When laptops, tablets, smartphones, TVs, and in-vehicle electronic devices are running, a "cheep" noise is sometimes heard. This phenomenon is called "howling" and may be caused by passive components such as capacitors and inductors. The principle of the 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 in power inductors, which are the main components of power supply circuits such as DC-DC converters, and effective countermeasures.
Reasons for howling of power inductors
1. Intermittent operation, frequency variable mode, load changes, etc. may cause human ears to audible frequency vibration
. "sound". In the power inductor of a DC-DC converter, when an alternating current or a pulse wave of a frequency within the audible range of the human ear flows, the main body of the inductor vibrates. This phenomenon is called "coil noise" and is sometimes heard into a howling phenomenon (Figure 1).
Figure 1: Howling mechanism of power inductors
As the functions of electronic equipment continue to increase, the power inductor of the DC-DC converter has also become one of the sources of noise. In DC-DC converters, switching devices are turned ON/OFF to generate pulsed current. By controlling the length of ON time (pulse width), a stable DC current with 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 relatively high, reaching several 100kHz to several MHz. Since the vibration at this frequency exceeds the audible range of the human ear, no noise will be felt. So, why does the power inductor of the DC-DC converter emit a "squeak" howling?
There are several possible reasons. The first possibility is that the DC-DC converter is intermittently operated for the purpose of saving battery power, or the DC-DC converter is switched from the PWM method to the PFM (Pulse Frequency Modulation) method. The case of operation in variable frequency mode. Figure 2 shows the basic principles of PWM and PFM.
Figure 2: PWM (pulse amplitude modulation) mode and PFM (pulse frequency modulation) mode
2. Howling caused by intermittent operation of DC-DC converters such as PWM dimming
For the purpose of saving energy, etc., the automatic dimming function of the backlight of the liquid crystal display of the mobile device has introduced intermittent operation of the DC-DC converter. This is a system that automatically adjusts the brightness of the backlight according to the illuminance of the usage environment to extend the battery life.
There are many ways for this dimming, among which, the way of controlling the LED lighting time and the length of the light-off time is called PWM dimming. The advantage of the PWM dimming system is that there is less chromaticity change caused by dimming, and it is mainly used in the backlight of notebook computers and tablet computers.
PWM dimming makes the DC-DC converter intermittently operate at a low frequency of about 200Hz, and adjusts the brightness by repeatedly turning on/off the light. In a constant on/off cycle, turning the light on longer will make it brighter and shorter it will make it darker. In the intermittent work of around 200Hz, the eyes will basically not notice the strobe of the backlight. However, since it is at a frequency audible to the human ear, when an intermittently operating current flows through a power inductor mounted on a substrate, the main body of the inductor will vibrate due to the frequency, resulting in howling.
Note: duty cycle
In a DC-DC converter, the ratio of the ON time to the switching cycle (ON time of the switching device + OFF time) is called the duty ratio. When performing PWM dimming on LEDs, the lighting time/(lighting time + light-off time) is called the duty cycle and represents the brightness.
3. Howling caused by frequency variable mode DC-DC converter
The characteristic of PWM type DC-DC converter is that its efficiency can be as high as about 80~90% or more in normal operation. However, under light load conditions such as standby time, the efficiency will be seriously reduced. 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, the switching frequency will gradually decrease by extending the OFF time. Since switching loss is proportional to frequency, efficiency can be improved at light loads by lowering the frequency. However, the reduced frequency will enter the range of about 20~20kHz that can be heard by the human ear, and the power inductor will whistle at this time.
4. Howling caused by load
Various power-saving technologies are used in mobile devices such as notebook computers for the purpose of saving battery power, which may cause howling in inductors. For example, for the purpose of both low power consumption and processing power, there is a mode of periodically changing the current consumption in the CPU of a notebook computer. When the cycle is in the frequency range audible to the human ear, the power inductor may be affected Produce howling.
Note: The role of power inductors in DC-DC converters.
Inductors allow direct current to flow smoothly, while for changing currents such as alternating current, through self-induction, an electromotive force is generated in a direction that prevents the change, and resistance is exerted. role. At this time, 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.
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. By coordinating with capacitors, the high frequency generated by switching devices ON/OFF Pulses are smoothed out.
Since a large current flows through 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 core, a high inductance value can be realized with a small number of windings, and the product can be further miniaturized. 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 chopping method)
Mechanism of Power Inductor Body Vibration and Noise Amplification
When a current with a frequency in the audible range of the human ear flows, vibrations generated in the power inductor body cause howling. There are several possible reasons for the vibration and noise.
Causes of vibration
➀ Magnetostriction (magnetic strain) of the magnetic core
➁ Magnetization of the magnetic core leads to mutual attraction
➂ Leakage flux causes winding vibration and noise
amplification
➀ Contact with other components
➁ Leakage flux causes an effect on the surrounding magnetic body
➂Same as the natural vibration number of the entire package including the substrate.
The vibration causes and noise amplification causes of power inductor noise are summarized in Figure 4. The main contents of these reasons are explained below.
Figure 4: Causes of vibrations that cause howling in power inductors and causes of their expansion
Various causes and effects of vibration
Causes of vibration ➀: Magnetostriction (magnetic strain) of a magnetic body core.
When a magnetic field is applied to a magnetic body to magnetize it, its shape changes slightly. This phenomenon is called "magnetostriction" or "magnetic strain". In an inductor with a magnetic core such as ferrite, the AC magnetic field generated by the winding expands and contracts the magnetic core, and vibrations may be detected.
Figure 5: Magnetostrictive (magnetic strain) action of a magnetic body
Magnetic bodies are collections of small areas called domains (Figure 5). The magnetic moments of the atoms inside the magnetic domain are in the same direction, so the magnetic domain is a tiny magnet with a constant spontaneous magnetization direction, 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 spontaneous magnetization cancels each other out, and thus appear to be in a demagnetized state.
When a magnetic field is applied to the magnetic body in this demagnetized state from the outside, the spontaneous magnetization direction of each magnetic domain is unified to the direction of the external magnetic field, so the range of the magnetic domain gradually changes. This phenomenon is caused by the movement of magnetic walls, the boundaries between magnetic domains. Thus, as the magnetization proceeds, the dominant magnetic domain gradually expands its range, and finally becomes a single magnetic domain, facing the direction of the external magnetic field (saturation magnetization state). In this magnetization process, a minute positional change occurs at the atomic level, but at the macroscopic level, it appears as magnetostriction, that is, a change in the shape of the magnetic body.
The shape change caused by magnetostriction is extremely small, about 1/10,000 to 1/1,000,000 of the original size, but as shown in Fig. When the magnetic field is alternating, the magnetic body will expand and contract repeatedly and generate vibration. For this reason, in power inductors, the vibration of the magnetic core due to magnetostriction cannot be completely eliminated. Although the vibration level of a single power inductor is small, when it is mounted on a substrate, if its vibration is consistent with the natural vibration number of the substrate, the vibration will be amplified, and howling will be heard.
Cause of vibration➁: Magnetization of the core of the magnetic body leads to mutual attraction
When the magnetic body is magnetized by an external magnetic field, it will exhibit magnetic properties, thereby attracting each other with the 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 shield core (ring core), and noise may be emitted from there. When an alternating current flows through the winding, the magnetized drum core and shield core will attract each other due to the magnetic force generated by the generated magnetic field. 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 sealed 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.
Figure 6: The drum core and the shield core attract each other and cause howling
Cause of Vibration ➂: Leakage Flux Causes Winding Vibration
In an unshielded power inductor without a shielded core, there will be no howling due to the mutual attraction caused by the magnetization of the drum core and the shielded core mentioned above. But other problems can occur in unshielded products. Since the unshielded product has an open magnetic circuit structure, the leakage flux will affect the winding. Due to the current flowing in the winding, a force acts on the winding according to Fleming's left hand rule. For this reason, when an alternating current flows through the winding, the winding itself vibrates, causing howling (Fig. 7).
Figure 7: Magnetic flux causes winding to vibrate
Various Causes of Noise Amplification
Causes of Noise Amplification➀ Contact with Other Components
In a power circuit board on which many electronic components and devices are mounted at high density, if the inductor comes into contact with other components, the tiny vibration of the inductor will be amplified , so that howling will be heard.
Causes of Noise Amplification ➁ Effects on surrounding magnetic bodies due to leakage flux
When there is a magnetic body such as a shield near the inductor, the magnetic body will vibrate due to the influence of the leakage flux of the inductor, resulting in howling.
Causes of Noise Amplification ➂ Same as the natural vibration number of the whole component including the substrate
Usually, the air vibration caused by the magnetostriction of a single small magnetic body core used in products such as inductors is hardly recognized For howling. However, the inductor is composed of multiple components, and when it is mounted on the substrate, it will generate natural vibration numbers of multiple audible frequencies of the human ear, and the vibration will form a howling after the vibration is amplified. At the same time, if it matches the multiple natural vibration numbers of the whole module, howling may occur after being installed in the module.
Figure 8 shows an example of analyzing the vibration of a board on which a power inductor is mounted using a computer simulator using FEM (Finite Element Method). In the analysis model used, the power inductor is placed in the center of the substrate (FR4), and the two long sides of the substrate are fixed.
In general, 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 source of the vibration. Among them, the vibration frequency of the primary mode is basically the same as the vibration frequency of a single power inductor. However, it is worth noting that the secondary mode, which vibrates significantly in the Z direction (height direction), appears at a high frequency in the case of a single power inductor, but appears at an extremely low frequency when it is fixed on a substrate.
《Analysis model》The power inductor is placed in the center of the substrate (FR4).
Boundary conditions: 2 sides of the long side of the fixed substrate.
1st mode: 2034Hz~
2nd mode: 2262Hz~
5 times mode: 4048Hz~
18 times mode: 16226Hz~
Figure 8: Example of analyzing the vibration of "power inductor + substrate" with a computer simulator
Noise Countermeasures for Power Inductors
The following summarizes key points for countermeasures against noise from power inductors for DC-DC converters.
Key point 1: Avoiding the flow of audible frequency current to the human ear
Avoiding the flow of audible frequency current to the human ear is the most basic countermeasure.
However, if it is unavoidable to conduct electricity at frequencies audible to the human ear, such as intermittent operation for the purpose of energy saving, etc., or a DC-DC converter in variable frequency mode, try the following noise reduction measures.
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. If it is necessary to approach it as a last resort, use a shielded (closed magnetic circuit structure) inductor with less leakage flux, and pay attention to the placement direction.
Key point 3: Staggering the natural vibration number
Sometimes the howling can be reduced by staggering the natural vibration number or increasing the vibration number. For example, by changing conditions such as inductor shape, type, layout, and substrate fastening, the natural vibration frequency of the entire package including the substrate will change. In addition, howling is common in large power inductors with a size of 7mm or more. By adopting a small power inductor of 5mm or less, the natural vibration frequency will be increased, thereby reducing howling.
Point 4: Replacing with a metal integral molded type
As mentioned above, in a fully shielded power inductor, the drum core and the shield core are magnetically attracted to each other, and noise occurs in the gap. Meanwhile, in unshielded type power inductors, vibration of wires caused by leakage flux can cause howling.
Replacing this type of power inductor with a metal one-piece molded type is an effective solution to the noise problem of such power inductors. 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 will not attract each other. At the same time, since the coil is integrated with the magnetic body when fixing the coil, the problem of winding vibration due to magnetic flux can also be avoided. Not only that, TDK's products also use metal magnetic materials with low magnetostriction, so vibration caused by magnetostriction can be suppressed, and it is expected to reduce howling by replacing unshielded or fully shielded products.
Noise comparison between fully shielded and metal
integral type power inductors (TDK products, about 6mm in size) and fully shielded and metal integral molded power inductors (TDK products, about ) was used as a measurement sample to investigate the occurrence of noise. Install a microphone inside the anechoic box, energize the measurement sample installed on the substrate with a sine wave current from 0A to the rated current for 60 seconds, and perform a frequency sweep at an audible frequency of 20Hz to 20kHz, and record its peak sound pressure ( Figure 8).
As shown in the graph, when comparing fully shielded and semi-shielded types, it can be seen that the sound pressure level will vary depending on the frequency.
The difference is more noticeable when comparing fully shielded and metal unibody products. In the fully shielded type, noise at a level of about 30 to 50 dB is generated in a wide frequency band. On the other hand, the metal unibody type has the same low level as the background noise in a wide frequency band, and even in the peak part, it suppresses about 20dB compared with the full-shield type. Suppression of 20dB means only 1/10 of the level, so it can be seen that replacing it with a metal integral molding type is an effective countermeasure.
Figure 9: Example of noise evaluation for various types of power inductors
Source: TDK
Comic Inductors
Of course, there is also the most unique method. A netizen said that this method is "everything has been tested", that is, plug the customer's ears (*￣︶￣).
-End-
"Diffuse if useful"
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