"When laptops, tablets, smart phones, televisions and on-board electronic devices are running, sometimes you can hear" Ji "noise. This phenomenon is called "whistling", which may be caused by passive components such as capacitors and inductors. The principle of capacitor whistling is different from that of inductor, especially the whistling of inductor. Its causes are diverse and complex. In this paper, the main components of power circuits such as DC-DC converter - power inductors, the causes of howling and effective countermeasures will be introduced.
Causes of power inductor whistling
1. Intermittent operation, variable frequency mode, load variation, etc. may cause audible frequency vibration of human ears
Sound wave is an elastic wave propagating in the air. Human hearing can hear "sound" in the frequency range of about 20 ~ 20kHz. In the power inductor of DC-DC converter, when the AC current and pulse wave flowing through the audible range frequency of human ears, the main body of the inductor will vibrate. This phenomenon is called "coil noise", and sometimes it will be heard as howling phenomenon (Fig. 1).
Figure 1: power inductor whistling mechanism
With the continuous strengthening of the functions of electronic equipment, the power inductor of DC-DC converter has also become one of the sources of noise. The DC-DC converter is on / off through the switching device, thereby generating a pulsed current. By controlling the time length (pulse width) of on, a stable DC current with constant voltage can be obtained. This method is called PWM (pulse amplitude modulation), which is widely used as the mainstream of DC-DC converter.
However, the switching frequency of DC-DC converter is high, reaching several 100kHz ~ several MHz. Since the frequency vibration exceeds the audible range of human ears, noise will not be felt. Then, why does the power inductor of DC-DC converter emit "Ji" scream?
There are several possible reasons. First, it is possible to make the DC-DC converter work intermittently for the purpose of saving battery power, or switch the DC-DC converter from PWM mode to PFM (pulse frequency modulation) mode and operate in variable frequency mode. Figure 2 shows the basic principles of PWM mode and PFM mode.
Figure 2: PWM (pulse amplitude modulation) mode and PFM (pulse frequency modulation) mode
2. Whistling caused by intermittent operation of DC-DC converter such as PWM dimming
For the purpose of energy saving, the automatic dimming function of LCD backlight of mobile devices introduces the intermittent operation of DC-DC converter. This is a system that automatically dims the brightness of the backlight according to the ambient illumination, so as to prolong the service time of the battery.
There are many ways of dimming, among which the way to control the LED on time and off time is called PWM dimming. The advantage of PWM dimming system is that there is less chromaticity change caused by dimming. 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 about 200Hz, and adjusts the brightness by repeatedly turning on / off the lamp. In the constant cycle of lighting / extinguishing, the time of dimming will be brightened and the dimming will be made when it is short. In the intermittent operation of about 200Hz, the eyes basically will not detect the backlight stroboscopic condition. However, because it is in the audible frequency of human ears, when the intermittent working 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 howling.
Note: duty cycle
In the DC-DC converter, the on time ratio relative to the switching cycle (on time + off time of the switching device) is called the duty cycle. When PWM dimming the LED, the light on time / (light on time + light off time) is called the duty cycle and represents the brightness.
3. Whistling caused by frequency variable mode DC-DC converter
The characteristic of PWM DC-DC converter is that its efficiency can be as high as about 80 ~ 90% in ordinary work. However, under light load conditions such as standby time, the efficiency will be seriously reduced. The loss caused by the switch is proportional to the frequency. Therefore, constant switching loss will occur under light load, so the efficiency will be reduced.
Therefore, in order to improve this problem, a DC-DC converter that automatically replaces PWM mode with PFM (pulse frequency modulation) mode is used under light load. PFM mode is to control the switching frequency under the condition of fixed on time in combination with load reduction. Since the on time is constant, the switching frequency will gradually decrease by extending the off time. Since the switching loss is directly proportional to the frequency, high efficiency can be achieved under light load by reducing the frequency. However, the reduced frequency will enter the audible range of about 20 ~ 20kHz, and the power inductor will howl.
4. Whistling caused by load
For the purpose of saving battery power, various power-saving technologies are used in mobile devices such as notebook computers, which may lead to the roar of inductors. For example, for the purpose of taking into account low power consumption and processing capacity, the laptop CPU has a mode of periodically changing the current consumption. When the cycle is in the audible frequency range of human ears, the power inductor may howl due to this effect.
Note: function of power inductor in DC-DC converter
The inductor can make the DC current flow smoothly, while for the current with changes in AC current, it can generate electromotive force in the direction of preventing changes through self induction and play the role of resistance. At this time, the inductor converts electrical energy into magnetic energy, accumulates it, and releases it after converting it into electrical energy. The energy is proportional to the inductance of the inductor.
Power inductors, also known as power coils and power chokes, are the main components used in switching power supply circuits such as DC-DC converters. Through coordination with capacitors, the high-frequency pulses generated by switching devices on / off are smoother.
Because the power inductor of the power supply circuit will flow large current, the winding type is the mainstream product. This is because by using a magnetic body with high permeability (ferrite or soft magnetic metal) in the magnetic core, the high inductance value can be achieved with less records, so that the product can be more miniaturized. Fig. 3 shows the basic circuit of DC-DC converter (non insulated and chopper mode) using power inductor.
Figure 3: basic circuit of DC-DC converter (non insulated and chopper mode)
Mechanism of main body vibration and noise expansion of power inductor
When the current flows through the audible frequency range of human ears, the vibration of the main body of the power inductor will cause howling. There are several possible causes of vibration and noise.
Causes of vibration
➀ magnetostrictive (magnetic strain) effect of magnetic core
➁ magnetization of magnetic core leads to mutual attraction
➂ winding vibration caused by magnetic flux leakage
Causes of noise amplification
➀ contact with other elements
➁ magnetic flux leakage leads to the effect on the surrounding magnetic body
➂ consistent with the overall natural vibration number of components including the base plate
The vibration causes of power inductor howling and the causes of noise expansion are summarized in Figure 4. The main contents of these reasons are described below.
Figure 4: vibration causes and expansion causes of power inductor whistling
Various causes and effects of vibration
Vibration cause ➀: magnetostriction (magnetic strain) of magnetic core of magnetic body
After applying a magnetic field to the magnetic body to magnetize it, its shape will change slightly. This phenomenon is called "magnetostriction" or "magnetic strain". In inductors with ferrite and other magnetic cores, the AC magnetic field generated by the winding will stretch the magnetic core, and sometimes its vibration sound will be detected.
Figure 5: magnetostriction (magnetic strain) effect of magnetic body
Magnetic bodies are small-scale aggregates called magnetic domains (Fig. 5). The atomic magnetic moment in the magnetic domain faces the same direction, so the magnetic domain is a small magnet with constant spontaneous magnetization, but the magnetic body as a whole does not show the characteristics of a magnet. This is because the arrangement of a plurality of magnetic domains constituting a magnetic body cancels the spontaneous magnetization, so it is in a degaussing state on the surface.
When a magnetic field is applied to the magnetic body in the degaussing state from the outside, each magnetic domain will unify the spontaneous magnetization in the direction of the external magnetic field, so the magnetic domain range will gradually change. This phenomenon is caused by the movement of the magnetic wall, the boundary between magnetic domains. Thus, with the progress of magnetization, the dominant magnetic domain gradually expands its range, finally becomes a single magnetic domain, and faces the direction of the external magnetic field (saturated magnetization state). In this magnetization process, small position changes will occur at the atomic level, while at the macro level, it will show magnetostriction, that is, the shape change of the magnetic body.
The shape change caused by magnetostriction is extremely small, about 1 / 10000 ~ 1 / 1000000 of the original size. However, as shown in Fig. 5, the current flows when the coil is wound on the magnetic body. When the generated AC magnetic field is applied, the magnetic body will expand and contract repeatedly and produce vibration. Therefore, the magnetic core vibration caused by magnetostriction cannot be completely eliminated in the power inductor. Although the vibration level of the single power inductor 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 scream will be heard.
Cause of vibration ➁: mutual attraction caused by magnetization of magnetic core of magnetic body
When the magnetic body is magnetized by an external magnetic field, it will show the properties of a magnet and attract each other with the surrounding magnetic body. Fig. 6 shows an example of a full screen power inductor. This is a power inductor with closed magnetic circuit structure, but there is a gap between the drum core and the shielding core (annular core), and the noise will sometimes be emitted from there. When AC current flows through the winding, the drum core and shielding core magnetized by the generated magnetic field will attract each other due to magnetic force. If the vibration is within the audible frequency range of human ears, noise will be heard.
The gap between the drum core and the shielding magnetic core is closed by adhesive, but in order to prevent cracking due to stress, hard materials will not be used, so the vibration caused by mutual attraction cannot be completely suppressed.
Figure 6: drum core and shielding core attract each other, resulting in howling
Vibration cause ➂: winding vibration caused by magnetic flux leakage
In the unshielded power inductor without a shielded magnetic core, there will be no howling due to the mutual attraction caused by the magnetization of the drum core and the shielded magnetic core. However, other problems will occur in unshielded products. Because the unshielded product is of open magnetic circuit structure, the magnetic flux leakage will have an effect on the winding thickness. Due to the current flowing in the winding, according to Fleming's left-hand rule, the force will act on the winding. Therefore, when AC current flows through the winding, the winding itself will vibrate, resulting in howling (Fig. 7).
Figure 7: winding vibration caused by magnetic flux
Various causes of noise amplification
Cause of noise amplification ➀ contact with other components
In the power supply circuit board with multiple electronic components and equipment pasted on a high density, if the inductor contacts with other components, the small vibration of the inductor will be amplified and a roar will be heard.
Noise amplification cause ➁ magnetic flux leakage causes the effect on the surrounding magnetic bodies
When there are magnetic bodies such as shielding cover near the inductor, the magnetism will vibrate and scream due to the influence of inductor magnetic flux leakage.
The reason for noise amplification ➂ is consistent with the overall natural vibration number of components including the base plate
Generally, the air vibration caused by magnetostriction of small magnetic core monomer used in inductors and other products will not be recognized as howling. However, when the inductor is composed of multiple components and mounted on the substrate, it will produce the natural vibration number of the audible frequency of multiple human ears, and the roar will be formed after the vibration is amplified. At the same time, if it is consistent with multiple natural vibration numbers of the whole assembly, whistling may occur after installation in the assembly.
Fig. 8 shows an example of analyzing the vibration of a substrate with a power inductor by using a computer simulator using FEM (finite element method). In the analysis model used, the power inductor is arranged in the center of the substrate (FR4), and the two sides of the long side of the substrate are fixed.
In general, there are many natural values (natural vibration number) of structural resonance. Accordingly, there will be a variety of vibration modes. In the "power inductor + substrate" analysis model, with the increase of frequency, various vibration modes will appear in each natural vibration number. In the 1st, 2nd, 5th and 18th vibration modes shown in Fig. 8, the power inductor may be the vibration source. The vibration frequency of the primary mode is basically the same as that of the single power inductor. However, it should be noted that the secondary mode with significant vibration in Z direction (height direction) has a high frequency in the case of power inductor monomer, but a very low frequency after fixed on the substrate.
The power inductor of the analysis model is configured in the center of the substrate (FR4).
Boundary conditions: 2 sides on the long side of the fixed base plate.
Primary mode: 2034hz~
Secondary mode: 2262hz~
5 times mode: 4048hz~
18 times mode: 16226hz~
Figure 8: example of vibration analysis of "" power inductor + substrate "" by computer simulator
Howling Countermeasures of power inductors
The following summarizes the key countermeasures of power inductor howling of DC-DC converter.
Key 1: avoid the current flowing through the audible frequency of human ears
The most basic countermeasure is to avoid the current flowing through the audible frequency of human ears.
However, when intermittent operation for the purpose of energy saving and DC-DC converter with variable frequency mode cannot avoid the power on of human ear audible frequency, please try the following mute countermeasures.
Key 2: no magnetic body is placed around
Do not place magnetic objects (shield, etc.) that may be affected by magnetic flux leakage near the inductor. When it is necessary to approach, a shielded inductor (closed magnetic circuit structure) with less magnetic flux leakage shall be used, and the placement direction shall be paid attention to.
Key 3: stagger the number of natural vibrations
Sometimes the howling can be reduced by staggering the natural vibration number or increasing the vibration number. For example, by changing the shape, type, layout, substrate fastening and other conditions of the inductor, the overall natural vibration number of the component containing the substrate will change. In addition, howling is common in large power inductors with a size of more than 7mm. By using a small power inductor less than 5mm, the natural vibration number will be increased, so as to reduce the howling.
Key point 4: replace with metal integrated type
As described above, in the full screen power inductor, the drum core and the shielding core will attract each other due to magnetism, resulting in howling at the gap. At the same time, in the unshielded power inductor, the wire vibration caused by magnetic flux leakage will lead to howling.
For this kind of power inductor whistling problem, it is an effective solution to replace it with metal integrated type. This is a power inductor formed by embedding a hollow coil in a soft magnetic metal magnetic powder. Because there is no gap, the magnetic cores will not attract each other. At the same time, because the coil is integrated with the magnetic body when fixing the coil, the problem of winding vibration caused by magnetic flux can also be avoided. Moreover, TDK products also use metal magnetic materials with small magnetostriction, so it can suppress the vibration caused by magnetostriction. It is expected to reduce the howling by replacing unshielded or full screen products.
Comparison of noise between full screen type and metal integrated type
In the following, full screen and semi screen power inductors (TDK products, about 6mm size) and full screen and metal integrated power inductors (TDK products, about 12mm size) are taken as measurement samples to investigate the occurrence of noise. A microphone is installed inside the silencing box, and the measurement sample installed on the substrate is connected with a sine wave current of 0A ~ rated current
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