"In laptop, tablet, smartphone, TV, and car electronic equipment, sometimes" "" "" noise. This phenomenon is called "whistling", resulting in the cause of this phenomenon. It may take passive components such as capacitors and inductors. The principle of the craftshot of the capacitor and the inductor is different, especially the whistlement of the inductor, which is a variety of reasons, very complicated. This article will be on the DC-DC converter, etc. The main components of the power circuit - the causes of the power inductor and the effective countermeasures.
Power inductor whistling reason
1. Intermittent work, frequency variable mode, load change, etc. may result in human ear audible frequency vibration
Sound waves are elastic waves propagating in air, and people's auditory can hear "" sound "" in about 20 to 20 kHz frequency range. In the power inductor of the DC-DC converter, when the AC current and pulse wave flowing through the human ear, the inductor main body occurs, which is called "" coil noise ", sometimes it will Listed to the phenomenon (Figure 1).
Figure 1: Power inductor whistling mechanism
As the function of the electronic device is constantly strengthened, the power inductor of the DC-DC converter has also become one of the noise generators. The DC-DC converter is ON / OFF through the switching device, thereby generating a pulse current. By controlling the time (pulse width) of the ON, a stable DC current of the voltage can be obtained. This means is called PWM (pulse amplitude), which is widely used as the mainstream mode of the DC-DC converter.
However, the switching frequency of the DC-DC converter is high, reaching 100kHz ~ Number MHz, because the frequency vibration exceeds the human ear audible range, so noise will not feel. So why is the power inductor of the DC-DC converter issues "" "" "叫?
There are several possible reasons, first possible to save battery power, etc., let DC-DC converters perform intermittent work, or switch DC-DC converters from PWM mode to PFM (pulse FM) mode, The situation running in frequency variable mode. Figure 2 shows the basic principle of the PWM mode and the PFM mode.
Figure 2: PWM (pulse amplitude) mode and PFM (pulse FM) mode
2. How to call the DC-DC converter during PWM dimming
For energy saving, etc., mobile device liquid crystal display backlight automatic dimming function introduces DC-DC converter intermittent work. This is based on the use of ambient illumination, and the backlight brightness is automatically dimmed, thereby extending the system of battery usage time.
This dimming has a variety of ways, where controlling the LED light time and the length of the lamp are called PWM dimming. The advantage of the PWM mode dimming system is that the chromaticity changes caused by dimming, which are mainly used in backlights of laptops and tablets.
PWM dimming is intermittently operated by lower frequencies of about 200 Hz, and adjusts the brightness by repeated light / off operation. In the constant loop of the light / extinguished, the position will change when the light is light, and the shorter is diminished. In the intermittent work of about 200 Hz, the eyes are basically not aware of the backlight. However, since it is in a human ear-term frequency, when the current flows through the intermittent working in the substrate, the inductor body will vibrate due to the frequency of the frequency, resulting in a whistling.
Note: Duty cycle
In the DC-DC converter, the ON time ratio of the switch cycle (the switch device is only + OFF time) is called a duty cycle. When PWM dimming is performed on the LED, the light time / (light time + turn off time) is called a duty cycle and represents the brightness.
3. Frequency variable mode DC-DC converter caused by whiskers
The PWM mode DC-DC converter is characterized in that in normal work, its efficiency can be as high as approximately 80 to 90%. But under light load such as standby time, efficiency will be severely reduced. The loss caused by the switch is proportional to the frequency. To this end, constant switching losses occur in light load, thereby reduced efficiency.
Therefore, in order to improve this problem, use the DC-DC converter that automatically replaces the PWM mode with the PFM (pulse frequency modulation) method in light load. The PFM mode is a method of controlling the switching frequency in the case of a fixed ON time. Since the ON time is constant, the switching frequency will gradually decrease by extending OFF time. Since the switching loss is proportional to the frequency, it can be highly efficient in light load by lowering the frequency. However, the reduced frequency will enter the range of about 20 to 20 kHz of the human ear, and the power inductor will hamper.
4. Howling caused by load
For the purpose of saving battery power, there are various types of electricity technology such as mobile devices such as laptops, which may cause the inductor to scream. For example, for the purpose of taking into account low power consumption and processing power, the laptop CPU has periodic changed consumption current mode. When the cycle is in a human ear-repetable frequency range, the power inductor may be due to the influence. Send a whistle.
Note: The role of the power inductor in the DC-DC converter
The inductor can smoothly flow through the DC current, and the current changed by the alternating current, by self-inductance, the electromotive force is generated in the direction where the change occurs, and the effect is exerted. At this time, the inductor converts the electrical energy into magnetic energy, accumulates it, and releases it after converting into electrical energy. The size of this energy is proportional to the inductor value of the inductor.
The power inductor is also referred to as the power coil, the power choke, is a main element in the switching mode of the DC-DC converter, and coordinated with the capacitor, so that the high frequency generated by the switching device ON / OFF The pulse is smoother.
Since the power inductor of the power supply circuit flows through a large current, the winding type is mainstream product. This is because the high electrical effect is achieved by minimizing the magnetic body (ferrite or soft magnetic metal) in the magnetic core, so that the product is achieved to make the product more miniaturize. Figure 3 shows the Basic circuit of the DC-DC converter (non-insulated and chopper method) using a power inductor.
Figure 3: DC-DC converter (non-insulated type and chopping method) basic circuit
Mechanism of power inductor body vibration and noise expansion
When the current flowing through the human ear can be listened, the vibration of the power inductor body causes a whisker. The cause of vibration and the following possibility are possible.
Vibration cause
➀ magnetic magnetic core magnet telescopic (magnetic strain)
➁ Magnetic core magnetization leads to mutual attraction
➂ leakage flux causes winding vibration
Noise zoom
➀ Contact with other components
➁ Leak fluxing causes effects on peripheral magnetic properties
➂ The number of components including the substrate is consistent with the total number of vibrations
The cause of the vibration of the power inductor, and the reason for noise expansion are summarized as shown in Figure 4. The main contents of these reasons will be described below.
Figure 4: Causes of vibration caused by power inductors and expansion reasons
Various causes and roles of vibration
Vibration Cause ➀: Magnetic core magnet telescopic (magnetic strain)
After the magnetic field is applied to the magnetic field, it is slightly changed after the magnetization is magnetized. This phenomenon is called "magnetostrictive" or "magnetocarns". Among the inductors such as ferrite such as ferrite, the AC magnetic field produced by the winding causes the magnetic core to telescopic, and sometimes it detects its vibration.
Figure 5: Magnetic magnetostrictive (magnetic strain)
The magnetic body is a smaller assembly called magnetic domains (Fig. 5). The atomic magnet in the magnetic domain is the same, so the magnetic domain is a spontaneous magnetization towards a constant minute magnet, but the magnetic body does not exhibit the characteristics of the magnet. This is because, constituting the plurality of magnetic domains of the magnetic body, which arranges spontaneous magnetization unsearmed each other, so it is in an uniform state from the surface.
When a magnetic field is applied from the outside of the magnetic body in this degauss, each magnetic domain will unify the spontaneous magnetization as an external magnetic field direction, so the magnetic domain range will gradually change. This phenomenon is caused by the movement of the magnetic wall of the magnetic domain. Thereby, as magnetization progresses, the dominant magnetic domain gradually expands its range, and finally becomes a single magnetic domain, and facing the direction of the outer magnetic field (saturated magnetization). During this magnetization, a small position change occurs at the atomic level, and at the macroscopic level, it will appear as magnetostrictive, that is, the shape of the magnetic body.
The shape changes caused by magnetostrictiveness are extremely small, approximately 1 to 1 million in the original size of 1,000, but as shown in FIG. 5, flow over current in the state of the coil in the magnetic body, when applied When the AC magnetic field, the magnetic body will telescopically, and vibration is generated. To this end, in the power inductor, the magnetic body magnetic core vibration caused by magnetostrictiveness cannot be completely eliminated. The power inductor monomer vibration level is small, but when it is attached to the substrate, if the vibration is consistent with the inherent vibration number of the substrate, the vibration will be amplified, so that the whisker will be heard.
Causes of vibration: Magnetic core magnetization leads to mutual attraction
The magnetic body will show the magnet properties when magnetization is magnetized by an external magnetic field, thereby attracting each other with ambient magnetic body. Figure 6 shows an example of a full shielded power inductor. This is a power inductor that closes the magnetic circuit structure, but a gap is provided between the drum core and the shielded magnetic core (ring magnetic core), and the noise is sometimes emitted from the site. When the AC current flows through the AC, the magnetized drum and the shielding core of the magnetized magnet will be attracted to each other due to the magnetic force. If the vibration is within the human ear, the noise is heard.
The gap between the drum core and the shield core is closed by an adhesive, but in order to prevent cracking due to stress, a hard material is not used, so that the vibration caused by mutual attraction cannot be completely suppressed.
Figure 6: Drum core and shielding cores attract each other
Vibration Cause ➂: Leakage magnetic flux causes winding vibration
In the unshielded power inductor with a shielded core, a whistle is produced by mutual attraction caused by the aforementioned drum core and shielding core magnetization. But other problems will occur in the unshielded product. Since the unshielded product is an open magnetic circuit structure, the leakage magnetic flux will work around the coarse. Since the current flows through the current, according to the Buddha to make the left hand, the force will work on the winding. To this end, when the AC current flows through the windings, the windings themselves vibrate, thereby producing a whistle (Figure 7).
Figure 7: Magnetic flux causes winding vibration
Various reasons for noise amplification
Noise enlargement reasons for contact with other components
In the power supply circuit board of multiple electronic components and devices in high-density, if the inductor is in contact with other components, the slight vibration of the inductor will be enlarged, so that the whisker will be heard.
Noise amplification causes ➁ Leak flux flux results in a peripheral magnetic body
When there is a magnetic body such as a shield in the vicinity of the inductor, the magnetic experience is vibrating due to the influence of the inductor leakage flux.
Noise amplification reasons are consistent with the total number of vibrations, including components including substrates.
Typically, a small magnetic magnetic core monomer in a product such as an inductor, which is substantially not recognized as a whisker. However, the inductor is combined by a plurality of components and is attached to the substrate, and the number of inherent vibrations of multiple people can be generated, and the vibration will form a whistler. At the same time, if the number of inherent vibrations is consistent with the number of components, there may be a whistling after installation to the component.
Figure 8 shows an example of analyzing the vibration of the substrate of the power inductor by the computer simulator using the FEM (finite element). In the analytical model used, the power inductor is disposed in the center of the substrate (FR4) and is fixed to the long side of the substrate.
Under normal circumstances, the solid value (number of inherent vibrations) of the structural resonance has a plurality of, which corresponds to a wide variety of vibration modes. In the analysis model of the "Power Induction + Substrate", as the frequency is improved, the number of intrinsic vibrations will also have a variety of vibration modes. In Fig. 8, the power inductor may be a vibration source in FIG. 8, 2 times, 5, 18 vibration mode. Among them, the vibration frequency of 1 mode is basically the same as the vibration frequency of the power inductor monomer. However, it is worth noting that 2 modes of the Z direction (high direction) vibration have a higher frequency in the case of a power inductor monomer, but a very low frequency is formed after being fixed to the substrate.
The "Analysis Model" power inductor is disposed in the center of the substrate (FR4).
Boundary condition: Fixed substrate long side 2 surfaces.
1 mode: 2034Hz ~
2 mode: 2262Hz ~
5 times mode: 4048Hz ~
18 times mode: 16226Hz ~
Figure 8: Example of analyzing the vibration of "Power Inductor + Substrate" by computer simulator
Screaming countermeasure for power inductor
The power inductor of the DC-DC converter is summarized below.
Focus 1: Avoid flowing through the human ear to listen to frequencies
Avoiding human ear to listen to frequencies is the most basic countermeasure.
However, when energy saving, the DC-DC converter such as energy saving and the DC-DC converter of the frequency variable mode cannot avoid the power of the human ear to hear, please try the following silence countermeasures.
Focus 2: Do not place magnetic body around
Magnetic bodies (shields, etc.) that may be leaked in the vicinity of the inductor. If you need to be close, you should use a small shield type (closed magnetic circuit structure) of the leakage (closed magnetic circuit structure), and should also pay attention to the direction of placement.
Focus 3: Stick to the number of inherent vibrations
Sometimes the number of vibrations can be reduced by the number of vibrations or improved vibration. For example, by changing the inductor shape, type, layout, substrate fastening, etc., the number of components containing the substrate will vary. In addition, the whistle is often seen in a large power inductor above 7mm sizes. By using a small power inductor of 5mm or less, the number of inherent vibrations will increase, thereby reducing the screaming.
Focus 4: Replacement is a metal integral shape
As described above, in the full shielded power inductor, the drum core and the shielding core are attracted to each other by magnetism, thereby screaming at the gap portion. At the same time, wire vibration caused by leakage fluxing causes a whisker in the unshielded power inductor.
For such power inductors, the problem is replaced as a metal-integrated molding type is an effective solution. This is a power inductor that is integrally formed after embedding the hollow metal magnetic powder. Since there is no gap, the core does not attract each other, and it is integrated with the magnetic body during the fixed coil, so it can also avoid problems with the winding vibration due to the magnetic flux. Not only that, TDK products also use a small metal magnetic material, which can inhibit vibration caused by magnetosformation, and it is expected to reduce the shackles by replacementless or full shielding products.
Full shielded and metallic integrated noise comparison
The full shielded and semi-shielded power inductor (TDK product, about 6 mm size), and full shield type and metal integrated power inductor (TDK product, about 12 mm size) are measured as measurement samples, and the occurrence of noise We conducted a survey. Install the microphone inside the muzzle box, a sine wave current of 0a to rated current to the measurement sample installed on the substrate60 seconds, and the human ear can be scanned by the human ear audition frequency 20Hz ~ 20kHz, and its peak sound pressure is recorded here (Fig. 8).
As shown in the chart, compare full shielded and semi-shielded types can be found after the sound pressure is different due to frequencies.
When the full shield type is integrated with metal, the difference is more significant. In full shielding, there is a noise of about 30 to 50 dB in a wide range of frequency bands. In the metallic integral shape, in a large range of frequency bands, it is in the same low level, even in the peak portion, it also suppresses about 20 dB compared to the full shielding type. Inhibition of 20 dB means only 10, one level, which is visible, and the metal-integral forming type is an effective countermeasure.
Figure 9: Example of noise assessment of various types of power inductor
Source: TDK
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