Discharge tube driving device and piezoelectric transformer therefor

Abstract: A driving device for driving a discharge tube (20) includes a piezoelectric transformer (18). A voltage, which is periodically inverted, is applied to input electrodes (38, 40) of the piezoelectric transformer (18). This design makes it possible to derive a high voltage from the piezoelectric transformer due to the piezoelectric effect, ignite the discharge tube (20), and keep the discharge tube ON. Thus, a smaller and thinner driving device for a discharge tube can be achieved.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a driving device and a piezoelectric transformer therefor for use in a discharge tube such as a cold-cathode tube which is employed for, e.g. a liquid crystal display. Description of the Related Art In general, liquid crystal displays do not emit light by themselves, and therefore, backlit type liquid crystal displays are currently dominant. In the backlit type liquid crystal display, a discharge tube such as a cold-cathode tube is provided at the rear or side of the liquid crystal display. Driving such a discharge tube requires a high AC voltage of more than several hundreds of volts although the voltage depends on the length and diameter of the discharge tube. Typically used for the driving device of the discharge tube for producing the high AC voltage is a high-frequency inverter which employs an electromagnetic transformer shown in Fig. 19. In the discharge tube driving device shown in Fig. 19, high output voltage V 4 generated in a secondary winding 75 of the electromagnetic transformer 70 is applied to a discharge tube 60 such as a cold-cathode tube via a ballast capacitor 81 which restricts tube current Io, thus causing the discharge tube 60 to emit light. As another method, a discharge tube driving device employing a piezoelectric transformer has been disclosed in Japanese Patent Publication No. 55-26600, Japanese Patent Laid-Open No. 52-113578, and Japanese Patent Laid-Open No. 5-114492. The piezoelectric transformer has a significantly simpler construction than the electromagnetic transformer and therefore it enables the achievement of a smaller and thinner driving device at lower cost. The principle and features of the piezoelectric transformer are disclosed in "Characteristics and Applications of Piezoelectric Transformer" in the July 1971 issue of "Electronic Ceramics" which is a technical journal published by GAKKENSHA. The principle of a Rosen type piezoelectric transformer will now be described with-reference to the Rosen piezoelectric transformer shown in Fig. 20. A pair of input electrodes 4 and 5 is formed by silver-baking them onto the top and bottom surfaces of the left half of a plate-shaped piezoelectric element 2 made of a lead-zirconate-titanate (PZT) material. In the same manner, an output electrode 6 is formed on the right end surface. The left half of the ceramic element 2 is polarized in the direction of thickness (downward in the figure); the right half is polarized in the direction of length (leftward in the figure) beforehand. In the piezoelectric transformer thus configured, when an AC voltage of approximately the same frequency of the lengthwise natural resonant frequency of the ceramic element 2 is applied across the input electrodes 4 and 5 from an AC voltage source 8, the ceramic element 2 develops intense mechanical vibration lengthwise. This causes the right-half power generating section to produce electric charge due to the piezoelectric effect, and output voltage Vo appears between the output electrode 6 and one of the input electrodes, e.g. the input electrode 5. The chart given in Fig. 21 shows the load resistance characteristic with respect to output voltage of the Rosen type piezoelectric transformer; the chart given in Fig. 22 shows the load resistance characteristic with respect to efficiency. In this case, AC input voltage Vi is 10 V, the driving mode is the full-wavelength mode, the piezoelectric material used is HCEPC-28 (made by Hitachi Metals, Ltd.), and the ceramic element measures 28 mm long, 7.5 mm wide, and 1.0 mm thick. As may be seen from the charts, the Rosen type piezoelectric transformer is capable of providing an output voltage 700 V, a 70-fold step-up ratio in a zone of a relatively high load resistance, e.g. 3 MΩ although the efficiency is low, only 50% or less. Conversely, the Rosen type piezoelectric transformer is capable of accomplishing high efficiency of 90% or higher in a zone around a load resistance of 100 kΩ although the output voltage is 85 V, a 8.5-fold step-up ratio. In the 1970s, a piezoelectric transformer for high-voltage power source for use in a TV receiver was a popular research topic. The piezoelectric transformer made use of the zone of the relatively high load resistance stated above. This type of piezoelectric transformer, however, has not yet been put in practical use because it cannot surpass the electromagnetic transformer mainly because of the problem of the heat generated by the element due to the operation in the low-efficiency zone. On the other hand, a typical apparatus, which incorporates the piezoelectric transformer based on the other zone where high efficiency can be obtained for relatively low resistance, requires a high DC input voltage of 25 V, unavoidably resulting in a battery assembly which was too large for use in a battery-driven, portable information terminal or personal computer. Portable information terminals or personal computers which use liquid crystal displays are required to operate on low voltage at high efficiency to achieve smaller power battery assemblies and prolonged service. Accordingly, the driving devices for the discharge tubes used for such applications are required to provide a high step-up ratio and high efficiency to enable them to produce high output voltage from low input voltage. The discharge tube driving device employing the conventional Rosen type piezoelectric transformer cannot meet the two requirements stated above. It has been proposed to improve a piezoelectric transformer by using a plurality of piezoelectric layers. A so obtained multilayer piezoelectric transformer is disclosed in FR-A-2 278 165 from which the first part of claim 1 starts out. In this transformer, the layers are polarised in thickness direction within a first region (driving region) and in longitudinal direction within a second region (driven region). The upper and lower surfaces of each layer are provided within the first region with a metallisation. The layers are then stacked so that the metallisations of adjacent layers come into contact and the contacting metallisations are bonded either by use of an electrically conductive adhesive or by sintering of the metallisation. Other multilayer piezoelectric transformers are disclosed in EP-A-605 901 and Electronics Letters, vol. 24, No. 7, March 1998, pages 444 to 445. Manufacturing of such conventional transformers starts out typically from individual plates of sintered piezoelectric ceramics which are bonded e.g. by an epoxy resin. SUMMARY OF THE INVENTION It is an object of the invention to provide a multilayer piezoelectric transformer having a high step-up ratio and a high efficiency. This object is solved by the transformer set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. Claims 3 to 9 relate to a preferred driving device for a discharge tube, employing the piezoelectric transformer of the invention. A discharge tube driving device permits reduced size and thickness by employing a piezoelectric transformer. According to an embodiment of such a driving device, an exciting means applies a voltage, which is periodically inverted, to a pair of input electrodes provided on the piezoelectric transformer. This causes the piezoelectric transformer to develop intense mechanical vibration due to the electrostrictive effect, and voltage is generated because of the piezoelectric effect from the mechanical vibration. In this case, the capacitance of a power generating section of the piezoelectric transformer is smaller than a driving section thereof, producing high output voltage which ignites a discharge tube. Once discharge is initiated, stable discharge is continued at an intersection of the characteristic of the piezoelectric transformer and that of the discharge tube. In particular, the use of a self-excited type, which is designed to feed back a part of the output of the piezoelectric transformer so as to self-excite the aforesaid exciting means, makes it possible to provide stable outputs even if the natural resonant frequency of the piezoelectric transformer changes due to a temperature change, time-dependent change or the like because the frequency of the applied voltage can be changed accordingly. Further, a tube current detection section may be provided for detecting the tube current of the discharge tube, so that the tube current can be controlled according to detected values, thereby controlling the light emission from the discharge tube. Such a discharge tube driving device can achieve both high step-up ratio and high efficiency, when an electromagnetic transformer is connected with a primary winding thereof to a DC input power source; a switching means is connected to the other end of the primary winding; the piezoelectric transformer is connected to a secondary winding of the electromagnetic transformer; and a driving/oscillating means causes the switching means to be self-oscillated by using a feedback signal received from the piezoelectric transformer, outputs of the piezoelectric transformer being supplied to the discharge tube. In such a device, a voltage, which has been boosted in accordance with the turn ratio of the primary winding to the second winding of the electromagnetic transformer, is applied to the input side of the piezoelectric transformer. Hence, the piezoelectric transformer outputs high AC voltage even for relatively low load resistance, thus allowing a discharge tube such as a cold-cathode tube to be driven at high efficiency. Even higher efficiency and step-up ratio can be achieved when the resonant frequency based on the inductance of the primary winding or secondary winding of the electromagnetic transformer and a parallel capacitance, which includes the parasitic capacitance of the switching means, or a parallel capacitance which includes the input capacitance of the piezoelectric transformer, is set to nearly coincide with the natural resonant frequency of the piezoelectric transformer itself. In use of the multilayer piezoelectric transformer of the present invention, in the first region, i.e. the driving section of the multilayer type piezoelectric transformer, input voltage is applied to the respective laminated thin layers. Therefore, in comparison with the conventional single-board type, the multilayer type is capable of multiplying the step-up ratio approximately by the number of laminated layers. Hence, the multilayer type piezoelectric transformer provides high voltage even for relatively low load resistance, enabling a discharge tube such as a cold-cathode tube to be driven at high efficiency. Even higher efficiency and step-up ratio can be accomplished when the resonant frequency based on the inductance of an inductor and the parallel capacitance, which includes the parasitic capacitance of the switching means or the input capacitance of the multilayer type piezoelectric transformer, is set to nearly coincide with the natural resonant frequency of the multilayer type piezoelectric transformer itself. Other objects and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying figures and claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram which shows the construction of an example of a discharge tube driving device in accordance with the present invention; Fig. 2 is a circuit diagram which shows an example of a self-excited driving circuit of the driving device shown in Fig. 1 useful to understand the invention; Fig. 3 is a circuit diagram of a current detecting circuit shown in Fig. 1; Fig. 4 is the characteristic curves which show the characteristics of a discharge tube and piezoelectric transformer; Fig. 5 is a perspective view of a multilayer type piezoelectric transformer in accordance with the present invention; Fig. 6 shows the tube current characteristic with respect to the input voltage of the multilayer type piezoelectric transformer in accordance with the present invention; Fig. 7 shows the tube current characteristic with respect to the efficiency of the multilayer type. piezoelectric transformer in accordance with the present invention; Fig. 8 is a circuit diagram of the discharge tube driving device which employs the multilayer type piezoelectric transformer in accordance with the present invention; Fig. 9 is a circuit diagram of the discharge tube driving device in accordance with the present invention, wherein a feedback signal is detected through a feedback electrode provided near an output electrode; Fig. 10 is a circuit diagram of the discharge tube driving device in accordance with the present invention, wherein a feedback signal is detected through an antenna disposed near an output electrode; Fig. 11 is a diagram showing the circuit construction of an embodiment of the present invention, which is provided with a light controller for controlling the light emission of a discharge tube; Fig. 12 shows the waveforms of the input voltage and the output voltage of the multilayer type piezoelectric transformer in the discharge tube driving device in accordance with the present invention; 13 Fig. 13 shows the tube current characteristic with respect to the input voltage of the discharge tube driving device in accordance with the present invention; Fig. 14 shows the tube current characteristic with respect to the efficiency of the discharge tube driving device in accordance with the present invention; Fig. 15 is a perspective view of a multilayer type piezoelectric transformer of another embodiment in accordance with the present invention; Fig. 16 is a lengthwise cross-sectional view of the multilayer type piezoelectric transformer of another embodiment in accordance with the present invention; Fig. 17 is a perspective view of a multilayer type piezoelectric transformer of still another embodiment in accordance with the present invention; Fig. 18 is a circuit diagram of a discharge tube driving device which employs the multilayer type piezoelectric transformer in accordance with the present invention; Fig. 19 is a circuit diagram of a discharge tube driving device which employs a conventional electromagnetic transformer; Fig. 20 is an explanatory view which is illustrative of the operating principle of a Rosen type piezoelectric transformer; Fig. 21 is a characteristic diagram illustrative of the load resistance characteristic with respect to output voltage of the Rosen type piezoelectric transformer; and Fig. 22 is a characteristic diagram illustrative of the load resistance characteristic with respect to efficiency of the Rosen type piezoelectric transformer. DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is the block diagram which shows an example of a discharge tube driving device ; Fig. 2 is the circuit diagram which shows a possible self-excited driving circuit of the driving device shown in Fig. 1, useful to understand the invention; Fig. 3 is the circuit diagram of a current detecting circuit shown in Fig. 1; and Fig. 4 is the characteristic curves showing the characteristics of the discharge tube and of the piezoelectric transformer. As illustrated, a driving device 10 for a discharge tube has a chopper circuit 14 for increasing or decreasing a DC voltage V1 supplied from a DC power source 12 to a desired voltage. An output voltage V2 of the chopper circuit 14 is supplied to an exciter circuit 16 serving as the exciter which characterizes the present invention. The exciter circuit 16 functions to apply a voltage V3, which inverts periodically, to a piezoelectric transformer 18. The piezoelectric transformer 18 consists of a piezoelectric ceramic element, which will be discussed later; it generates high electric power at the output side when it is subjected to the application of the aforesaid voltage. The output of the piezoelectric transformer 18 is applied to a discharge tube 20 such as a cold-cathode tube to ignite it and continue the discharge. Connected to the discharge tube 20 is a tube current detecting circuit 22 serving as a tube current detector for detecting the current flowing through it, the detected values being connected to a light emission controller 27 which controls the output of the piezoelectric transformer 18 so as to adjust the light emission of the discharge tube 20. More specifically, the light emission controller 27 is constituted by an error amplifier circuit 26 amplifying the voltage difference between the voltage, which is received from the tube current detecting circuit 22 and which indicates a detected value, and a pulse width modulator (PWM) circuit 28, for example, which performs pulse width modulation wherein the width of output pulse is changed in accordance with the error voltage received from the circuit 26. The control signal issued from the PWM circuit 28 is used to control the pulse width of the chopper circuit, thereby regulating the output voltage V2. The circuit configuration of the exciter circuit 16 and the piezoelectric transformer 18 will now be described in conjunction also with Fig. 2. As illustrated, the exciter circuit 16 is self-excited; it includes two transistors of, e.g. a first switch transistor 30 consisting of a pnp transistor, and a second switch transistor 32 consisting of an npn transistor, the two switch transistors being connected through the collectors thereof to provide complementary outputs. The emitter of the first switch transistor 30 is connected to one terminal A1 to which the voltage V2 is applied. The emitter of the second switch transistor is connected to the other terminal A2 to which the voltage V2 is applied. A transistor 34 consisting of, e.g. a pnp transistor, is provided to control the base currents of the two transistors 30 and 32. The terminal A1 is connected to the base of the second switch transistor 32 via a first resistor R1 and a first diode D1, the forward direction of the first diode D1 being the direction toward the aforesaid base. The base of the first switch transistor 30 is connected to the emitter of the transistor 34 via the second resistor R2. A second diode D2, the forward direction of which is the direction toward the emitter, is connected between the emitter of the transistor 34 and a point where the first resistor R1 and the first diode D1 are connected. The collector of the transistor 34 is connected to the terminal A2 and the base thereof is connected to a third resistor R3. In this example, useful to understand the invention, the piezoelectric transformer 18 shares the same construction as that shown in Fig. 20, it has a thin-plate piezoelectric ceramic element 36 made of, e.g. a lead-zirconate-titanate material which is fired. The ceramic element 36 measures, for example, 28 mm long, 7.5 mm wide, and 2.0 mm thick approximately. A pair of input electrodes 38 and 40 is formed by, for example, silver-baking them onto the top and bottom surfaces of the driving section (on the left side in the drawing) of the ceramic element 36. Further, an output electrode 44 is formed on the end surface of the power generating section which is the right half of the ceramic element 36, a feedback electrode 42 being provided apart from the input and output electrodes. The areas of the respective electrodes 38, 40, and 44 are set so that the capacitances generated among the electrodes are set to optimum values. One of the input electrodes 38 is connected to the terminal A1 and the other input electrode 40 is connected to the point where the collectors of the first switch transistor 30 and the second switch transistor 32 are connected. The feedback electrode 42 is connected to the third resistor R3. The terminal A1 is connected to one electrode 46 of the discharge tube 20 via the tube current detecting circuit 22 and the output electrode 44 is connected to the other electrode 48. The tube current detecting circuit 22 is constituted by a pair of diodes D3 and D4 which are connected in parallel so that the forward directions thereof are opposite from each other as shown in Fig. 3. A variable resistor R5 is connected in series with a diode D3 which is provided so that the forward direction thereof becomes the output electrode 44 side of the ceramic element 36, the detected voltage, which indicates the current value, being taken out through a moving terminal 50 thereof. The following describes the operation of the embodiment thus constructed. The DC voltage V1 supplied from the DC power source 12 is stepped up or down by the chopper circuit 14 in accordance with the control signal received from the PWM 28 to provide the DC voltage V2 which is supplied to the exciter circuit 16. The exciter circuit 16 applies the voltage V3, which is inverted periodically, to the piezoelectric transformer 18 to expand or contract the piezoelectric transformer 18. This causes the piezoelectric transformer 18 to generate voltage due to the piezoelectric effect; a part of the generated voltage is returned as the self-oscillation feed signal to the exciter circuit 16, the majority of the generated voltage being supplied to the discharge tube 20 to ignite it and continue the discharge. The then tube current of the discharge tube is detected by the tube current detecting circuit 22 and the detected voltage is compared with a reference voltage 24 by the error amplifier circuit 26 to output an error voltage. Based on the error voltage, the PWM 28 carries out pulse width modulation to produce a control signal which is supplied to the chopper circuit 14 so as to control the output voltage V2, thus regulating the light emission in the discharge tube 20 as stated above. Specific details of the operation in the exciter circuit 16 will now be given. In Fig. 2, when the DC voltage V2 is applied to the terminals A1 and A2, currents are allowed to flow into the base of the second switch transistor 32 via the first resistor R1 and the first diode D1 to turn the transistor 32 ON. This causes the voltage V3 (V2) to be applied to the input electrodes 38 and 40 of the piezoelectric transformer 18 to charge an input capacitance thereof. Due to the charging, negative voltage appears at the feedback electrode 42 and the base of the transistor 34 is subjected to forward bias through the third resistor R3, turning the transistor 34 ON. The moment the transistor 34 is turned ON, the first switch transistor 30 is turned ON and the second switch transistor 32 is turned OFF, causing the input capacitance of the piezoelectric transformer 18 to be discharged. Due to the discharging, positive voltage appears at the feedback electrode 42 and therefore the base of the transistor 34 is subjected to inverse bias through the third resistor R3, turning the transistor 34 OFF. Hence, the moment the transistor 34 is turned ON, the first switch transistor 30 is turned OFF and the second switch transistor 32 is turned back ON. After that, the same series of operations is repeated and high-frequency voltage is applied to the input electrodes 38 and 40 of the piezoelectric transformer 18. As a result, a boosted high-frequency voltage appears between the input electrode 38 and the output electrode 44, the high-frequency voltage being supplied to the electrodes 46 and 48 to cause the discharge tube 20 such as a cold-cathode tube to emit light. In this case, the inversion of the voltage applied to the input electrodes 38 and 40 is the self-oscillation implemented by the voltage generated at the feedback electrode 42 provided on the piezoelectric transformer 18. This means that the operating frequency is determined by the natural resonant frequency of the piezoelectric transformer 18 itself. For this reason, even if the resonant frequency of the piezoelectric transformer 18 varies due to temperature changes, time-dependent changes, or changes in load, the operating frequency varies with such changes, making it possible to drive the piezoelectric transformer at an optimum frequency for efficiency at all times. Further, as shown in Fig. 4, the characteristic of the piezoelectric transformer is expressed by the relationship between the output current and the output voltage; the output voltage slowly decreases as the output current increases. The characteristic of the discharge tube is expressed by the relationship between the current and the voltage, wherein the voltage shows a sudden change at a certain point as illustrated. Hence, the piezoelectric transformer supplies a high output voltage during a non-discharging period of time. As a result, the discharge tube 20 initiates discharging. The moment the discharging is started, the output voltage from the piezoelectric transformer 18 goes down. After that, stable discharging is continuously carried out at intersection P between the characteristic curve of the piezoelectric transformer and the characteristic curve of the discharge tube. As it is obvious from the characteristic curves, although the piezoelectric transformer 18 is not capable of providing large current, it is capable of generating high voltage with no load, making it extremely compatible with the characteristic of the discharge tube 20 such as a cold-cathode tube which has a negative characteristic. Further, since the piezoelectric transformer 18 cannot provide large current as stated above, no excessive short-circuit current flows through it even if, e.g. the discharge tube, which is the secondary, is short-circuited. This eliminates the need of providing a protective circuit which is required when an electromagnetic transformer is employed. There is still another advantage. Most backlighting systems for liquid crystal displays to which the discharge tube 20 is applied require light controllers for controlling the brightness of the backlight. The embodiment enables the operator to change the value of the variable resistor R5 to a proper value to change the output voltage V2 supplied from the chopper circuit 14 to a proper value, thereby increasing or decreasing the tube current, i.e. increasing or decreasing the emission of light. For instance, increasing the output voltage V2 increases the output voltage V3 provided by the exciter circuit 16. As a result, the power supplied from the piezoelectric transformer 18 to the discharge tube 20 increases, leading to an increased tube current. Fig. 5 is the perspective view of the multilayer type piezoelectric transformer in accordance with an embodiment of the present invention. To fabricate the multilayer type piezoelectric transformer, a green sheet made of PZT ceramic (made by Hitachi Metals, Ltd.; Trade name: HCEPC-28) is first produced. Then, platinum internal electrodes (input internal electrodes) 357 and 358 are printed on a part of the green sheet by employing the screen printing technique. In the next step, the sheet with the electrodes printed on is laminated and pressed together and sintered. After that, the laminated, sintered sheet is cut and polished, input external electrodes 351, 352, and an output electrode 355 are provided by silver-baking, and the internal electrode 357 is connected to the external electrode 351 and the internal electrode 358 to the external electrode 352. At this time, in order to prevent the internal electrode 357 from contacting the external electrode 352 and the internal electrode 358 from contacting the external electrode 351, an insulating layer 359 is provided between these electrodes. Polarizing treatment is carried out in the direction of the thickness of the driving section and in the direction of the length of the power generating section. This completes the multilayer type piezoelectric transformer. When this multilayer type piezoelectric transformer is driven in the full-wavelength mode which is generally used, a portion at a quarter of the full length exhibits the smallest vibration displacement. Hence, when the external electrodes 351 and 352 are installed at that portion, the characteristics of the multilayer type piezoelectric transformer are not impaired even when the external electrodes 351 and 352 are fixed with external terminals. Fig. 6 shows the tube current characteristic with respect to the input voltage when a cold-cathode tube is the load which uses the number of layers of the multilayer type piezoelectric transformer thus fabricated as the parameter thereof; Fig. 6 shows the tube current characteristic with respect to efficiency. The cold-cathode tube measures 3.0 mm, in diameter and 210 mm in length. The multilayer type piezoelectric transformer measures 30 mm long, 6.5 mm wide, and 1.2 mm thick, the resonant frequency being approximately 111 kHz in the full-wavelength mode although it depends on the tube current. The cold-cathode tube used is the type which is installed along the length of a liquid crystal display of a notebook personal computer. According to the characteristic shown in Fig. 6, to obtain a tube current of 5 to 7 mA, an AC input voltage of more than 23 to 90 V is required for a fewer laminated layers. A multilayer type piezoelectric transformer of six layers or more provides the tube current required for the liquid crystal display for the notebook personal computer from an AC input voltage of 14 V or less. According to the characteristics shown in Fig. 20, transformer efficiency of 90% or higher is obtained in the zone of a tube current of 3 mA or more, the efficiency being comparable to that of the conventional electromagnetic transformer. Other embodiments of the multilayer type piezoelectric transformers according to the present invention are shown in Fig. 15 and Fig. 16. Fig. 15 is the perspective view; Fig. 16 is the lengthwise cross-sectional view. By slightly shifting the opposing internal electrodes 357 and 358 in the direction of the width, the internal electrodes and the external electrodes are connected without the need of the insulating layer 359. Still another embodiment of the multilayer type piezoelectric transformer in accordance with the present invention is shown in Fig. 17. In the case of this embodiment, the opposing internal electrodes 357 and 358 are slightly shifted lengthwise to connect the internal electrodes to the external electrodes without using the insulating layer 359; the external electrodes 351 and 352 are provided on the end surface and at the portion located at the half point of the full length which are both subjected to the largest vibration displacement in the full-wavelength mode. In this case, the external electrodes are connected with the external terminals via lead wires, which have sufficient strength, rather than directly connecting them. Portable information equipment such as a notebook personal computer uses DC voltage for the input power supply. An inverter, which converts DC to AC, is added to the multilayer type piezoelectric transformer according to the present invention, so that it is employed as the discharge tube driving device for the portable information equipment. Fig. 8 is the circuit diagram of the discharge tube driving device in accordance with the present invention. The following describes the operation of the circuit. The moment the DC voltage V 2 is applied, the positive voltage (V 2 ) is applied to the input electrodes 351 and 352 of the multilayer type piezoelectric transformer 350 via an inductor 330; immediately after this, a starting current comes in through the resistor R 1 , turning ON a transistor 322 and a MOS field-effect transistor (hereinafter referred to as "MOSFET") 321, and input voltage V 3 of the multilayer type piezoelectric transformer becomes zero. Specifically, during that period, a pulse-shaped voltage is applied to the input electrodes of the multilayer type piezoelectric transformer; therefore, a positive voltage appears at the output electrode 355 due to the operating principle, turning a transistor 324 ON via the feedback resistor R 2 . When the transistor 324 is turned ON, the transistor 322 is turned OFF and a transistor 323 is turned ON; therefore, the MOSFET 321 is momentarily turned OFF, causing the positive voltage to be applied again to the input electrode 351 of the multilayer type piezoelectric transformer. This causes the output voltage of the multilayer type piezoelectric transformer to be inverted to negative, turning the transistor 324 OFF, the transistor 322 ON, and the transistor 323 OFF via the feedback resistor R 2 ; therefore, the MOSFET 321 is momentarily turned ON. After that, the same series of operations is repeated and high-frequency voltage, which has been stepped up, is generated between the output electrode 355 and a common electrode 352 of the multilayer type piezoelectric transformer and the high-frequency voltage is applied to electrodes 361 and 362 to cause a discharge tube 360 to emit light. The inverting operation for turning ON/OFF the MOSFET 321 is the self-oscillating operation performed using the voltage generated at the output electrode of the multilayer type piezoelectric transformer 350. In this self-oscillation, the feedback signal detector exhibits the greatest mechanical displacement. To obtain maximum output, the mechanical displacement of the right end surface, wherein the output electrode is present, needs to be maximized. As shown in Fig. 8, the feedback signal can be obtained either directly from the output electrode of the multilayer type piezoelectric transformer or through an electrode or antenna which is provided near the output electrode as in the embodiment which will be discussed later. The example given in Fig. 8 has an E-class amplifier circuit composed of an inductor 330, MOSFET 321, and the multilayer type piezoelectric transformer 350. The E-class amplifier circuit was proposed by N. O. Sokal et al. in the United States in 1975; it is explained on pages 153 to 155 of "Story about Amplification" written by Genzaburo Kuraishi, published by Nikkan Kogyo Shimbunsha. The resonance based on the inductance L 1 of the inductor 330, the output capacitance C OS of the MOSFET 321, and the input capacitance C 01 of the multilayer type piezoelectric transformer 350 causes the input voltage V 3 applied to the multilayer type piezoelectric transformer 350 to exhibit a half-wave sinusoidal voltage waveform as illustrated in Fig. 12, resulting in lower switching loss of the MOSFET 321. This indicates the characteristics of the E-class amplifier circuit. Further, when the frequency of the resonance is set to approximately coincide with the natural resonant frequency f 0 of the multilayer type piezoelectric transformer 350 as shown by the expression given below, especially high efficiency and step-up ratio can be obtained because there will be a smaller harmful higher harmonic component and a larger fundamental wave component: f 0 ≒ 1 2π L 1 (C 0S +C 01 ) [Hz] If the output capacitance C OS of the MOSFET 321 and the input capacitance C 01 of the multilayer type piezoelectric transformer 350 do not allow the resonant frequencies to approximately coincide with each other, then the capacitor C 1 may be connected in parallel to the respective elements. As another alternative, a capacitor may be connected in parallel to the inductor 330 for equivalent effect. Fig. 13 shows the characteristic of this embodiment in tube current with respect to input voltage observed when 6-layer and 12-layer piezoelectric transformers are used; the transformers being the same type as that shown in Fig. 5 and having the circuit configuration shown in Fig. 8; Fig. 14 shows the characteristic of this embodiment in tube current with respect to efficiency. The same cold-cathode tube measuring 3.0 mm in diameter and 210 mm in length stated above was used. According to the characteristics shown in Fig. 13, a tube current of 5 to 7 mA can be obtained from a DC input voltage of 14 V or less in the case of the 6-layer piezoelectric transformer, or from a DC input voltage of 8 V or less in the case of the 12-layer piezoelectric transformer. The DC input voltage can be further decreased by increasing the number of the layers. For example, a 30-layer piezoelectric transformer is capable of providing a tube current of 5 to 7 mA from a DC input voltage of 3 V or less. As it is understood from the characteristics shown in Fig. 14, an inverter efficiency of 80% or more is obtained in the zone of the required tube current of 5 mA or more. This efficiency is as high as that obtained with the inverter which uses an electromagnetic transformer. In this embodiment, the resistance values of the feedback resistor R 2 and a starting resistor R 1 are adjusted to obtain a proper I/O phase. High voltage of high frequency is generated at the output electrode of the multilayer type piezoelectric transformer 350; therefore, induced voltage may be generated in the parts or wiring disposed in the vicinity thereof, leading to oscillation which deviates from the foregoing I/O phase with a resultant lower efficiency and step-up ratio. The voltage waveform of this embodiment is shown in Fig. 12. Fig. 9 shows the circuit diagram of the discharge tube driving device in accordance with the present invention, wherein the feedback signal is detected through a feedback electrode 353 provided near the output electrode. The maximum output can be obtained through the output electrode as in the example shown in Fig. 8. Fig. 10 shows the circuit diagram of the discharge tube driving device in accordance with the present invention, wherein the feedback signal is detected through an antenna 354 disposed near the output electrode. The maximum output can be obtained through the output electrode as in the example shown in Fig. 10. Fig. 11 shows an embodiment of the present invention which is provided with a light controlling function for regulating the amount of light emitted by the discharge tube. The embodiment is composed of the embodiment shown in Fig. 10 to which a tube current detector 3200 and a preceding-stage voltage controller 3100 have been added. The tube current detector 3200 is provided to detect the current flowing through the discharge tube 360. The tube current is detected through the variable resistor R5; the level of a detection signal 3210 can be changed by varying the resistance value. Based on the detection signal, the preceding-stage voltage controller controls the supply voltage V 2 applied to the inverter. To be more specific, when the resistance of the variable resistor R5 is decreased, the preceding-stage voltage controller increases the supply voltage V 2 to increase the tube current so as to ensure a constant detection signal. The preceding-stage voltage controller controls the supply voltage V 2 by employing a step-down type, step-up type, or inverting-type chopper control or dropper control which is a publicly known art. There is another light control method wherein the tube current is controlled through duty control for continuing the application of the supply voltage V 2 . Another embodiment of the present invention will now be descried, referring to an accompanying drawing. Fig. 18 is the circuit diagram of a discharge tube driving device in accordance with the present invention. When the DC voltage V 2 is applied, high-frequency voltage, which has been boosted, appears between the output electrode 355 and the common electrode 352 and the high-frequency voltage is applied to the electrodes 361 and 362, causing the discharge tube 360 to emit light. The inverting operation for turning ON/OFF the MOSFET 321 is the self-oscillating operation performed using the voltage generated at the output electrode of the piezoelectric transformer. The DC voltage V 2 is stepped up by a turn ratio (n 1 + n 2 )/n 1 of a primary winding 3131 (the number of turns is n 1 ) to a secondary winding 3132 (the number of turns is n 2 ) of an electromagnetic transformer 3130 before it is applied to the piezoelectric transformer. Hence, a cold-cathode tube can be driven on a 3-volt input voltage which is used in portable information terminal equipment. Thus, the present invention can be implemented in various forms. Further, it is apparent that the present invention can be implemented with different polarizing directions or vibration modes of the piezoelectric transformer, or constructions of the switching means or driving/oscillating means from those illustrated herein. According to the present invention, the driving section of the multilayer type piezoelectric transformer consists of multiple layers; therefore, it is capable of driving a discharge tube such as a cold-cathode tube at high efficiency even from a low input voltage used in portable electronic equipment. Even higher efficiency and step-up ratio can be obtained when the resonance frequency based on the inductor provided in series with the multilayer type piezoelectric transformer and a parallel capacitance, which includes the parasitic capacitance of the switching means, or a parallel capacitance, which includes the input capacitance of the multilayer type piezoelectric transformer is set to nearly coincide with the natural resonant frequency of the multilayer type piezoelectric transformer itself. Moreover, since the multilayer type piezoelectric transformer has a high output impedance, there is no need for the ballast capacitor for restricting the tube current which used to be employed for the conventional electromagnetic transformer type. In addition, the use of the multilayer type piezoelectric transformer enables a smaller and thinner discharge tube driving device to be achieved because it is significantly simpler and in construction and smaller than the conventional electromagnetic transformer. It is to be understood that the present invention is not limited to the specific embodiments thereof and that other modifications are possible within the scope of the appended claims.

Claims: An elongated plate-like piezoelectric transformer, comprising: a plurality of stacked, sheet-like elongated piezoelectric layers polarised within a first region of the transformer (350) in the direction of their thickness and within a second region of the transformer (350) in the direction of their length, a plurality of internal electrodes (357, 358) interposed between the piezoelectric layers within the first region of the transformer (350), two first external electrodes (351, 352) to which the internal electrodes (357,358) are alternately connected, and a second external electrode (355) formed on a surface of the second region, characterised in that the piezoelectric layers are green sheets of piezoelectric ceramic which are laminated, with the internal electrodes interposed therebetween within the first region, and then sintered together. A transformer according to claim 1, wherein the internal electrodes (357, 358) are connected with the first external electrodes (351, 352) at points spaced from an end surface of the first region by approximately a quarter of the full length of the transformer over the first and second regions. A device for driving a discharge tube, comprising: a piezoelectric transformer (18, 350) according to claim 1 or 2, and exciting means (16) for periodically inverting the voltage applied to one of said first external electrodes of the piezoelectric transformer, acting as an input electrode, wherein an output of the piezoelectric transformer is supplied to the discharge tube (20). A device for driving a discharge tube, comprising: a piezoelectric transformer (18, 350) according to claim 1 or 2, a DC input power source (V 2 ), an electromagnetic transformer (3130) having a primary winding (3131) with its one end connected to the DC input power source and the other end connected to a switching means (321), and a secondary winding (3132) with its one end connected to the piezoelectric transformer, and driving means (322 - 324) for causing the switching means (321) to be self-oscillating by feedback from an output of the piezoelectric transformer, wherein an output of the piezoelectric transformer is supplied to the discharge tube (360). A device for driving a discharge tube, comprising: a piezoelectric transformer (18, 350) according to claim 1 or 2, a DC input power source (V 2 ), an inductance element (330) having a coil with its one end connected to the DC input power source, switching means (321) connected to the other end of the coil of the inductance element (330) and connected in parallel to the piezoelectric transformer and driving means (322 - 324) for driving the switching means (321) to be self-oscillating by feedback from an output of the piezoelectric transformer, wherein an output of the piezoelectric transformer is supplied to the discharge tube (360). A device in accordance with claim 3, wherein said exciting means (16) comprises driving means which is self-oscillating by feedback from an output of the piezoelectric transformer (18, 350). A device in accordance with any of claims 4 to 6, wherein the driving means is self-oscillating by feedback from an electrode (353) provided in the vicinity of the second external electrode (355) of the piezoelectric transformer. A device in accordance with any of claims 4 to 6, wherein the driving means is self-oscillating by feedback from an antenna (354) disposed in the vicinity of the second external electrode (355) of the piezoelectric transformer. A device in accordance with any of claims 4 to 6, wherein the driving means is self-oscillating by feedback from part of the electric power generated at the second external electrode (355) of the piezoelectric transformer.

Inventors: ABE TOORU, JOMURA SHIGERU, TAMAKAI TOSHIYUKI, ABE, TOORU, JOMURA, SHIGERU, TAMAKAI, TOSHIYUKI
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1995
Main classification: H01L41
IPC codes: H05B41, H01L41