JPH05235434A - Thickness longitudinal vibration piezoelectric porcelain transformer and driving method therefor - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to provide a piezoelectric transformer capable of operating in a high frequency band, in particular, a piezoelectric transformer for an on-board power supply, which is required to be downsized and have low noise. [Structure] As shown in FIG. 1, a piezoelectric transformer according to the present invention has a plurality of internal electrode layers 13 and 14, an insulating layer 15, and piezoelectric ceramic layers 16, 17 and 18, which are alternately laminated. The piezoelectric ceramic layers 16 and 17 that are adjacent to each other via a layered structure are polarized bodies that are polarized in opposite directions, and the external electrodes include two pairs of low impedance portions 11 and 11 ′ and a pair of high impedance portions 12, Low impedance part 1
1 and 11 'are arranged adjacent to each other with an insulating layer 15 interposed therebetween. The piezoelectric transformer of the present invention can be used in a wide band in a high frequency band of several MHz or more, is small in size and has high efficiency, and can remarkably contribute to downsizing of a power supply.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer capable of operating in a high frequency band, and more particularly to a piezoelectric transformer for an on-board power supply which is required to be compact and low in noise and a driving method thereof.

[0002]

2. Description of the Related Art In recent years, electromagnetic transformers have been used as switching power supplies in order to miniaturize the power supply circuits of electronic devices. To reduce the size of the switching power supply, it is desirable to increase the switching frequency. However, when the switching frequency is increased, the hysteresis loss of the magnetic material used for the electromagnetic transformer, the eddy current loss, and the loss due to the skin effect of the conductor wire are rapidly increased, and the efficiency of the transformer is extremely low. For this reason, the upper limit of the practical frequency band of the electromagnetic transformer was at most 500 KHz.

On the other hand, the laminated piezoelectric transformer is used in a resonance state, and (1) has a higher energy density at the same frequency than a general electromagnetic transformer, so that it can be miniaturized.

(2) Incombustibility can be achieved.

(3) No noise due to electromagnetic induction. It has many advantages.

FIG. 4 shows the structure of a Rosen type piezoelectric transformer, which is a typical conventional laminated type piezoelectric transformer. Hereinafter, description will be given with reference to the drawings. When a high voltage is to be taken out, in the piezoelectric plate provided with electrodes on the surface, a portion indicated by 41 is a low impedance driving portion of the piezoelectric transformer, and electrodes 43 and 44 are provided on the upper and lower surfaces thereof, and this portion is It is polarized in the thickness direction as indicated by 46 in the figure. Also,
Similarly, a portion indicated by 42 is a high-impedance power generation portion, and an electrode 45 is provided on the end face thereof, and the power generation portion 42 is polarized in the length direction of the piezoelectric plate as indicated by 47 in the figure. The operation of this piezoelectric transformer is as follows.
44 longitudinal vibration is excited when a voltage is applied by the transverse effect 31 mode through electromechanical coupling factor k _{3 1,} the entire transformer vibrates. In addition power portion 42, longitudinal effect longitudinal vibration mode by electromechanical coupling coefficient k _{3 3} (33 mode)
As a result, a high voltage is taken out from the output electrode 45. On the other hand, when a high voltage is input and a low voltage is output, it is clear that the vertical effect high impedance portion 42 should be the input side and the horizontal effect low impedance portion 41 should be the output side. The other types of piezoelectric transformers also use the same extension vibration of a flat plate and the radial vibration of a circular plate as in the Rosen type, and the applicable frequency is up to 200.
It is up to about kHz.

On the other hand, the piezoelectric ceramic transformer known in Japanese Patent Laid-Open No. 3-173484 has a structure in which piezoelectric ceramic plates polarized in the thickness direction are stacked, and is driven at the resonance frequency of the thickness longitudinal vibration to generate the MHz band. Is possible.

[0008]

As shown in the above conventional example, the applicable frequency range of the piezoelectric transformer is 200 kHz.
Only in the low frequency region below. Also, the Rosen type piezoelectric transformer has a significantly smaller electromechanical coupling coefficient k in the transverse effect vibration mode than the electromechanical coupling coefficient in the longitudinal effect.
_Since there is no choice but to use _{3 1} , there is a drawback that the bandwidth is small. Further, the piezoelectric ceramic transformer disclosed in Japanese Patent Laid-Open No. 3-173484 has problems in high power, small size, and high output.

[0009]

The present invention has been made to overcome the above drawbacks. According to the present invention, a plurality of internal electrode layers and piezoelectric ceramic layers are alternately laminated,
The piezoelectric ceramic layers adjacent to each other via the internal electrode layers are laminated bodies that are polarized in opposite directions, and are thickness vibration piezoelectric ceramic transformers that include two low impedance portions and one high impedance portion. The impedance oscillating piezoelectric ceramic transformer is characterized in that the impedance portions are arranged adjacent to each other via an insulating layer. Further, as a driving method, driving is performed so that the half wavelength becomes the same as a predetermined size of the present transformer.

[0010]

The present invention has been made to provide a piezoelectric ceramic transformer having a low loss and a sufficient function at a high frequency of 1 MHz or more. FIG. 1 is a perspective view of a piezoelectric ceramic transformer of the present invention, and FIG. 2 is a sectional view. 1 and 2, a piezoelectric ceramic transformer according to the present invention comprises two pairs of low impedance parts 11 and 11 'in which a large number of piezoelectric ceramic layers 16 polarized in the thickness direction are laminated, and an insulating layer 15 and a piezoelectric ceramic in the middle thereof. It is composed of the high impedance portion 12 made of the layer 17. Further, electrodes are evenly arranged on the ceramic layer 16 of each of the low impedance portions 11 and 11 '. The electrodes 13 of the low impedance portion 11 are externally alternately connected to serve as output terminals 22 and 23, and the other low impedance portion 1
The electrode 13 of 1'is also connected to the outside alternately and output terminals 24, 2
It was set to 5. Next, the upper and lower electrodes 14 of the high impedance portion 12
Then, the input terminals 20 and 21 were taken out. Further, the thickness of each piezoelectric ceramic layer of the high impedance portion 12 and each of the low impedance portions 11 and 11 'are configured to be the same. Here, the piezoelectric ceramic layers 16 and 17 are arranged such that the polarization directions of adjacent layers are opposite. Further, an electric field can be applied to the piezoelectric ceramic plates 10 in the thickness direction between the piezoelectric ceramic plates by the internal electrodes 13. A piezoelectric ceramic transformer having such an internal multi-layer electrode can be manufactured by a laminated ceramic technique (doctor blade method) used in a laminated piezoelectric ceramic capacitor, a laminated piezoelectric actuator, or the like. In the piezoelectric ceramic transformer, it is possible to reduce the layer spacing to about 25 μm. Therefore, even if the thickness longitudinal resonance vibration of the half-wavelength mode (fundamental mode with free ends) or the 3/2 wavelength mode (third-order free third-order mode) is used, it is possible to use the laminated ceramic technology in the 2-10 MHz band It is also possible to realize a piezoelectric ceramic transformer that operates in the ultra-high frequency range.

FIG. 2 shows a sectional view of the present piezoelectric transformer used for thickness longitudinal vibration. The + terminals 22, 24 and the one terminal 23 of the output terminals 22 to 25 of the low impedance section 11, 11 ',
25 were connected to each other to serve as output terminals. In such a connection, when a high voltage having a frequency equal to the resonance frequency of thickness longitudinal vibration is applied between the electric terminals 20 and 21 on the high impedance side, the entire piezoelectric ceramic transformer mechanically operates due to the inverse piezoelectric effect of the high impedance section 12. 2 (a) vibration distribution, and the low impedance portions 11 and 11 'generate a voltage having the same frequency as the input voltage by the positive piezoelectric effect and output it to the output terminal. At this time, due to the difference in impedance between the input side and the output side, the voltage at the output terminal is
It will be lower than the voltage across one. However, by connecting two pairs of low impedances in parallel, the charge distribution (Fig. 2 (b)
As is clear from the charge distribution), the output voltage can be taken out without canceling the charges, and the output voltage does not change and the current doubles. Further, when the output voltages 22, 23, and 24, 25 are taken out from the two low impedances, two outputs are obtained for one input. Furthermore, different voltages can be freely output by changing the electrode spacing of low impedance. When converting a low voltage to a high voltage, if a low voltage is applied to the output terminal, the high voltage is output from the input terminals 22 and 23. If the insulating plate 15 is arranged between the low impedance part 11 and the low impedance part 11 ′ and the insulating plate 15 is arranged between the low impedance part 11 ′ and the high impedance part 12, the input terminals 20 and 21 are connected. Output terminals 22, 23 and 24,
Since 25 can be electrically separated, the degree of freedom of peripheral circuits can be increased.

The resonance frequency of the thickness longitudinal vibration is expressed by the following equation, ignoring the decrease of the resonance frequency due to the piezoelectric longitudinal effect.

F _r = mv _t / 2t (n = 1, 2, 3 ...) where f _r : resonance frequency of thickness longitudinal vibration n: mode order v _t : thickness direction longitudinal wave Sound velocity t: thickness If the piezoelectric ceramic transformer can be manufactured so that v _t and t are constant, the resonance frequency f _r of the thickness longitudinal vibration is also constant, but it is not always as designed due to variations in contraction rate during integral firing. It does not have a resonance frequency. Therefore, a structure capable of adjusting the resonance frequency after firing is desirable. It is not easy to adjust after firing in a piezoelectric transformer having electrodes on the upper and lower main surfaces, but in the piezoelectric transformer of the present invention, since a frequency adjusting layer that can be polished is arranged on the upper and lower sides, it is not possible to perform polishing after firing. The resonance frequency of the period is obtained, and it can be exactly matched with the drive frequency of the external circuit.

Next, the efficiency will be described. Most of the loss of the piezoelectric ceramic transformer is mechanical loss, and the mechanical loss is larger as the mechanical quality factor Qm is smaller. The mechanical quality factor Qm is largely present in the parallelism and flatness of the upper and lower principal surfaces. The surface of a piezoelectric transformer that has just been fired has poor parallelism and flatness, and its mechanical quality factor Qm is at most 1
00. On the other hand, in the piezoelectric transformer of the present invention, by performing parallel plane polishing, it is possible to obtain parallelism and flatness within several μm, and a mechanical quality factor Qm of 1000 or more can be easily obtained.

[0015]

EXAMPLES As an example of a piezoelectric ceramic transformer according to the present invention, a piezoelectric ceramic transformer having the structure shown in FIGS. 1 and 2 was manufactured by a green sheet method. 1 and 2 are a perspective view and a sectional view, respectively, of an embodiment of a piezoelectric ceramic transformer of the present invention. The 6-terminal type piezoelectric transformer shown in FIGS. 1 and 2 uses the thickness longitudinal third-order mode in the 2.4 MHz band, and the PbTiO ₃ system piezoelectric material is used as the piezoelectric material. In FIG. 2, the thickness of one layer of the piezoelectric ceramics of the low impedance parts 11 and 11 'was about 0.18 mm. The same material is used for the frequency adjustment layer 18 and the thickness is about 0.25 mm. The frequency adjusting layer and the insulating layer may be made of other materials as long as they can be integrally fired with the low impedance part and the high impedance part. The Pt paste was printed by a screen printing method and integrally sintered with a piezoelectric ceramic to form Pt internal electrodes 13 and 14.
After baking, the external electrodes are baked to make electrode terminals 20, 21, 2
2, 23, 24 and 25 are formed respectively. Electrode terminal 2
7kV / m between 0 and 21, between 22 and 23, between 24 and 25
A direct current voltage of m is applied to polarize. Finally, parallel-plane polishing is performed so as to have a thickness necessary for adjusting the resonance frequency. In this example, since the piezoelectric transformer is driven at 2.4 MHz, parallel plane polishing was performed so that the thickness of the piezoelectric transformer was 3.3 mm. Next, the + terminals 22 and 24 and the one terminals 23 and 25 of the electric terminals of the low impedance portion were connected to each other to serve as output terminals. Then, the high impedance part 12
It was evaluated as a step-down transformer in which a high-frequency / high-voltage signal for exciting a longitudinal third-order mode is input from the electric terminals 20 and 21 of the above, and the output terminal of the low impedance part is terminated with an appropriate resistance load to take out the output. FIG. 3 shows measured values of frequency-gain and efficiency characteristics. The resonance frequency is 2.4 MHz, the mechanical quality factor Qm value is 850, and the maximum energy conversion efficiency is 9
7% was obtained. In particular, a power capacity of 10 W was realized, and a power capacity about twice that of a piezoelectric transformer having a low impedance part was obtained. The 1-input and 2-output piezoelectric transformers in FIG. 2 were evaluated in the same manner as in the case of the low impedance parallel connection described above. The electrical terminals 20, 21 of the high impedance part are input and the electrical terminals 22, 23 and 2 of the low impedance are input.
Outputs 4 and 25 were used. As a result, a maximum energy conversion efficiency of 97% and a power capacity of 5 W were realized at each of the two output terminals, and two completely independent output voltages were obtained. Furthermore, it has been confirmed that a desired voltage can be obtained by changing the layer spacing of one layer in the low impedance part. It is clear that the prototype piezoelectric transformer has the step-down function of the transformer and high energy conversion efficiency, and in particular realizes high power and multiple outputs. It goes without saying that the input terminal can be used as a boosting transformer by taking out the low impedance portion and the output terminal from the high impedance portion.

[0016]

As described above, the piezoelectric ceramic transformer according to the structure of the present invention can be used in a wide band in a high frequency band of 1 MHz or more, is small in size, highly efficient, and
It is clear that high power and multiple outputs are realized,
It has advantages that conventional piezoelectric transformers do not have, and it has great industrial value.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention.

FIG. 2 is a connection diagram, vibration and charge distribution diagram showing an embodiment of the present invention.

FIG. 3 is a characteristic diagram of the piezoelectric transformer of the present invention.

FIG. 4 is a diagram showing a conventional example.

[Explanation of symbols]

10 piezoelectric ceramics 11,11 'low impedance part of piezoelectric transformer 12 high impedance part of piezoelectric transformer 13,14 internal electrode 15 insulating layer 16,17,18 piezoelectric ceramic layer 20,21,22,23,24,25 electric terminal 41 Low impedance part of conventional Rosen type piezoelectric transformer 42 High impedance part of conventional Rosen type piezoelectric transformer 43,44,45 Electrodes 46,47 Polarization direction

Claims (2)

[Claims] 1. A laminated body in which a plurality of internal electrode layers and piezoelectric ceramic layers are alternately laminated, and adjacent piezoelectric ceramic layers via the internal electrode layers are polarized in opposite directions to each other, and two low impedance parts are provided. A thickness longitudinal oscillating piezoelectric ceramic transformer comprising: and a high impedance portion, wherein two low impedance portions are arranged adjacent to each other with an insulating layer interposed therebetween. 2. The piezoelectric ceramic transformer according to claim 1, wherein
A driving method of a piezoelectric ceramic transformer characterized by driving at a resonance frequency of a thickness longitudinal vibration third-order mode in which half the wavelength is equal to the thickness of the piezoelectric ceramic layer of the high impedance portion and the thickness of each low impedance portion. ..