MOUNTING STRUCTURE OF PIEZOELECTRIC TRANSFORMER AND METHOD OF MOUNTING PIEZOELECTRIC TRANSFORMER

Abstract: In a structure in which a piezoelectric transformer element (11) is mounted on a circuit board (27) equipped with a circuit component for driving the piezoelectric transformer element (11), at least one of a fixing member (33, 35) and a supporting member (37, 39, 41) made of an elastic material having flexibility is interposed between the piezoelectric transformer element (11) and the circuit board (27) so as to cover vibration node portions (29, 31) of the piezoelectric transformer element (11) and at least a part of the remaining portion except the vibration node portions (29, 31). Thus, the piezoelectric transformer element (11) is fixed to the circuit board (27).

Description: Technical Field This invention relates to a structure in which a piezoelectric transformer element for use in a piezoelectric inverter of a backlight for a liquid crystal display or the like is mounted on a circuit board equipped with circuit components and a method of mounting the same. Background Art As a conventional mounting method of a piezoelectric transformer element, there is known a mounting method in which the piezoelectric transformer element is contained in a case and an external terminal of the case is connected by soldering or the like to a circuit board equipped with a circuit component. Another mounting method is also known in which no casing is carried out to avoid a problem of heat generation following an increase in output power but a vibration node is fixed by the use of a flexible elastic material, for example, silicone. In order to adjust the brightness of a backlight, it is required to carry out control of an inverter electrically connected to the backlight. As a circuit system for such brightness adjustment, a burst adjustment is generally used. This results in a problem of generation of an audible sound, which has not been caused in the conventional method of fixedly supporting the piezoelectric transformer used in the inverter. The reason is as follows. The piezoelectric transformer is generally driven in a single mode. However, by addition of a brightness adjusting function, the piezoelectric transformer itself is driven in a mode other than the single mode. Even if the piezoelectric transformer is fixedly supported at a nodal point in the single mode, a nodal point in the other mode will be deviated from the supporting point. This results in generation of the audible sound in a mounted state. Further, from the EP-A-0 847 093 a mounting structure of a piezoelectric transformer element is known comprising the features of the preamble of claim 1. In addition, from the JP-A-8 306 981 a piezoelectric transformer element is known wherein the supporting member supporting it on a plane is adhered to the entire surface of the piezoelectric transformer element. Therefore, it is a first object of this invention to provide a mounting structure of a piezoelectric transformer element, which can minimize generation of an audible sound even if it is used in vibration modes of different frequencies. It is a second object of this invention to provide a method of mounting the above-mentioned piezoelectric transformer element. Disclosure of the Invention According to this invention, there is provided a mounting structure of a piezoelectric transformer element comprising the features of claims 1 or 4. According to this invention, there is also provided a mounting method of a piezoelectric transformer element comprising the steps of claims 5 or 8. Further advantageous features are the subject matters of the dependent claims. Thus, in this invention, as a method of mounting a piezoelectric transformer element on a circuit board equipped with at least one circuit component for driving the piezoelectric transformer element, the piezoelectric transformer element is fixed at vibration nodes by the use of a flexible elastic material (preferably, silicone or the like). Furthermore, another flexible elastic material is inserted in a portion other than the vibration nodes in a spot-like fashion or in parallel to the vibration nodes to be used as a spacer. In this manner, the piezoelectric transformer has a structure hard to transmit the vibration to the circuit board upon excitation which is not a single mode excitation. Thus, it is possible to suppress a heat generation and an audible sound. Brief Description of the Drawing Fig. 1(a) is a plan view showing a conventional mounting structure of a piezoelectric transformer element; Fig. 1(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in Fig. 1(a); Fig. 2(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a first embodiment of this invention; Fig. 2(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in Fig. 2(a); Fig. 3(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a second embodiment of this invention; Fig. 3(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in (a); Fig. 4(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a third embodiment of this invention; Fig. 4(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in Fig. 4(a); Fig. 5(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a fourth embodiment of this invention; Fig. 5(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in Fig. 5(a); Fig. 6(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a fifth embodiment of this invention; Fig. 6(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in (a); Fig. 7(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a sixth embodiment of this invention; Fig. 7(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in (a); Fig. 8(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to a seventh embodiment of this invention; Fig. 8(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in (a); Fig. 9(a) is a plan view showing a mounting structure of a piezoelectric transformer element according to an eighth embodiment of this invention; and Fig. 9(b) is a side view showing the mounting structure of the piezoelectric transformer element illustrated in (a). Best Mode for Embodying the Invention Before describing the best mode for embodying this invention, description will at first be made about a conventional mounting structure of a piezoelectric transformer element with reference to Fig. 1 in order to facilitate an understanding of this invention. Referring to Fig. 1(a) and Fig. 1(b), a conventional piezoelectric transformer element 11 comprises a rectangular piezoelectric ceramic plate or a multilayer structure (hereinafter referred to as a rectangular piezoelectric plate 13) including internal electrode layers and piezoelectric-ceramic layers, outer electrodes 15 and 17 formed on both of the upper and the lower surfaces of the rectangular piezoelectric plate 13 to extend over an approximate half at the side of one end as depicted by hatching, and an outer electrode 19 formed on an end surface of the rectangular piezoelectric plate 13 in the remaining approximate half not provided with the outer electrodes 15 and 17 as similarly depicted by hatching. The outer electrodes 15, 17, and 19 are connected to lead wires 21, 23, and 25 by soldering or the like, respectively. In order to mount the piezoelectric transformer element 11 on a circuit board 27, they are bonded through fixing members 33 and 35 made of a flexible elastic material (a silicone-based adhesive or the like) and arranged on the lower surface of the piezoelectric transformer element 11 beneath vibration nodes 29 and 31 thereof. One of the lead wires 21 and 23 connected to the outer electrodes 15 and 17 is used as an input terminal while the lead wire 25 connected to the outer electrode 19 is used as an output terminal. The other of the lead wires 21 and 23 connected to the outer electrodes 15 and 17 is used as a common terminal. When the input side is applied with an AC voltage having a frequency equal to a resonance frequency, the piezoelectric transformer element 11 resonates. By this resonance, an AC voltage raised by a step-up ratio of the piezoelectric transformer element is produced between the input terminal and the common terminal. Now, description will be made about embodiments of this invention with reference to Fig. 2 through Fig. 8. Referring to Fig. 2(a) and Fig. 2(b), a piezoelectric transformer element 11 according to a first embodiment of this invention has a structure similar to that of the conventional one illustrated in Fig. 1. Specifically, the piezoelectric transformer element 11 comprises a rectangular piezoelectric plate 13, outer electrodes 15 and 17 formed on both of the upper and the lower surfaces of the rectangular piezoelectric plate 13 to extend over an approximate half at the side of one end, and an outer electrode 19 formed on an end surface of the rectangular piezoelectric plate 13 in the remaining approximate half not provided with the outer electrodes 15 and 17. Like in Fig. 1, lead wires 21, 23, and 25 are provided although not illustrated in Fig. 2(a) and Fig. 2(b). In the first embodiment of this invention, the piezoelectric transformer element 11 is mounted on a circuit board 27 via fixing members 33 and 35 made of a flexible elastic material (a silicone-based adhesive or the like) and introduced at and adhered to the lower surface of the piezoelectric transformer element 11 beneath vibration nodes 29 and 31 thereof. Furthermore, in the first embodiment of this invention, disk-shaped spacer members 37 and 39 made of completely dry silicone such as silicone rubber as a flexible elastic material are introduced under an edge portion at one end of the piezoelectric transformer element 11 and adhered by the silicone-based adhesive. In addition, a disk-shaped spacer member 41 made of silicone as a flexible elastic material is also introduced under, and adhered by the silicone-based adhesive to, an edge portion at the other end. Thus, the piezoelectric transformer element is supported by the three spacer members 37, 39, and 41. Referring to Fig. 3(a) and Fig. 3(b), a mounting structure of a piezoelectric transformer element according to a second embodiment of this invention is different from the structure illustrated in Fig. 2 in that spacer members 41 and 43 also made of silicone as a flexible elastic material are introduced under, and adhered by silicone-based adhesive to, an edge portion at the other end of the piezoelectric transformer element 11, thus the piezoelectric transformer element being supported by four points in total including those at the one end. Except the above-mentioned difference, the structure is similar to that of the first embodiment illustrated in Fig. 2. Referring to Fig. 4(a) and Fig. 4(b), in a mounting structure of a piezoelectric transformer element 11 according to a third embodiment of this invention, the piezoelectric transformer element 11 is similar in structure to each of the conventional one illustrated in Fig. 1 and the first and the second embodiments illustrated in Fig. 2 and Fig. 3. As shown In Fig. 4(a) and Fig. 4(b), strip-like fixing members 33 and 35 made of an elastic material having flexibility are arranged on the lower surface of the piezoelectric transformer element 11 beneath vibration nodes 29 and 31 thereof. Spacer members 45 and 47 each of which serves as a single strip-like supporting member are inserted and adhered at the input side and the output side in parallel to the fixing members 33 and 35 at the vibration nodes. Referring to Fig. 5(a) and Fig. 5(b), in a mounting structure of a piezoelectric transformer element 11 according to a fourth embodiment of this invention, the piezoelectric transformer element 11 is similar in a structure to each of the conventional one illustrated in Fig. 1 and the first through the third embodiments illustrated in Fig. 1 through Fig. 4. As shown in Fig. 5(a) and Fig. 5(b), strip-like fixing members 33 and 35 made of an elastic material having flexibility are arranged on the lower surface beneath vibration nodes 29 and 31. Spacer members 45 and 47 each of which is a single strip-like member are introduced under, and adhered to, the input side and the output side and extend in parallel to the fixing members 33 and 35 at the vibration nodes. The fourth embodiment is different from the embodiment illustrated in Fig. 4 in that a strip-like spacer 49 similar to the spacer members 45 and 47 is introduced under, and adhered at, the center of the piezoelectric transformer element 11 in parallel to the fixing members 33 and 35 at the vibration nodes. As specific examples according to the first through the fourth embodiments described above, piezoelectric transformer elements having a three-dimensional size of 53/83/3.3 mm were mounted on boards. Thereafter, comparison of characteristics was carried out between the first embodiment of this invention and the third embodiment of this invention as shown in the following Table 1. Item Conventional Product Products of Present Invention First Embodiment Second Embodiment Step-up Ratio 90 91 90 Driving Frequency(kHz) 64.5 64.5 64.5 Heat Generation (°C) 18 17 18 Audible Sound (dB) 70 61 62 As shown in the above Table 1, no substantial change and no substantial deterioration were observed in heat generation, step-up ratio, and driving frequency of the piezoelectric transformer, as compared with the conventional mounting method. In addition, the level of the audible sound was reduced by 8 dB or more, as compared with the conventional product Thus, the characteristics of a practically acceptable level were obtained. Referring to Fig. 6(a) and Fig. 6(b), in a mounting structure of a piezoelectric transformer element according to a example which does not form a part of this invention, the piezoelectric transformer element 11 has a structure similar to that of the conventional one illustrated in Fig. 1. Specifically, the piezoelectric transformer element 11 comprises a rectangular piezoelectric plate 13, outer electrodes 15 and 17 formed on both of the upper and the lower surfaces of the rectangular piezoelectric plate 13 to extend over an approximate half at the side of one end, and an outer electrode formed on an end surface of the rectangular piezoelectric plate 13 in the remaining approximate half having none of the outer electrodes 15 and 17. In addition, the outer electrodes 15, 17 are connected by soldering to lead wires 21, 23, and 25, respectively. In the example, the piezoelectric transformer element 11 is mounted on a circuit board 27 by a fixing member 51 made of a flexible elastic material (silicone or the like) introduced under, and adhered to, the entire surface of the piezoelectric transformer element 11 including the positions beneath vibration nodes 29 and 31. Referring to Fig. 7(a) and Fig. 7(b), in a mounting structure of a piezoelectric transformer element according to an other example which does not form a part of this invention, the piezoelectric transformer element 11 is similar in structure to those illustrated in Fig. 1 through Fig. 6. Specifically, the piezoelectric transformer element 11 comprises a rectangular piezoelectric plate 13, outer electrodes 15 and 17 formed on both of the upper and the lower surfaces of the rectangular piezoelectric plate 13 to extend over an approximate half at the side of one end, and an outer electrode 19 formed on an end surface of the rectangular piezoelectric plate 13 in the remaining approximate half having none of the outer electrodes 15 and 17. In addition, the outer electrodes 15, 17, and 19 are connected by soldering to the lead wires 21, 23, and 25, respectively. This example is different in structure from that illustrated in Fig. 6 in that the piezoelectric transformer element 11 is mounted on a circuit board 27 via a spacer member 53 made of silicone as a flexible elastic material and serving as a supporting member, which is introduced under the entire surface of the piezoelectric transformer element 11 including the positions beneath the vibration nodes 29 and 31 thereof, and adhered thereto by a silicone-based adhesive. Referring to Fig. 8(a) and Fig. 8(b), in a mounting structure of a piezoelectric transformer element according to a fifth embodiment of this invention, the piezoelectric transformer element 11 has a structure similar to that of the conventional one illustrated in Fig. 1. Specifically, the piezoelectric transformer element 11 comprises a rectangular piezoelectric ceramic plate or a multilayer structure (hereinafter referred to as a rectangular piezoelectric plate 13) including internal electrode layers and piezoelectric ceramic layers, outer electrodes 15 and 17 formed on both of the upper and the lower surfaces of the rectangular piezoelectric plate 13 to extend over an approximate half at the side of one end, and an outer electrode 19 formed on an end surface of the remaining approximate half having none of the outer electrodes 15 and 17. The outer electrodes 15, 17, and 19 are connected by soldering to lead wires 21, 23, and 25, respectively. In addition, a circuit board 27 is provided with a groove 57 formed between base portions 55 and 55 corresponding to vibration nodes 29 and 31 of the piezoelectric transformer element 11, and grooves 59 and 59 formed outside of the base portions 55 to extend over the width beyond the ends of the piezoelectric transformer element 11. In the fifth embodiment of this invention, the piezoelectric transformer element 11 is mounted on the circuit board 27 in a manner such that the lower surface of the piezoelectric transformer element 11 is adhered to the base portions of the circuit board 27 at the positions beneath the vibration nodes 29 and 31 via fixing members 33 and 35 made of a flexible elastic material (silicone or the like). The above-mentioned seventh embodiment of this invention can also provide the effect similar to those in the first through the fourth embodiments. In addition, it is possible to prevent the contact between the ends of the piezoelectric transformer element 11 and the circuit board 27 due to warping of the circuit board 27 resulting from the deviation in resonance frequency of the piezoelectric transformer element 11. Therefore, generation of the audible sound can be reduced. Referring to Fig. 9(a) and Fig. 9(b), in a mounting structure of a piezoelectric transformer element according to an sixth embodiment of this invention, it is mounted on a circuit board 73 equipped with components for driving the piezoelectric transformer element 71 in a power supply circuit. The above-mentioned piezoelectric transformer element is driven in vibration modes of (1/2)λ or λ and comprises outer electrodes 75, 77, 79, and 81 formed on both of the front and the back side surfaces of the transformer. The piezoelectric transformer 71 is mounted on the circuit board 73 via fixing members 87 and 89 made of a flexible elastic material and interposed therebetween at the positions beneath vibration nodes 83 and 85 of the piezoelectric transformer as well as supporting members 91, 93, and 95 similar to the spacer members in the first embodiment to prevent generation of the sound. Thus, the mounting structure of the sixth embodiment shows that the mounting structures of the first through the seventh embodiments can be applied to a piezoelectric transformer element having a different structure. In the eighth embodiment of this invention, the fixing members 87 and 89 are disposed at the portions of the vibration nodes (nodal points) 83 and 85. As described above, according to this invention, in the structure in which the piezoelectric transformer element for use in the piezoelectric inverter is mounted on the circuit board equipped with the circuit components for driving the piezoelectric transformer element, use is made of the mounting method in which the flexible elastic material is introduced under, and adhered as the spacer to, the position except the vibration nodes. In this manner, it is possible to reduce generation of the audible sound without changing the characteristics such as the heat generation, the step-up ratio, and the driving frequency. In addition, according to this invention, it is possible to provide the mounting method for obtaining the mounting structure similar to that described above. Industrial Applicability As described above, the mounting structure and the mounting method of a piezoelectric transformer according to this invention is adapted to the use of the piezoelectric transformer element as a piezoelectric inverter of a backlight of a liquid crystal display or the like by mounting it on a circuit board equipped with a circuit component.

Claims: A mounting structure of a piezoelectric transformer element (11; 71) which is mounted on a circuit board (27; 73) equipped with at least one circuit component for driving said piezoelectric transformer element, wherein said piezoelectric one transformer element is fixed to said circuit board by the use of at least one elastic fixing member (33, 35; 87, 89) interposed between said piezoelectric transformer element and said circuit board so as to cover vibration node portions (29, 31; 83, 85) of said piezoelectric transformer element characterized in that at least one supporting member (37, 39, 41, 43; 45, 47; 49; 91, 93, 95) made of an elastic material is provided between the piezoelectric transformer element and said circuit board so as to cover a part of a remaining portion outside said vibration node portions, at least under an edge portion at, at least one end of the piezoelectric transformer element. A mounting structure of a piezoelectric transformer element (11; 71) as claimed in claim 1, wherein said at least one fixing member is strip-like shaped. A mounting structure of a piezoelectric transformer element (11; 71) as claimed in claim 2, wherein said at least one supporting member has a disk shape or a strip-like shape extending in parallel to said at least one fixing member. A mounting structure of a piezoelectric transformer element (11) which is mounted on a circuit board (27) equipped with at least one circuit component for driving said piezoelectric transformer element said piezoelectric transformer element (11) being fixed to said circuit board (27) by the use of at least one fixing member (33, 35) made of an elastic material and interposed between said piezoelectric transformer element and said circuit board at vibration node portions (29, 31) of said piezoelectric transformer element characterized in that said circuit board (27) is provided with a groove (57), formed between portions (55, 55) corresponding to said vibration node portions of said piezoelectric transformer element (11) and grooves (59, 59) formed outside of the above-mentioned portions (55) to extend over the width beyond the ends of said piezoelectric transformer element (11). A mounting method of a piezoelectric transformer element (11; 71) which is mounted on a circuit board (27; 73) equipped with at least one circuit component for driving said piezoelectric transformer element, wherein said piezoelectric transformer element is fixed to said circuit board by the use of at least one elastic fixing member (33, 35; 87, 89) interposed between said piezoelectric transformer element and said circuit board so as to cover vibration node portions (29, 31; 83, 85) of said piezoelectric transformer element characterized in that at least one supporting member (37, 39, 41; 43; 45, 47; 49; 91, 93, 95) made of an elastic material is disposed between the piezoelectric transformer element and said circuit board so as to cover a part of a remaining portion outside said vibration node portions at least under an edge portion at, at least end of the piezoelectric transformer element. A mounting method of a piezoelectric transformer element (11; 71) as claimed in claim 5, wherein said at least one fixing member is strip-like shaped. A mounting method of a piezoelectric transformer element (11; 71) as claimed in claim 6 wherein said at least one supporting member is disk shaped or a strip-like shaped and arranged in parallel to said at least one fixing member. A mounting method of a piezoelectric transformer element (11) which is mounted on a circuit board (27) equipped with at least one circuit component for driving said piezoelectric transformer element said piezoelectric transformer element (11) being fixed to said circuit board (27) by the use of at least one fixing member (33, 35) made of an elastic material and interposed between said piezoelectric transformer element and said circuit board at vibration node portions (29, 31) of said piezoelectric transformer element characterized in that said circuit board (27) is provided with a groove (57) formed between portions (55, 55) corresponding to said vibration node portions of said piezoelectric transformer element (11) and grooves (59, 59) formed outside of the above-mentioned portions (55) to extend over the width beyond the ends of said piezoelectric transformer element (11).

Inventors: KUMASAKA KATSUNORI, SAITO KOICHI, KUMASAKA, KATSUNORI, SAITO, KOICHI, KUMASAKA, Katsunori, Tokin Corporation, SAITO, Koichi, Tokin Corporation
Applicants: NEC TOKIN CORP, NEC TOKIN CORPORATION
Application year: 1999
Main classification: H01L41
IPC codes: H05K3, H01L41, H05K1