Sometimes, a "chirping" noise can be heard when laptops, tablets, smartphones, televisions, and in-vehicle electronic devices are running. This phenomenon is called "whistling," and it can be caused by passive components such as capacitors and inductors. The principles behind whistling differ between capacitors and inductors, especially the whistling of inductors, which has a variety of complex causes. This article will introduce the causes of whistling from power inductors—a key component in power circuits such as DC-DC converters—and effective countermeasures.
Causes of power inductor whistling
Intermittent operation, variable frequency modes, and load variations can cause vibrations at frequencies audible to the human ear.
Sound waves are elastic waves that propagate through the air. Human hearing can perceive "sound" in the frequency range of approximately 20 to 20 kHz. In the power inductor of a DC-DC converter, when alternating current and pulse waves of frequencies within the audible range flow through it, the inductor body vibrates. This phenomenon is called "coil noise" and is sometimes heard as a whistling sound (Figure 1).
Figure 1: Power Inductor Whistling Mechanism
As electronic devices become increasingly sophisticated, the power inductors in DC-DC converters have become a source of noise. DC-DC converters use switching devices to turn ON/OFF, thereby generating pulsed current. By controlling the ON duration (pulse width), a stable DC current with constant voltage can be obtained. This method is called PWM (Pulse Width Modulation), and it is widely used as the mainstream method for DC-DC converters.
However, DC-DC converters operate at high switching frequencies, ranging from several hundred kHz to several MHz. Since these frequencies are beyond the range of human hearing, the noise is not perceptible. So, why do the power inductors in DC-DC converters emit a "chirping" sound?
There are several possible reasons. First, it could be that the DC-DC converter is operating intermittently to save battery power, or that the DC-DC converter is being switched from PWM mode to PFM (Pulse Frequency Modulation) mode, operating in a variable frequency mode. Figure 2 shows the basic principles of PWM and PFM modes.
Figure 2: PWM (Pulse Amplitude Modulation) and PFM (Pulse Frequency Modulation) methods
Howling caused by intermittent operation of DC-DC converters such as PWM dimming
For energy conservation and other purposes, mobile devices have introduced intermittent operation of DC-DC converters for features such as automatic backlight dimming in LCD displays. This system automatically adjusts the backlight brightness based on the ambient light level, thereby extending battery life.
There are several dimming methods, among which the method of controlling the LED's on and off time is called PWM dimming. The advantage of PWM dimming systems is that they cause less color change during dimming, and they are mainly used in the backlighting of laptops and tablets.
PWM dimming uses a low frequency of around 200Hz to cause the DC-DC converter to operate intermittently, adjusting brightness by repeatedly turning the light on and off. In this constant on/off cycle, increasing the on time makes the light brighter, and decreasing it makes it dimmer. During this intermittent operation at around 200Hz, the backlight flicker is virtually imperceptible to the human eye. However, because it falls within the audible frequency range, when the intermittent current flows through the power inductor mounted on the substrate, the inductor body will vibrate due to the frequency, resulting in a whistling sound.
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 cycle. When performing PWM dimming on an LED, the ratio of the lighting time to (lighting time + light-off time) is called the duty cycle and represents the brightness.
Whistling caused by frequency variable mode DC-DC converter
The characteristic of PWM DC-DC converters is that their efficiency can reach approximately 80-90% or higher under normal operating conditions. However, the efficiency will drop significantly under light load conditions such as standby time. Switching losses are proportional to the frequency. Therefore, constant switching losses occur under light load conditions, thus reducing efficiency.
Therefore, to improve this problem, a DC-DC converter that automatically replaces PWM (Pulse Frequency Modulation) with PFM (Pulse Frequency Modulation) is used under light load conditions. PFM controls the switching frequency while maintaining a fixed ON time to accommodate reduced load. Since the ON time is constant, the switching frequency gradually decreases by extending the OFF time. Because switching losses are proportional to frequency, reducing the frequency achieves higher efficiency under light load conditions. However, the reduced frequency will fall into the audible range of approximately 20-20kHz, at which point the power inductor will emit a whistling sound.
Whistling caused by load
To conserve battery power, various power-saving technologies are used in mobile devices such as laptops, which may cause inductors to whistle. For example, to balance low power consumption and processing power, laptop CPUs have a pattern that periodically changes the current consumption. When this cycle is within the range of human hearing, the power inductor may whistle due to this effect.
Note: The role of the power inductor in a DC-DC converter
An inductor allows direct current to flow smoothly, while for alternating current or other changing currents, it generates an electromotive force (EMF) in the direction that opposes the change through self-induction, thus acting as a resistor. In this process, the inductor converts electrical energy into magnetic energy, stores it, and releases it after converting it back into electrical energy. The magnitude of this energy is directly proportional to the inductor's inductance.
Power inductors, also known as power coils or power chokes, are key components in switching power supply circuits such as DC-DC converters. By coordinating with capacitors, they smooth out the high-frequency pulses generated by the ON/OFF operation of switching devices.
Because power inductors in power circuits carry large currents, wound-type inductors are the mainstream product. This is because by using a high-permeability magnetic material (ferrite or soft magnetic metal) in the core, a high inductance value can be achieved with fewer turns, thus allowing for product miniaturization. Figure 3 shows the basic circuit of a DC-DC converter (non-isolated and chopper type) using a power inductor.
Figure 3: Basic circuit of DC-DC converter (non-isolated and chopper type)
Mechanisms of vibration and noise amplification in power inductors
When a current flows through a frequency within the audible range, the vibration of the power inductor body can cause a whistling sound. The causes of this vibration and noise can be summarized in several possibilities.
Vibration cause
1.➀ Magnetostriction (magnetic strain) of the magnetic core of a magnetic body
2. ② Magnetization of the magnetic core of a magnetic body leads to mutual attraction.
3. ➂ Leakage flux causes winding vibration
Noise amplification
1.➀ Contact with other components
2. ② Leakage flux causes an effect on surrounding magnetic materials.
3. ➂ Consistent with the overall natural vibration coefficient of the component, including the substrate.
Figure 4 summarizes the causes of vibration and noise amplification that lead to the whistling sound of the power inductor. The main points of these causes are explained below.
Figure 4: Causes of vibration leading to power inductor whistling and their amplification
Various causes and effects of vibration
Cause of vibration ➀: Magnetostriction (magnetic strain) of the magnetic core of the magnetic material
When a magnetic field is applied to a magnetic material and it is magnetized, its shape will undergo subtle changes. This phenomenon is called "magnetostriction" or "magnetic strain." In inductors with magnetic cores such as ferrite, the alternating magnetic field generated by the windings causes the magnetic core to stretch or contract, and sometimes a vibration sound can be detected.
Figure 5: Magnetostriction (magnetic strain) of magnetic materials
A magnetic body is a collection of small areas called magnetic domains (Figure 5). The atomic magnetic moments within a magnetic domain are aligned in the same direction, making each domain a tiny magnet with a constant spontaneous magnetization orientation. However, the magnetic body as a whole does not exhibit the characteristics of a magnet. This is because the arrangement of the multiple domains that make up the magnetic body causes their spontaneous magnetization to cancel each other out, thus appearing to be in a demagnetized state.
When a magnetic field is applied to a magnetic body in this demagnetized state from the outside, the individual magnetic domains will spontaneously magnetize and align themselves with the direction of the external magnetic field, thus gradually changing the domain extent. This phenomenon is caused by the movement of the domain boundaries—the magnetic walls. As magnetization progresses, the dominant domain gradually expands its extent, eventually becoming a single domain and aligning itself with the external magnetic field (saturation magnetization). During this magnetization process, minute positional changes occur at the atomic level, while at the macroscopic level, this manifests as magnetostriction, i.e., changes in the shape of the magnetic body.
The shape change caused by magnetostriction is extremely small, approximately 1/10,000 to 1/1,000,000 of the original size. However, as shown in Figure 5, when current flows through a coil wound around a magnetic material, and an alternating magnetic field is applied, the magnetic material will repeatedly expand and contract, generating vibration. Therefore, in power inductors, it is impossible to completely eliminate the vibration of the magnetic core caused by magnetostriction. Although the vibration level of a single power inductor is small, when mounted on a substrate, if its vibration matches the natural vibration coefficient of the substrate, the vibration will be amplified, resulting in a whistling sound.
Vibration cause ➁: Magnetization of the magnetic core of the magnetic body leads to mutual attraction.
Figure 6: The whistling sound is caused by the mutual attraction between the drum core and the shielding magnetic core.
When a magnetic material is magnetized by an external magnetic field, it will exhibit magnetic properties and attract surrounding magnetic materials. 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 shielding core (toroidal core), from which noise can sometimes be emitted. When alternating current flows through the winding, the drum core and the shielding core, which are magnetized by the generated magnetic field, will attract each other due to magnetic force. If this vibration is within the audible frequency range, noise will be heard.
The gap between the drum core and the shielding core is sealed with adhesive, but in order to prevent cracking due to stress, a harder material is not used, thus making it impossible to completely suppress vibrations caused by mutual attraction.
Vibration cause ➂: Leakage flux causes winding vibration
In unshielded power inductors without a shielded core, there is no whistling due to the mutual attraction between the magnetized core and the shielded core. However, other problems arise in unshielded products. Because unshielded products have an open magnetic circuit structure, leakage flux will affect the winding. Since current flows through the winding, according to Fleming's left-hand rule, a force will act on the winding. Therefore, when alternating current flows through the winding, the winding itself will vibrate, thus producing a whistling sound (Figure 7).
Figure 7: Magnetic flux causes winding vibration
Various reasons for noise amplification
Noise amplification cause ➀ Contact with other components
In a power circuit board with multiple electronic components and devices mounted at high density, if an inductor comes into contact with other components, the inductor's minute vibrations will be amplified, resulting in a whistling sound.
Noise amplification is caused by: ① Leakage flux affecting surrounding magnetic materials.
When there are magnetic objects such as shields near an inductor, the magnetic objects will vibrate due to the leakage flux of the inductor, resulting in a whistling sound.
The reason for noise amplification is consistent with the overall natural vibration coefficient of the component, including the substrate.
Normally, the air vibrations caused by the magnetostriction of small magnetic cores used in products such as inductors are not perceived as whistling. However, inductors are composed of multiple components and, when mounted on a substrate, generate multiple inherent vibrations at audible frequencies. These vibrations, when amplified, can create whistling. Furthermore, if these vibrations coincide with the inherent vibrations of the entire assembly, whistling may occur after the inductor is installed in the assembly.
Figure 8 shows an example of analyzing the vibration of a substrate with a mounted power inductor using a computer simulator employing the FEM (Finite Element Method). In the analysis model used, the power inductor is positioned in the center of the substrate (FR4), and the two long sides of the substrate are fixed.
Generally, a structure has multiple natural values (natural vibration numbers) that can cause resonance, resulting in various vibration modes. In this "power inductor + substrate" analysis model, as the frequency increases, various vibration modes also 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. The 1st mode's vibration frequency is basically the same as that of a single power inductor unit. However, it is noteworthy that the 2nd mode, which exhibits significant vibration in the Z-direction (height direction), shows a higher frequency in the case of a single power inductor unit, but a very low frequency when fixed to the substrate.
The following is a summary of key countermeasures for power inductor whistling in DC-DC converters.
Key Point 1: Avoid current flowing through frequencies audible to the human ear.
The most basic countermeasure is to avoid current flowing through frequencies audible to the human ear.
However, when intermittent operation for energy saving or other purposes, or DC-DC converters with variable frequency modes, cannot avoid powering on frequencies audible to the human ear, please try the following noise reduction measures.
Key Point 2: Do not place magnetic objects nearby.
Do not place magnetic objects (shielding covers, etc.) that may be affected by leakage flux near the inductor. If it is unavoidable to place it near the inductor, use a shielded inductor (closed magnetic circuit structure) with less leakage flux, and also pay attention to the placement direction.
Key Point 3: Stagger the natural vibration numbers
Sometimes, whistling can be reduced by shifting or increasing the natural frequency. For example, changing the inductor's shape, type, layout, and substrate mounting conditions will alter the overall natural frequency of the assembly, including the substrate. Furthermore, whistling is common in large power inductors larger than 7mm. By using smaller power inductors (5mm or less), the natural frequency can be increased, thereby reducing whistling.
Key Point 4: Replacement with unibody metal molding
As mentioned above, in fully shielded power inductors, the drum core and the shielding core attract each other magnetically, causing a whistling sound at the gap. Meanwhile, in unshielded power inductors, leakage flux causing wire vibration can also lead to a whistling sound.
For this type of power inductor whistling issue, replacing it with a one-piece metal inductor is an effective solution. This is a power inductor formed by embedding a hollow coil within soft magnetic metal powder and then molding it into a single unit. Because there are no gaps, the magnetic cores do not attract each other. Furthermore, since the coil is integrated with the magnetic material during fixing, the problem of winding vibration caused by magnetic flux is avoided. Moreover, TDK products use metallic magnetic materials with low magnetostriction, thus suppressing vibrations caused by magnetostriction. Replacing with unshielded or fully shielded products can potentially reduce whistling.
Figure 8: Examples of noise assessment for various power inductors
TDK's one-piece molded metal power inductors effectively combat whistling and have minimal leakage flux, making them suitable for placement near signal lines and other locations.
Meanwhile, TDK power inductors using ferrite cores are characterized by a wider variety of inductors and the ability to handle higher inductance values. They also offer excellent mass production capabilities and are widely used in various types of equipment.
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