JP2701515B2 - Multilayer ceramic capacitors - Google Patents

Description

The present invention relates to a multilayer ceramic capacitor, and more particularly, to a multilayer ceramic capacitor.
Regarding its structure.

[Conventional technology]

At present, miniaturization and high performance of various electronic devices such as computers are rapidly progressing, and accordingly, there is a strong demand for miniaturization of electronic components used therein.

In such a trend, a switching power supply (hereinafter, referred to as a power supply) which is often used as a power supply of the electronic device described above is no exception, and downsizing has been achieved by increasing a switching frequency.

For this reason, it is required to reduce the size of a large-capacity output smoothing capacitor used in the power supply.

4.7 to 10μF
A large capacitance is required.

As a capacitor for output smoothing as described above,
Conventionally, a multilayer ceramic capacitor is used. In this type of multilayer ceramic capacitor, as shown in FIG. 2, the dimension between the terminal electrodes 1 is L, the dimension in the width direction of the capacitor element 2 is W, and the thickness is When T is set, L = 10 mm, W = 6 mm, T = 2 mm in order to obtain a large capacitance of 10 μF.
A degree of shape was used.

[Problems to be solved by the invention]

At present, the switching frequency of the power supply is shifting from the conventional 100 to 200 kHz to 500 kHz in order to miniaturize the power supply.

When a multilayer ceramic capacitor having the above-described conventional shape is used as an output smoothing capacitor for the high-frequency power supply, for the reason described below,
A phenomenon occurs in which the equivalent series resistance becomes remarkably high near the switching frequency of 500 kHz.

If such an increase in the equivalent series resistance of the output smoothing capacitor near the switching frequency causes adverse effects such as an increase in ripple current and heat generation as a power supply, this capacitor is practically used for output smoothing. Can not.

The phenomenon described above, like multilayer ceramic capacitors,
This is a peculiar phenomenon when a ferroelectric is used as the dielectric material.

Generally, the Curie point of ferroelectric materials for capacitors is
In order to increase the dielectric constant at room temperature, it is usually designed to be near room temperature. For this reason, near room temperature, a so-called electrostrictive effect in which a distortion occurs in the dielectric in proportion to the square of the electric field appears.

Here, a case where an AC voltage is applied to this capacitor element as an electric field is considered.

At this time, if no DC bias is applied, substantially no vibration occurs even if a small AC voltage of 1 V or less is applied.

However, when a DC bias is applied, a DC and an AC electric field are applied to the capacitor element in a superimposed manner, and a distortion proportional to the square of the electric field is generated, so that strong vibration appears.

At this time, resonance occurs when the mechanical natural frequency of the capacitor element matches the frequency of the applied AC voltage,
The equivalent series resistance increases rapidly.

The frequency at which the above-described resonance occurs is expressed by a relational expression between the resonance frequencies of the generally known piezoelectric longitudinal effect and the transverse effect, and is obtained by the following equations (1) and (2), respectively.

However, f; resonance frequency [rho; Density l; length _{^{▲ S E 11 ▼, ▲ S}} D 33 ▼; it is compliance.

Here, in the dielectric of the lead-based composite perovskite compound of the component and composition conventionally used, It is about.

Using the above numerical values, for example, L × W × T = 10 mm × 6 m
When the resonance frequency of a multilayer ceramic capacitor having a shape of m × 2 mm is determined, the resonance frequency of the L component of the piezoelectric transverse effect is approximately 160 kHz, and the resonance frequency of the W component is approximately 260 kHz.

For this reason, the three-fold vibration of the L component and the two-fold vibration of the W component appear around 500 kHz, which is a practical problem.

At present, among the largest and widely used multilayer ceramic capacitors (L × W × T = 4.5 mm × 3.2 mm × 2.0 mm) among the multilayer ceramic capacitors that can be automatically mounted on a substrate, Similarly, when the resonance frequency is obtained, the resonance frequency of the L component becomes 350 kHz and the resonance frequency of the W component becomes 490 kHz. In this case, the resonance frequency of the W component comes close to 500 kHz, which is a serious problem in practical use.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a multilayer ceramic capacitor which is as small as possible and does not exhibit resonance at around 500 kHz which is a switching frequency of a power supply.

In the above description, the piezoelectric lateral effect has been described for simplicity.

Strictly speaking, compliance in the piezoelectric longitudinal effect and
Although the compliance in the lateral effect is not exactly the same, the difference is small, and for the resonance frequency,
Since they work in the 1/2 power, each resonance frequency may be regarded as the same.

[Means for solving the problem]

The multilayer ceramic capacitor according to the present invention comprises a capacitor element in which thin layers of a lead-based composite perovskite compound and internal electrodes are alternately laminated, and a pair of terminal electrodes provided on the outer surface of the capacitor element. When the dimension between the terminal electrodes of the multilayer ceramic capacitor is L and the dimension and thickness in the width direction of the capacitor element are W and T, respectively, the multilayer ceramic capacitor has a capacitance of 4.7 μF or more, And L = 4.88 mm to 5.88 mm W = 2.44 mm to 2.94 mm T ≦ 2.44 mm.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

First, magnesium / lead niobate Pb (Mg
_1/3 Nb _2/3 ) O ₃ , Lead Nickel Niobate Pb (Ni _1/3 Nb _2/3 )
A ternary composite perovskite compound was selected such that the molar ratio of O ₃ and lead titanate PbTiO ₃ was 0.2, 0.6, 0.2.

The starting materials were weighed and mixed so as to have the above composition, pre-fired at a certain temperature, and pulverized by a ball mill to obtain a dielectric powder.

Next, an organic binder and an organic solvent were added to and mixed with the dielectric powder to form a slurry, a 20 μm-thick film was formed using an ordinary doctor blade, and cut to form a green sheet.

Further, an internal electrode paste is formed on the green sheet by a screen printing method, and a plurality of green sheets are formed as shown in FIG. After alternately laminating them so as to form counter electrodes, chips were obtained by cutting to predetermined dimensions. The chip in this state is called a raw chip. (Note that
FIG. 3 is a view showing a cross section of the multilayer ceramic capacitor shown in FIG. 2, and the terminal electrode 1 has not yet been formed at the stage of a raw chip. Next, the raw chip was processed at a certain temperature to disperse and scatter the organic binder, and then fired at a predetermined temperature to obtain a capacitor element 2. The terminal electrode 1 was formed on the capacitor element 2 by applying. Thus, the multilayer ceramic capacitor shown in FIGS. 2 and 3 was prepared.

The actual dimensions of the multilayer ceramic capacitor thus obtained were L = 5.65 mm, W = 2.7 mm, and T = 2.05 mm.

To measure the frequency dependence of the equivalent series resistance of the created multilayer ceramic capacitor, this multilayer ceramic capacitor was fixed to an alumina substrate with solder, and 100 kHz with 25 VDC applied using a 4194A impedance analyzer manufactured by Yokogawa Hewlett-Packard Company. The equivalent series resistance was measured in the range from to 10 MHz.

FIG. 4 shows the measurement results. According to FIG. 4, the resonance due to the natural vibration of the L component of the multilayer ceramic capacitor is around 300 kHz, and the resonance due to the double vibration is 600 kHz.
Appears around Hz.

In addition, resonance due to the natural vibration of the W component appears around 700 to 750 kHz.

However, resonance does not appear around the problematic 500 kHz, and it can be seen that there is no problem even if the switching frequency of the power supply is 500 kHz.

Next, for comparison, a multilayer ceramic capacitor having a shape different from that of the example was produced under the same manufacturing conditions as in the example, using dielectric powders having the same components and compositions as those used in the example.

The dimensions of the multilayer ceramic capacitor were L = 4.
20 mm, W = 3.30 mm and T = 2.05 mm.

FIG. 3 shows the result of measuring the frequency dependence of the equivalent series resistance of this multilayer ceramic capacitor as in the case of the above-described embodiment.

The resonance due to the natural vibration of the L component appears around 400 kHz, and the resonance due to the double vibration appears around 800 kHz.

Also, resonance due to the natural vibration of the W component appears around 510 kHz.

With this shape, resonance occurs around 500 kHz, and it cannot be used for a power supply with a switching frequency of 500 kHz.

Here, the switching frequency of the switching power supply is 50
The reason why the size of the multilayer ceramic capacitor is limited at 0 kHz will be described below.

 First, the upper limits of the L and W dimensions will be described.

As described above, using 20 PMN-60PNN/20PT as a dielectric material, a green sheet having a thickness of 20 μm is prepared and L: 5.65 × W: 2.
If a 70 × T: 2.05mm capacitor is made, this is A Ne
w Dielectric Ceramic Material for Capacitors with
High Specific Capacitance (NEC RESEARCH & DEVELOP
MENT, No. 92, pp. 8-17, Januaru 1989)
= 17000 (conventional method) dielectric material, green sheet thickness 1
Since 5 μm is 10.5 μm after sintering, the shrinkage can be said to be 70%.
Therefore, the green sheet of the embodiment becomes 14 μm after sintering.

Next, a capacitor of L: 10 mm × W: 6 mm × T: 2 mm will be considered as a conventional example. By substituting density ρ = 8.2 and compliance S ₁₁ ^E = 1.2 × 10 ^-11 into the equation for calculating the reference vibration with this capacitor, the reference vibration corresponding to L, W, and T is 159 kHz, 266.
Appears at kHz and 797kHz. Therefore, at double oscillation, 319kHz, 5
31kHz, 1594kHz, 478kHz, 797kHz, 2291kHz for triple vibration
Will appear. In the conventional example, in this double vibration
Since the problem is 531 kHz and 478 kHz in the triple vibration, that is, the vibration is 478 kHz: problematic, 531 kHz: problematic, 500 ± 30 kHz should be considered problematic.

Next, the density ρ is 8.014 for lead titanate pbTiO ₃ (PT) and lead magnesium niobate Pb (Mg _1/3 Nb _2/3 ) O ₃ (PbTiO ₃ (PT)) for each component of the dielectric material described in the above embodiment. PMN) is 8.19 and lead nickel niobate Pb (Ni _1/3 Nb _2/3 ) O ₃ (PNN) is 8.57, for example, Structure, Properties and Preparation of Pero
vskite-type Compounds (PERGAMON PRESS, pp146,149)
It is written in. Therefore, the density of the composite perovskite compound described in the examples is 8.01 to 8.
It becomes 57. At this time, since S ₁₁ ^E hardly changes depending on the composition, a numerical value of S ₁₁ ^E = 1.2 ^* 10 ^-11 is used. At that time, the shape where the vibration appears at 470kHz to 530kHz is: Reference vibration: 3.04 to 3.43mm (ρ = 8.01) + 2.94 to 3.32mm
(Ρ = 8.57) Double vibration: 6.08-6.86mm (ρ = 8.01) +5.88-6.64mm
(Ρ = 8.57).

In order to avoid such resonance at a peculiar frequency, L, W, and T of the multilayer ceramic capacitor are required to have the above-mentioned reference vibration, double vibration, triple vibration,... N-fold vibration (n is an integer). Although it is only necessary to avoid the range, it is preferable to set L = W, L = 2W, L = 3W, etc. in order to reduce the frequency band in which resonance occurs. However, when L = W, it is difficult to determine the outlet of the internal electrode (that is, after forming the external electrode) at the time of manufacturing the capacitor, and L = 3
Extremely large L / W ratio, such as W, 4W or more,
In other words, a long and narrow capacitor has a problem that the strength is increased. For the above reasons, it is most preferable to set L = 2W. Therefore, in order to avoid the dimensions for which resonance is applied, it is necessary to set the L dimension to 5.88 mm or less and the W dimension to 2.94 mm or less.

Next, the lower limit of the L and W dimensions and the upper limit of the T dimension will be described.

For example, according to the multilayer ceramic capacitor general remarks (Gakudensha “Laminated ceramic capacitor”), L dimension 3.2mm × W: 1.6
For a capacitor having a shape larger than mm, the T dimension is required to be smaller than the W dimension. This is because there is an inconvenience if there is a T height higher than other components due to mounting problems. However, when trying to obtain a certain capacitor capacity, a certain volume is required. As described above, the upper limit of the T dimension depends on the W dimension, and as a result, the minimum required W dimension is required. The grounds for obtaining the minimum required W dimension are described below.

A New Dielectric Ceramic Material for
Capacitors with High Specific Capacitance (NEC RES
EARCH & DEVELOPMENT, No.92, pp.8-17, Januaru 1989)
In the example obtained from the shrinkage ratio of the green sheet, the dielectric interlayer thickness is 14 μm, the internal electrode thickness is about 3 μm from the photograph of the document, and the relative dielectric constant at 20 ° C. is about 17,000. The minimum capacitance of the capacitor required in the present invention is 4.7 μF.
It is. In a large-capacity multilayer ceramic capacitor used for a smoothing circuit of a switching power supply as in the embodiment,
In order to ensure reliability, the upper and lower protective layers in the T direction need a total of 800 μm in the upper and lower directions. Also, the smaller the side margin and the margin of the extraction portion, the larger the effective area of the internal electrode, so that the capacitance of the capacitor increases.

On the other hand, the side margin and the margin of the electrode take-out portion need to take into account the internal electrode printing accuracy and the green sheet lamination accuracy in addition to ensuring reliability. Therefore, a total of 1.2 mm is required for the side margin and the margin for taking out the electrode.

Next, from the capacitance value of 4.7 μF or more, the shape required to satisfy this condition is calculated.

Capacitor capacity 4.7μF, dielectric layer thickness 14μ
Thickness, upper and lower margin total 800μm, side margin and electrode take-out margin are each 1.2mm, number of lamination is n
Assuming that T (mm) is equal to the limit W (mm), L (mm) and T (mm) are L = 2W, T = W, and all W
It is represented by Here, the effective internal electrode area S of the capacitor per layer is as follows: S = (L−1.2) × (W−1.2) = (2W−1.2) (W−1.2) T dimension is 14μ when the number of layers is n
m, the electrode thickness is 3 μm, and the upper and lower margins are 800 μm, so T = W = (0.014 + 0.003) × n + 0.8, and n = (W−0.8) /0.017.

Next, the capacitance C (μF) of the capacitor is obtained by the following equation.

C = εε ₀ S (n−1) / d where the dielectric constant ε ₀ in vacuum = 8.856 × 10 ¹² (F / m) = 8.
Substituting 856 × 10 ^-9 (μF / mm) and relative permittivity ε = 17000, 4.7 = 17000 × 8.856 × 10 ^-9 × ｛(2W-1.2) (W-1.2) /0.014｝ × ｛(W- 0.8) /0.017-1)｝ (2W-1.2) (W-1.2) × {(W-0.8) /0.017-1)｝ = (4.7 × 0.014) / (17000 × 8.856 × ^10-9 ) = 437.05829f (W) = (2W−1.2) (W−1.2) × {(W−0.8) /0.017-1)} − 437.05829 = 0 When the real solution of the above cubic function of W is obtained by numerical calculation,
It can be seen that there is only one actual solution, and W = 2.4416. When W <2.4416, the condition of L = 2W is fixed and T>
W, which satisfies the condition described in the above-mentioned literature, the multilayer ceramic capacitor general remarks ("Gakuen Shuppan Co., Ltd." Therefore, assuming three significant figures, the lower limit of the W dimension is 2.44 mm, so the lower limit of the L dimension is 4.88 mm and the upper limit of the T dimension is 2.44 m.

Therefore, even when the multilayer ceramic capacitor having the above-described shape is used for a power supply having a switching frequency of 500 kHz, a sharp rise in the equivalent series resistance does not occur near the switching frequency.

The dimensional accuracy as described above can be sufficiently realized by conventional techniques.

〔The invention's effect〕

As described above, when the multilayer ceramic capacitor of the present invention is used for smoothing the output of a power supply having a switching frequency of 500 kHz, a sharp rise in the equivalent series resistance does not occur near the switching frequency.

Therefore, practical problems such as an increase in ripple current and heat generation do not occur at all as a power supply.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the frequency dependence of the equivalent series resistance of a multilayer ceramic capacitor according to an embodiment of the present invention, FIG. 2 is a perspective view showing the appearance of the multilayer ceramic capacitor, and FIG. FIG. 4 is a cross-sectional view illustrating a cross section of the multilayer ceramic capacitor, and FIG. 4 is a view illustrating frequency dependence of an equivalent series resistance of a multilayer ceramic capacitor prepared as a comparative example. 1 ... terminal electrode, 2 ... capacitor element, 3 ... internal electrode, 4 ... dielectric layer.

Claims (1)

(57) [Claims] 1. A multilayer ceramic comprising a capacitor element in which thin layers of a lead-based composite perovskite compound and internal electrodes are alternately laminated, and a pair of terminal electrodes provided on the outer surface of the capacitor element. The capacitor has a capacitance of 4.7 μF or more, where L is a dimension between terminal electrodes of the multilayer ceramic capacitor, and W and T are a dimension and a thickness in a width direction of the capacitor element, respectively. = 4.88mm to 5.88mm W = 2.44mm to 2.94mm A multilayer ceramic capacitor characterized by satisfying the following shape: T≤2.44mm.