Ball-shape noise filter

Bead noise filter

Technical field

The present invention relates to a bead type noise filter, which is used for signal lines and power lines mounted on a circuit board to suppress undesired radiation noise.

Background technique

As the size of electronic devices decreases and performance improves, high-frequency noise radiated from signal lines and power lines (hereinafter referred to as "signal lines") has become a major problem. Passing the signal line through a spiral tube or cylindrical closed-path magnetic core called a "bead core" can suppress most of these noises in the simplest way. In fact, usually the signal wire is inserted into the magnetic core to wind one turn (insert one turn) on the magnetic core. In some cases, the magnetic core can be provided with through holes that are often inserted, and the signal wire passes through these through holes and winds multiple turns on the magnetic core. .

The bead-shaped noise filter affecting the above-mentioned noise suppression method will be described below with reference to FIG. 1. Fig. 1 shows a closed-path magnetic core (bead core) 1 formed of a soft magnetic material with high resistivity. The magnetic core 1 is cylindrical and has a through hole 1a along its central axis, which is attached to the signal line 2 so that the signal line 2 passes through the through hole 1a, thereby functioning as a bead-shaped noise filter. Alternatively, the magnetic core 1 may be provided with a plurality of through holes 1a. (See Figure 2).

In order to suppress undesirable radiation noise of tens to hundreds of MHz, the magnetic core 1 is usually formed of Ni-Zn ferrite with a resistivity of 102 to 105 Ωm in order to operate properly in such a high frequency band. Moreover, the magnetic core 1 is required to have a high resistivity, so that even when the signal line 2 inserted into the through hole 1a has no insulating coating and may directly contact the through hole, it will not cause trouble. This is also the use of Ni-Zn to form the magnetic core Another reason for 1. Using Ni-Zn ferrite containing an expensive Ni material to form the magnetic core 1 increases the manufacturing cost of the bead noise filter, even if the structure is simple.

On the other hand, the conventional Mn-Zn ferrite is a cheap soft ferrite, but the resistivity is as low as 10-1-100Ωm. The eddy current loss increases sharply in a frequency band lower than the signal frequency band for which noise suppression is required. Therefore, the magnetic core 1 made of Mn-Zn ferrite cannot be used in the high frequency band. Moreover, another problem with the magnetic core 1 made of Mn-Zn ferrite is that when the signal line passes through the through hole 1a, the signal line 2 without an insulating coating must not directly contact the through hole 1a of the magnetic core 1 .

Mg-Zn ferrite is another inexpensive soft ferrite material. However, compared with other ferrite materials, Mg-Zn ferrite has poor soft magnetic properties. For example, the soft magnetic properties such as initial permeability and saturation magnetic flux density are worse than other ferrite materials. Therefore, when the magnetic core 1 is composed of Mg-Zn ferrite, in order to achieve the same performance as the bead noise filter, the magnetic core 1 composed of Mg-Zn ferrite must be increased in size. In particular, when used to suppress noise in the signal line 2 (especially power supply line) (in which fluctuating surge and surge noise in the signal line 2 become a problem), the size of the magnetic core 1 must be further increased to prevent magnetic saturation. Therefore, Mg-Zn ferrite cannot be used for bead noise filters.

Referring to FIG. 3, the equivalent circuit diagram of the bead noise filter shown in FIG. 3 is a parallel circuit composed of a series circuit composed of an inductance element L and a resistance element R and a capacitance element C. In the following, the inductance element L, the resistance element R, and the capacitance element C are simplified into an L element, an R element, and a C element.

In Figure 3, in the frequency band of the signal to be sent (signal frequency band), there is a relational expression L>>R, where L is the value of the L element, R is the value of the R element, and each element value is expressed as an impedance value|Z |. Therefore, the series circuit of the R and L elements in the bead noise filter has an inductance function (almost only the L element), so the signal to be transmitted does not have the loss caused by the R element. Therefore, the bead noise filter and the C component on the signal line constitute the low-pass filter in the circuit. However, since the value of the L component is small and the cutoff frequency is higher than the frequency band of the signal to be transmitted, the transmission loss of the signal can be reduced. can be ignored.

In the frequency band higher than the frequency band of the signal to be transmitted, there is a relational expression of L<<R. Therefore, the series circuit of R and L elements has a resistance function (almost only R elements), which can absorb noise due to heat generation, which is particularly helpful Effectively suppress unwanted radiation noise.

The impedance value of the bead noise filter |Z| is divided into the reactance X component (hereinafter referred to as the X component for ease of use) and the resistance R component, expressed by the following formula: |Z|=(X2+R2)]]> formula ( 1)

When an AC magnetic field is applied to the core of the bead noise filter, its complex permeability μ is expressed by the following formula: μ=(μ2+μ''2)]]>Equation (2) Where μ'is the real part. The X component of the bead noise filter is generated by the real part μ', and the R component of the bead noise filter is generated by the imaginary part μ".

Therefore, the X component plays a dominant role in the signal frequency band to be transmitted, that is, the bead noise filter combined with the C component on the signal line acts as a low-pass filter to prevent the noise component superimposed on the signal from passing through . However, the blocked and prevented noise components may affect other circuits. On the other hand, in the high frequency band where radiation noise is generated, the R component is the dominant part, and the noise component including the radiation noise is converted into heat energy to form a noise elimination factor. Compared with being blocked by a low-pass filter, being converted into heat energy can eliminate noise more safely and reliably.

The frequency at which the X component and the R component have the same value, that is, the frequency of the XR cross point, is the critical frequency of the change between the X component of reflected noise and the R component that converts noise into heat energy. Generally, the more the R component converts noise into heat energy, the better the noise can be eliminated. Therefore, if the impedance characteristics are equal, the lower the frequency of the XR crossover point, the better.

If the X component in the high frequency band is large and the R component is small, the capacitive element C on the signal line will produce LC resonance with a large Q value (Q is an indicator of inductance characteristics). Therefore, according to the bead noise filter In the connected circuit, the input digital signal will have waveform distortion, for example, a ring appears. Therefore, in the high frequency band, the smaller the X component, the better.

According to the above-mentioned frequency band and the relationship between the X component and the R component, the XR cross point of the bead noise filter composed of the magnetic core composed of the Ni-Zn ferrite approximately appears at 10MHz belonging to the high frequency band. , Its X reactance component still keeps increasing in the high frequency band. Therefore, if a bead noise filter composed of Ni-Zn ferrite is connected to the input signal line of a high-impedance digital circuit (such as a C-MOS inverter) with a few PF capacitance, then the input digital signal will be Waveform distortion occurs, for example, a circular ring shape, an undershooting signal, and an overshooting signal appear. These waveform distortions are caused by LC resonance with a relatively large Q value. Due to the waveform distortion and the high price of the aforementioned Ni-Zn, a magnetic core made of non-Ni-Zn ferrite is required to form a noise filter. In particular, a bead whose XR cross point frequency is lower than the XR cross point frequency of a bead noise filter composed of a magnetic core composed of Ni-Zn ferrite, and which mainly uses the R component to suppress noise more completely and reliably Shape noise filter.

The present invention has been described in accordance with the above requirements. The object of the present invention is to provide an inexpensive bead-shaped noise filter that can suppress noise while avoiding waveform distortion in transmission signals such as digital signals, so that in recent years Higher frequency and digitization of signals processed in electronic devices.

Specifically, the object of the present invention (according to claim 1) is to provide a bead-shaped noise filter composed of a magnetic core composed of inexpensive Mn-Zn ferrite, which has a significantly increased Resistivity, in order to obtain soft magnetic properties equivalent to the soft magnetic characteristics of Ni-Zn ferrite in the high-frequency band, thereby suppressing the radiated noise of tens to hundreds of MHz, and also provide a bead-shaped noise filter, which allows no If necessary, the signal wire or power wire of the insulating coating directly contacts the through hole of the magnetic core and passes through the through hole. Moreover, the purpose of the invention (according to claims 2 and 3) is to finally provide a high-performance bead-shaped noise filter that, after achieving the above-mentioned goal, mainly uses the R component to safely and reliably suppress the noise, so as not to cause the transmission of the signal. Waveform distortion.

Summary of the invention

According to claim 1 of the present invention, the bead-shaped noise filter includes a magnetic core with a through hole to form a closed magnetic circuit, and the bead-shaped noise filter is connected to one of the signal line and the power line, and the line passes through The closed magnetic circuit of the magnetic core, the magnetic core is made of soft magnetic material. The complex relative permittivity range of the soft magnetic material is: 1000-20000 at 1kHz frequency, and below 50 at 1MHz frequency; the resistivity is 150Ωm or higher. Therefore, a low-cost bead noise filter can be made, which has a high resistivity, and at the same time has soft magnetic characteristics equivalent to a bead noise filter made of Ni-Zn ferrite in the high frequency band, and can suppress Radiated noise of tens to hundreds of MHz.

According to claim 2 of the present invention, the magnetic core is made of Mn-Zn ferrite. The basic components of Mn-Zn ferrite include: 44.4-50.0 (not including 50.0) mol% Fe2O3; 4.0-26.5 mol% ZnO; 0.1-8.0 mol% of at least one of TiO2 and SnO2; and MnO is the remaining component. Therefore, it is possible to manufacture an inexpensive bead-shaped noise filter, which mainly uses the R component to suppress noise in a safer and more reliable manner, without causing distortion of the waveform of the transmitted signal.

According to the claims of the present invention, Mn-Zn ferrite is used to form the magnetic core. The basic composition of Mn-Zn ferrite includes: 44.4 to 50.0 (not including 50.0) mol% Fe2O3; 4.0-26.5 mol % ZnO; 0.1-8.0 mol% of at least one of TiO2 and SnO2; 0.1-16.0 mol% of CuO; and MnO are the remaining components. Therefore, it is possible to manufacture an inexpensive bead-shaped noise filter and achieve the same performance as the above-mentioned according to claim 2 of the present invention.

Description of the drawings

Fig. 1 is a perspective view of a bead noise filter according to the present invention; Fig. 2 is a perspective view of a bead noise filter according to the present invention; Fig. 3 is an equivalent circuit diagram of a bead noise filter; Fig. 4 is after use The impedance frequency characteristic curve of the bead noise filter of the magnetic core sample shown in Table 1 to be described; Fig. 5 is the impedance frequency of the example 1 of the present invention (the bead noise filter composed of the magnetic core numbered as sample 1) Characteristic curve; and Fig. 6 according to the impedance frequency characteristic curve of Comparative Example 2 (using a bead-shaped noise filter composed of a magnetic core numbered as sample 4).

detailed description

Hereinafter, a preferred embodiment of the present invention will be described with reference to FIGS. 1-6.

Referring to FIG. 1, the bead noise filter is composed of a magnetic core 1 (bead core), the magnetic core 1 has a high resistivity, and the signal line 2 (or power line) is inserted and passed through the magnetic core 1. The magnetic core 1 is cylindrical and has a through hole 1a along its central axis, and the signal line 2 passes through the through hole 1a. Figure 2 shows another bead-shaped noise filter. See Figure 2. The magnetic core 1 in Figure 2 is made of the same material as the magnetic core 1 shown in Figure 1. It also has a cylindrical shape, but has two through holes. 1a, the signal line 2 passes through one through hole 1a and turns to pass through the other through hole 1a.

Referring to FIG. 3, FIG. 3 is an equivalent circuit diagram of the bead noise filter. The equivalent circuit diagram shown in FIG. 3 is a series circuit formed by an inductance element L and a resistance element R and then a parallel circuit formed by the capacitive element C.

Fig. 1 and Fig. 2 show the bead noise filter according to the present invention, and Fig. 3 is an equivalent circuit diagram of the bead noise filter according to the present invention.

The conventional Mn-Zn ferrite containing 50.0 mol% or more of Fe2O3 is inexpensive and has good characteristics, but the resistivity is too low. Therefore, a bead-shaped noise filter with a magnetic core made of this Mn-Zn ferrite cannot be used in the high-frequency band, and it is not allowed to contact with signal lines without an insulating coating.

The inventor disclosed Mn-Zn ferrite in such as Japanese Patent Nos. 3108803 and No. 3108804, when the Mn-Zn ferrite contains less than 50.0 mol% Fe2O3 and an appropriate amount of at least one of TiO2 and SnO2 At this time, the resistivity of Mn-Zn ferrite increases significantly. However, according to the purpose of the present invention, that is, a high-performance bead noise filter that mainly uses the R component to suppress noise in a safe and reliable manner without causing distortion of the transmitted signal waveform, it is impossible to use a ferrite like Ni-Zn Such a high-resistivity ferrite can be realized. Specifically, since the XR cross point frequency of the core formed of Ni-Zn ferrite is at 10 MHz or higher, which belongs to the high-frequency band, as will be described below, the X reactance component will keep increasing in the high-frequency band. Therefore, if a bead-shaped noise filter composed of such a magnetic core is connected to the input signal line of a high-impedance digital circuit, it will cause the waveform distortion of the input digital signal, for example, a spike signal, a ring signal, etc. appear.

When an AC magnetic field is applied to the core of the bead noise filter whose equivalent circuit is shown in Figure 3, the permittivity ε is represented by the following complex number: ϵ=(ϵ'2+ϵ''2 )]]> Equation (3) where εis the real part of the complex relative permittivity, and ε" is the imaginary part of the complex relative permittivity. The imaginary part of the complex relative permittivity ε" is the component that causes dielectric loss.

Because this kind of bead noise filter is often connected to the signal line, the signal line passes through the magnetic core (bead core) and winds one turn (insert one turn) on the magnetic core. Due to the distance between the input end and the output end Therefore, the capacitance C component between the windings (the signal wires wound on the magnetic core) is small. There is not only a capacitance component C between the windings, but also a capacitance component C between the winding and the magnetic core, and therefore depends on the real part εof the complex relative permittivity of the magnetic core. For example, in the case of a Mn-Zn ferrite having a very large complex relative permittivity real part ε, a large capacitance component C is generated between the winding and the magnetic core. In particular, since the winding of the bead noise filter passes through the magnetic core in close contact, the capacitance component C mainly depends on the real part εof the complex relative permittivity of the magnetic core.

The inventors manufactured Mn-Zn ferrite bead noise filters with the same L component and R component, measured their impedance characteristics, and found the following facts.

In the conventional Mn-Zn ferrite with a large complex relative permittivity real part ε(large C component), it is found that the frequency of the XR cross point is close to the low frequency, but the impedance characteristic in the high frequency band is poor. On the other hand, in Ni-Zn ferrite with a small complex relative permittivity real part ε(small C component), it is found that the frequency of the XR cross point is close to the high frequency, but the impedance characteristic in the high frequency band is good . Moreover, the real part of the complex relative permittivity εof conventional Mn-Zn ferrite and Ni-Zn ferrite is basically kept constant, and there is only less than a single digit change from the low-frequency band to the high-frequency band (if any if).

Based on the above findings, the inventors of the present invention used a magnetic core formed of a soft magnetic material with a large change in the real part εof the complex relative permittivity from the low-frequency band to the high-frequency band to construct a bead-shaped noise filter. In the above-mentioned bead noise filter, since the real part of the complex relative permittivity εis large in the low frequency band to a certain extent, the frequency of the XR crossover point is close to the low frequency band, because the real part of the complex relative permittivity εis small in the high frequency band. , So the impedance characteristic is excellent in the high frequency band.

According to the present invention, the resistivity of the magnetic core is set to 150 Ωm or higher. It is not certain whether there will be a problem when a copper wire without an insulating coating is directly wound on the core, because it is related to the applied voltage, but in a wide range of applications, it is believed that a core with a resistivity of 150Ωm or higher Able to undertake appropriate work.

According to the present invention, the range of the real part εof the complex relative permittivity is set to be 1000 to 20000 at a frequency of 1 kHz, and 50 or less at a frequency of 1 MHz. As mentioned above, the real part εof the complex relative permittivity of this magnetic core changes significantly from the low frequency band to the high frequency band. The frequency of the XR cross point of the bead noise filter is close to the low frequency, for example, 5MHz.

Hereinafter, embodiments of the present invention will be explained. The range of the real part εof the complex relative permittivity of the soft magnetic material used to form the core of the bead noise filter is: 1000 to 20000 at a frequency of 1kHz, and 50 or less at a frequency of 1MHz, and resistivity It is 150Ωm or higher. Each magnetic core has a through hole, so a closed magnetic circuit is formed, and is attached to the signal line so that the signal line passes through the through hole of the magnetic core.

Example Process 1 Table 1 shows the basic components of the magnetic core samples. Some magnetic cores are made of soft magnetic materials according to the present invention, and other magnetic core samples are used as comparative examples and are made of conventional soft magnetic materials.

Table 1

Referring to Table 1, five magnetic core samples (Sample 1-Sample 5) were prepared. The material particles are selected from: Fe2O3, ZnO, TiO2, SnO2, CuO, MnO, NiO and MgO. The weight of each component is shown in Table 1. The selected components are mixed by ball milling to produce various mixtures, and the various mixtures are at 900 Calcined in the atmosphere at °C for 2 hours. The calcined various mixtures were ground into particles with an average particle diameter of 1.4 μm using a ball mill. Then, the various mixtures after grinding are mixed with the added polyvinyl alcohol, and then granulated, and molded into various green bodies with a spiral tube structure under a pressure of 80 MPa. To make the green bodies of samples 1 to 3, sinter them for 3 hours at a temperature of 1150°C in an atmosphere where nitrogen is injected to control the partial oxygen pressure; to make the green bodies of samples 4 and 5, Sintering in the atmosphere at a temperature of 1150°C for 3 hours.

Thus, samples 1 to 5 with an outer diameter of 3.2 mm, an inner diameter of 1.6 mm, and a height of 6 mm were prepared. Test samples 1-5 (core samples for bead noise filters) made in this way: initial permeability μi of 0.1 MHz; saturation magnetic flux density Bs of Z 1194A/m; resistivity ρν (Ωm); and The real part εof the complex relative permittivity at 1kHz and 1MHz. The test data is listed in Table 2.

Table 2

The initial magnetic permeability μi and the saturation magnetic flux density Bs of the sample 5 formed with Mg-Zn ferrite are both low, so compared with the samples 1-4, the sample 5 has no superiority. In particular, the bead-shaped noise filter is required to prevent magnetic saturation due to fluctuating current and surge noise, so it is necessary to increase the size of the sample 5 having a low saturation magnetic flux density Bs.

Sample 3 composed of ordinary Mn-Zn ferrite has good initial permeability μi and saturation magnetic flux density Bs, but the resistivity ρν is extremely low. So it will hinder its use in the high frequency band. Moreover, sample 3 must not be in direct contact with uninsulated signal wires, which inevitably limits its application.

Sample 1 formed with the Mn-Zn ferrite of the present invention (according to claim 1), Sample 2 formed with the Mn-Zn ferrite of the present invention (according to claim 3), and Ni-Zn ferrite The formed samples 4 all have excellent initial permeability μi, saturation magnetic flux density Bs, and resistivity ρν.

Process 2 Samples 1-5 (bead cores) constructed according to the above process 1 can all be connected to the signal line as a bead noise filter, and the signal line is passed through the magnetic core and wound on it by one turn (insert one turn). Examples of samples made in this way are shown in Table 3, where the frequency of XR crossover points was tested.

table 3Figure 4 shows the test results of the impedance frequency characteristics of Examples 1-5. The impedance frequency characteristic shown in Figure 4 is the actual characteristic of the bead noise filter.

As shown in Figure 4, the frequency above 10MHz is the key frequency band for noise suppression. Compared with other examples (Inventive Examples 1 and 2 and Comparative Examples 2 and 3), the impedance frequency characteristic of Comparative Example 1 is significantly lower. The reason is that the resistivity ρν of Mn-Zn ferrite is low.

Compared with other examples (Inventive Examples 1 and 2 and Comparative Examples 1 and 2), the impedance frequency characteristic of Comparative Example 3 is low. The reason is that the initial permeability μi of Mn-Zn ferrite is low.

In the high frequency band, Invention Examples 1 and 2 and Comparative Example 2 have excellent impedance characteristics.

The various test results of Inventive Examples 1 and 2 and Inventive Examples 1 and 2 in the foregoing Examples 1 and 2 seem to be preferred examples of the bead noise filter, and they are further examined.

The impedances of the invention examples 1 and 2 and the impedance of the comparative example 2 tested in the process 2 are divided into a reactance component X and a resistance component R according to the above-mentioned formula (1). The division components of Inventive Example 1 and Comparative Example 2 are shown by the curves in Figs. 5 and 6, respectively.

Referring to Fig. 5, the frequency of the XR cross point of Inventive Example 1 is around 5 MHz. The division components of Inventive Example 2 are not shown, and the result is basically the same as Inventive Example 1. On the other hand, the frequency of the XR cross point of Comparative Example 2 is around 10 MHz, as shown in FIG. 6. The reason is that, as shown in Table 2, in the frequency range between 1KHz and 1MHz, the complex relative permittivity real part εof sample 1 (invention example 1) and sample 2 (invention example 2) varies greatly, and Sample 4 (Comparative Example 2) has very little change in the real part εof the complex relative permittivity.

The above investigation found that although invention examples 1 and 2 and comparative example 2 have excellent impedance frequency characteristics in the high frequency band, that is, effective noise suppression can be performed, but the waveforms of the input digital signals of invention examples 1 and 2 The possibility of distortion is very low, so Inventive Examples 1 and 2 are superior to Comparative Example 2.

In the description of claim 1 of the present invention, the above-mentioned embodiment relates to a plurality of bead-shaped noise filters whose resistivity is 150Ωm or higher, and the real part εof the complex relative permittivity is 12320 and 10970 respectively at 1KHz. , It is 32 and 28 at 1MHz (invention examples 1 and 2), but it can be proved that the bead noise filter whose complex relative permittivity real part εis within the scope of the above-mentioned claim 1 has the same or substantially the same as the above-mentioned embodiment effect.

In the description of claim 2 of the present invention, the above-mentioned embodiment relates to a plurality of bead-shaped noise filters, the magnetic core of which is formed of Mn-Zn ferrite, and the basic composition of Mn-Zn ferrite includes: 47.0 mol % Fe2O3; 10.5 mol% ZnO; 1.0 mol% TiO2; and the remaining amount (41.5 mol%) of MnO (Invention Example 1), but it can be proved to have the basic components within the scope specified in claim 2 of the present invention The bead-shaped noise filter has the same or substantially the same effect as the above-mentioned embodiment.

In the description of claim 3 of the present invention, the above-mentioned embodiment relates to a plurality of bead-shaped noise filters, whose magnetic core is formed of Mn-Zn ferrite, and the basic composition of Mn-Zn ferrite includes: 47.0 mol% Fe2O3; 10.5mol% of ZnO; 0.5mol% of SnO2; 1.5mol% of CuO and the remaining amount (40.5mol%) of MnO (Invention Example 2), but it can be proved to be within the scope specified in claim 3 of the present invention The bead-shaped noise filter of the basic component has the same or substantially the same effect as the above-mentioned embodiment.

The bead-shaped noise filter according to the present invention helps to simply and inexpensively suppress high-frequency noise radiated from signal lines of new electronic devices with reduced size and improved performance, and is particularly suitable for suppressing noise while simultaneously inputting digital signals The waveform is not distorted.