material with high magnetic permeability (ferrite or soft magnetic metal) in the magnetic core, a high inductance value can be realized with a small number, and the product can be made more compact. Figure 3 shows the basic circuit of a DC-DC converter (non-isolated type and chopper type) using a power inductor.
Figure 3: Basic circuit of DC-DC converter (non-isolated type and chopper type)
Mechanism of Vibration and Noise Amplification of Power Inductor Body
Vibration that occurs in the body of the power inductor can cause howling when a current of frequencies in the audible range flows through the human ear. There are several possible reasons for the vibration and noise.
cause of vibration
1. Magnetostriction (magnetic strain) effect of magnetic core
2. ? Magnetic core magnetization leads to mutual attraction
3. Leakage flux causes winding vibration
The cause of noise amplification
1. Contact with other components
2. Leakage magnetic flux acts on surrounding magnetic bodies
3. Consistent with the natural vibration of the entire assembly including the base plate
Figure 4 summarizes the causes of vibration and noise amplification that lead to whistling in power inductors. The main contents of these reasons are explained below.
Figure 4: Vibration causes and expansion causes of whistling in power inductors
Various causes and effects of vibration
Cause of vibration?: Magnetic core magnetostriction (magnetic strain)
When a magnetic body is magnetized by applying a magnetic field, its shape changes slightly. This phenomenon is called “magnetostriction” or “magnetic strain”. In inductors with magnetic cores such as ferrites, the alternating magnetic field generated by the windings causes the magnetic core to expand and contract, and its vibration sound may be detected.
Figure 5: Magnetostriction (magnetic strain) effect of a magnetic body
Magnetic bodies are small-scale aggregates called domains (Figure 5). The magnetic moments of the atoms inside the magnetic domain are oriented in the same direction, so the magnetic domain is a tiny magnet with a constant spontaneous magnetization orientation, but the magnetic body as a whole does not exhibit the characteristics of a magnet. This is because the plurality of magnetic domains constituting the magnetic body are arranged so that the spontaneous magnetizations cancel each other, and thus appear in a demagnetized state when viewed from the surface.
When a magnetic field is applied to the magnetic material in this demagnetized state from the outside, the spontaneous magnetization directions of the respective magnetic domains are unified to the direction of the external magnetic field, so that the magnetic domain range gradually changes. This phenomenon is caused by the movement of the boundary between the magnetic domains, the magnetic wall. As a result, as the magnetization progresses, the dominant magnetic domain gradually expands its range, and finally becomes a single magnetic domain, oriented toward the direction of the external magnetic field (saturated magnetization state). In this magnetization process, tiny positional changes occur at the atomic level, while at the macroscopic level, it is manifested as magnetostriction, that is, changes in the shape of the magnetic body.
The shape change due to magnetostriction is extremely small, about 1/10,000 to 1/1,000,000 of the original size, but as shown in When an alternating magnetic field is applied, the magnetic body will repeatedly expand and contract and vibrate. Therefore, in the power inductor, the vibration of the magnetic core due to magnetostriction cannot be completely eliminated. Although the vibration level of the power inductor itself is small, when it is mounted on the substrate, if its vibration is consistent with the natural vibration number of the substrate, the vibration will be amplified, and a whistling will be heard.
Cause of vibration: The magnetization of the magnetic core of the magnetic body causes mutual attraction
Figure 6: The drum core and the shielded core attract each other causing howling
When a magnetic body is magnetized by an external magnetic field, it will exhibit the properties of a magnet and attract each other with surrounding magnetic bodies. Figure 6 shows an example of a fully shielded power inductor. This is a power inductor with a closed magnetic circuit structure, but there is a gap between the drum core and the shielded magnetic core (toroidal core), and noise is sometimes emitted from there. When an alternating current flows through the winding, the drum core and the shield core, which are magnetized by the generated magnetic field, will be attracted to each other by the magnetic force. If the vibration is within the audible frequency range of the human ear, noise will be heard.
The gap between the drum core and the shield core is closed with an adhesive, but in order to prevent cracking due to stress, a hard material is not used, and the vibration caused by mutual attraction cannot be completely suppressed.
Vibration reason: leakage magnetic flux causes winding vibration
In an unshielded power inductor without a shielded core, whistling does not occur due to the mutual attraction caused by the magnetization of the drum core and the shielded core. But other problems occur in unshielded products. Since the unshielded product has an open magnetic circuit structure, the leakage magnetic flux will have an effect on the winding thickness. Since current flows in the windings, according to Fleming’s left-hand rule, a force acts on the windings. For this reason, when an AC current flows through the windings, the windings themselves vibrate, which produces howling (Figure 7).
Figure 7: Magnetic Flux Causes Winding Vibration
Various causes of noise amplification
The cause of noise amplification?contact with other components
In a power supply circuit board where many electronic components and devices are mounted at a high density, if the inductor comes into contact with other components, the micro vibration of the inductor will be amplified, and a whistling will be heard.
Cause of noise amplification? 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.
The cause of noise amplification?Consistent with the natural vibration of the entire assembly including the base plate
Under normal circumstances, the air vibration caused by the magnetostriction of a small magnetic core used in products such as inductors is basically not recognized as howling. However, when an inductor is composed of multiple components and is mounted on a substrate, it will generate a number of natural vibrations at frequencies that are audible to the human ear, and the vibration will be amplified to form a whistle. At the same time, if the number of natural vibrations of the entire module matches, there is a possibility that howling may occur after installation in the module.
Fig. 8 shows an example of analyzing the vibration of a board mounted with a power inductor using a computer simulator using the FEM (Finite Element Method). In the analytical model used, the power inductor is placed in the center of the substrate (FR4), and the long sides of the substrate are fixed on two sides.
Generally, there are multiple natural values (natural vibration numbers) at which a structure resonates, and accordingly, there are various vibration modes. In this “power inductor + substrate” analysis model, as the frequency increases, various vibration modes appear for each natural vibration number. In the 1st, 2nd, 5th, and 18th vibration modes shown in Figure 8, the power inductor may be the vibration source. Among them, the vibration frequency of the first-order mode is basically the same as the vibration frequency of the power inductor itself. However, it is worth noting that the second-order mode, where the vibration in the Z direction (height direction) is more pronounced, has a high frequency in the case of a single power inductor, but has an extremely low frequency when it is fixed on the substrate.
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
Avoiding the flow of currents at frequencies audible to the human ear is the most basic countermeasure.
However, when intermittent operation for the purpose of energy saving, or DC-DC converters in variable frequency mode cannot avoid energization at frequencies audible to human ears, please try the following measures to reduce noise.
Point 2: Do not place magnetic objects around
Do not place magnetic objects (shields, etc.) that may be affected by leakage flux near the inductor. When approaching is unavoidable, a shielded (closed magnetic circuit structure) inductor with less leakage flux should be used, and attention should be paid to the placement direction.
Key point 3: Stagger the number of natural vibrations
Howling can sometimes be reduced by staggering the number of natural vibrations or increasing the number of vibrations. For example, by changing conditions such as the shape, type, layout, and substrate fastening of the inductor, the natural vibration frequency of the entire assembly including the substrate changes. In addition, howling is common in large power inductors over 7mm in size. By using a small power inductor of 5mm or less, the natural vibration frequency will be increased, which can reduce the howling.
Key point 4: Replacement with metal integral molding type
As described above, in a fully shielded power inductor, the drum core and the shielded magnetic core are magnetically attracted to each other, and whistling occurs at the gap. Meanwhile, in unshielded power inductors, the wire vibration caused by the leakage magnetic flux can cause howling.
For this kind of power inductor whistling problem, it is an effective solution to replace it with a metal integral molding type. This is a power inductor that is integrally molded by embedding an air-core coil in soft magnetic metal powder. Since there is no gap, the magnetic cores are not attracted to each other. At the same time, since the coil is integrated with the magnetic body when the coil is fixed, the problem of winding vibration caused by magnetic flux can also be avoided. Not only that, TDK’s products also use metal magnetic materials with small magnetostriction, so vibration caused by magnetostriction can be suppressed, and it is expected to reduce howling by replacing unshielded or fully shielded products.
Figure 8: Example of noise evaluation for various types of power inductors
TDK’s metal-integrated power inductors can effectively deal with howling, and at the same time, have very little leakage flux, so they are also suitable for placement near signal lines, etc.
At the same time, TDK’s power inductors using ferrite cores feature a wider variety of inductances and can cope with higher inductance values. It has excellent mass productivity and is mostly used in various types of equipment.
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