JPH1094263A - Piezoelectric-transformer driving apparatus, piezoelectric transformer and liquid-crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of a high-frequency voltage at the start of a driving operation by a method wherein a driving signal is set at a specific frequency, the sweep of a driving frequency is stopped when a load current becomes a set value and the load current is controlled within a set frequency range having a definite relationship with a resonance frequency at the actual driving operation of a piezoelectric transformer. SOLUTION: A current detection circuit 7 detects an inflow current to a load (e.g. a cold- cathode fluorescent lamp) 5. At the start of a driving operation, a frequency control circuit 8 sets a driving signal to a high frequency which is within 1.1 times (preferably within 0.7 times) a resonance frequency at the actual driving operation of a piezoelectric transformer 4, it starts the sweep of a driving frequency by using the driving signal, stops the sweep of the driving signal when a load current by the current detection circuit 7 becomes a set value, and controls the load current within a set frequency range having a definite relationship with the resonance frequency at the actual driving operation of the piezoelectric transformer 4 after stop of the sweep. Thereby, it is possible to suppress the generation of a high-frequency voltage from being generated at the start of the driving operation of the piezoelectric transformer, and its driving efficiency can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer driving device for converting an amplitude value of an AC voltage by a piezoelectric effect of a piezoelectric material such as a piezoelectric ceramic.

[0002]

2. Description of the Related Art Piezoelectric transformers developed in the late 1950's have been focused on as step-up transformers for high-voltage power supplies and have been developed. However, due to material limitations such as the breaking strength of piezoelectric ceramic materials, large-scale commercialization has been achieved. Development had been suspended without any progress. However, as the development of high-strength piezoceramics has recently progressed and the demand for smaller and thinner portable information devices such as notebook computers, electronic notebooks, and game consoles has increased, the LCD back- It has attracted much attention again as a boost transformer for light inverter power supplies.

An inverter for a liquid crystal backlight is used as a lighting power source for a cold cathode fluorescent lamp used as a backlight light source.
A conversion from a DC low voltage such as 2 V to a high frequency high voltage of about 1000 Vrms during lighting and about 500 Vrms during steady state is required. At present, electromagnetic winding transformers used in backlight inverters have been reduced in thickness by using transformers with a horizontal structure using a special core. There are limitations, and there is a disadvantage that the core loss and winding loss due to the use of a thin copper wire are large, and the efficiency is low.

On the other hand, a piezoelectric transformer is made of a piezoelectric ceramic material such as lead zirconate titanate (PZT) or a piezoelectric crystal material such as lithium niobate.
And an electrode on the secondary side (output side) is applied, and an AC voltage near the resonance frequency of the piezoelectric transformer is applied to the primary side electrode to mechanically resonate the piezoelectric transformer, and this mechanical vibration is converted by the piezoelectric effect. From the secondary electrode as high-voltage generated power. In addition, it is characterized in that it can be made smaller, especially thinner than an electromagnetic transformer, and that high conversion efficiency can be achieved.

Hereinafter, a conventional piezoelectric transformer driving device will be described with reference to the drawings. FIG. 16 is a schematic view of a Rosen type piezoelectric transformer, in which primary side (input side) and secondary side (output side) electrodes are attached to a rectangular plate made of a piezoelectric ceramic material such as lead zirconate titanate (PZT). It is composed. As shown by P in the figure, the primary side electrode portion is polarized in the thickness direction of the rectangular plate, and the secondary side electrode portion is polarized in the length direction. When an AC voltage near the resonance frequency of the piezoelectric transformer is applied to the primary electrode, the piezoelectric transformer causes mechanical vibration in the length direction, and this mechanical vibration is converted to a high voltage by the secondary electrode by the piezoelectric effect. And then take out as high voltage power.

Conventionally, as a driving method of the piezoelectric transformer, there are a self-excited oscillation circuit method in which the piezoelectric transformer is put in a loop of an oscillation circuit, and a separately-excited oscillation circuit method in which an oscillation circuit is separately provided. The self-excited oscillation circuit method has a drawback in that it has a problem in driving efficiency and cannot follow a large fluctuation of the load. Therefore, a recently-excited oscillation circuit method is often used in recent examples. FIG. 17 is a block diagram of a conventional separately-excited oscillation circuit type driving circuit of the Rosen type piezoelectric transformer shown in FIG. 16, that is, a conventional piezoelectric transformer driving device.

In FIG. 17, a variable oscillation circuit 101 generates a driving AC signal near the resonance frequency of the piezoelectric transformer 104, and when the output signal of the variable oscillation circuit 101 is a pulse signal, many high frequency signals other than the driving frequency component are generated. And these high-frequency signal components become loss or heat in the piezoelectric transformer 104, and significantly reduce the reliability of the piezoelectric transformer 104. Therefore, the waveform is substantially shaped into a sine wave by the waveform shaping circuit 102 in order to reduce the loss in the piezoelectric transformer 104. The waveform shaping circuit 102 is a low-pass filter in a simple case, and a band-pass filter is used in a case where efficiency is emphasized. The output of the waveform shaping circuit 102 is current-amplified or voltage-amplified by a drive circuit 103 to a level sufficient to drive the piezoelectric transformer. here,
The drive circuit 103 is composed of only a normal amplifier circuit composed of a transistor or a combination of an amplifier circuit and a step-up transformer. The output of the drive circuit 103 is stepped up by the piezoelectric transformer 104, and the cold cathode fluorescent lamp 10
5, the cold cathode fluorescent lamp 105 is turned on.

Here, the resonance frequency of the piezoelectric transformer 104 changes due to environmental changes such as temperature and load.
As shown in the circuit shown in FIG.
Is driven, the relationship between the resonance frequency of the piezoelectric transformer and the drive frequency changes. When the driving frequency is far away from the resonance frequency of the piezoelectric transformer, the voltage step-up ratio by the piezoelectric transformer is remarkably reduced, so that a sufficient current cannot be supplied to the cold cathode fluorescent lamp 105. High brightness cannot be maintained.

The circuit shown in FIG. 18 responds to changes in characteristics such as the resonance frequency of the piezoelectric transformer 104, and is another conventional drive circuit for the piezoelectric transformer 104 shown in FIG. It is a block diagram of a piezoelectric transformer drive device. Variable oscillation circuit 101, waveform shaping circuit 1
The functions of the drive circuit 102, the drive circuit 103, and the piezoelectric transformer 104 are the same as those of the circuit shown in FIG. However, a feedback resistor 106 having a small value is connected in series to the cold cathode fluorescent lamp 105, and the current flowing through the cold cathode fluorescent lamp 105 is detected by the feedback resistor 106. Feedback resistor 1 proportional to current
06 is input to the oscillation control circuit 107, and the oscillation control circuit 107 controls the variable oscillation circuit so that the voltage across the feedback resistor 106 is constant, that is, the current flowing through the cold cathode fluorescent lamp 105 is constant. 101 controls the frequency of the output signal. With this control, the cold-cathode fluorescent lamp 105 is turned on with almost constant luminance.

[0010] The circuit shown in FIG.
4 corresponds to a change in characteristics such as the resonance frequency of FIG.
6, another conventional drive circuit for the piezoelectric transformer 104 shown in FIG.
That is, it is a block diagram of a conventional piezoelectric transformer driving device. The functions of the variable oscillation circuit 101, the waveform shaping circuit 102, the drive circuit 103, and the piezoelectric transformer 104 are shown in FIG.
Is the same as the circuit shown in FIG. However, the cold cathode fluorescent lamp 10
5, a feedback resistor 106 having a small value is connected in series, and a current flowing through the cold cathode fluorescent lamp 105 is detected by the feedback resistor 106. When the load or environment changes, the piezoelectric transformer 1
When the characteristics such as the resonance frequency 04 change, the current flowing through the cold cathode fluorescent lamp 105 changes. The voltage across the feedback resistor 106 proportional to the current is applied to the pulse width control circuit 10.
8 and the pulse width control circuit 108
The pulse width of the output signal of the variable oscillation circuit 101 is varied by the pulse width variable circuit 109 so that the voltage across the terminal 06 is constant, that is, the current flowing through the cold cathode fluorescent lamp 105 is constant. Control. With this control, the cold-cathode fluorescent lamp 105 is turned on with almost constant luminance.

FIG. 20 shows the frequency characteristics of the admittance of the piezoelectric transformer at the time of actual driving. The arrows in FIG. 20 show the sweeping direction of the driving frequency. Hysteresis characteristics are shown in the vicinity. The operation becomes unstable in a hysteresis region indicated by oblique lines in FIG. 2, and it is necessary to drive the piezoelectric transformer avoiding the unstable operation region in order to operate the piezoelectric transformer stably. An example of driving while avoiding the hysteresis region in which the operation is unstable is disclosed in Japanese Patent Publication No. 5-28072 (Japanese Unexamined Patent Publication (Kokai) No. Sho 63-5).
6178), in order to drive while avoiding the unstable operation region, the drive frequency is swept downward from a frequency higher than the hysteresis region, and when the frequency reaches a predetermined frequency higher than the hysteresis region, the sweep of the drive frequency is stopped. The driving method is adopted. The same driving method is applied to the driving method of the piezoelectric transformer, and the driving frequency is swept from a frequency higher than the hysteresis area to a lower frequency, and when the frequency becomes a predetermined frequency higher than the hysteresis area, the driving frequency is stopped. Take the driving method.

As described above, the drive device of the separately excited oscillation circuit system has been described as a conventional example of the piezoelectric transformer drive device.

[0013]

As described above, the conventional piezoelectric transformer driving device sweeps the driving frequency from a frequency higher than a hysteresis region indicated by the piezoelectric transformer to a lower frequency, and a predetermined frequency higher than the hysteresis region. When the frequency is reached, the sweep of the drive frequency is stopped, and thereafter the output current of the piezoelectric transformer is controlled by varying the drive frequency of the piezoelectric transformer in the frequency range higher than the hysteresis region in order to keep the output current of the piezoelectric transformer almost constant. I was

However, in the conventional piezoelectric transformer driving method, a high-frequency voltage is applied to the piezoelectric transformer when the driving frequency is swept from a frequency higher than a hysteresis region near the resonance frequency at the start of driving, so that the material constant of the piezoelectric transformer deteriorates. And cause damage such as mechanical destruction.

Further, the input high-frequency voltage is boosted by the piezoelectric transformer, and a high-frequency voltage is applied to the load of the piezoelectric transformer to damage the load. Also, if the load is light or the secondary side of the piezoelectric transformer is opened or opened. When it is near, there is a problem that the piezoelectric transformer is mechanically broken by the high voltage of the high voltage that has been boosted.

In view of the above problems of the conventional transformer, the present invention suppresses the generation of high-frequency voltage generated at the start of driving, and provides a piezoelectric device having a condition of high reliability, long life, and high driving efficiency. It is an object to provide a transformer driving device.

[0017]

According to a first aspect of the present invention, there is provided a variable oscillation circuit for generating an AC signal, and a driving circuit for amplifying the AC signal to generate a driving signal. A circuit (3), a piezoelectric transformer (4) having an input electrode and an output electrode formed on a piezoelectric body, and a load (5) connected to the output electrode, and applying a drive signal to the input electrode of the piezoelectric transformer; A piezoelectric transformer driving device for boosting a drive signal and extracting it from an output electrode of a piezoelectric transformer to drive a load, comprising: a current detection circuit (7) for detecting a current flowing into the load; The drive signal is set to a frequency that is at least 1.1 times higher than the resonance frequency at the time of drive, and the drive frequency starts to be swept from the drive signal. When the load current by the current detection circuit reaches the set value, the drive frequency is set. Stop sweep In order to achieve the above object, a frequency control circuit (8) for controlling a load current within a set frequency range having a fixed relationship with the resonance frequency of the piezoelectric transformer during actual driving after the stop of the sweep is provided. The second embodiment of the present invention comprises a variable oscillation circuit (1) for generating an AC signal, a drive circuit (3) for amplifying the AC signal to generate a drive signal, and an input electrode and an output electrode formed on a piezoelectric body. A piezoelectric transformer (4) and a load (5) connected to an output electrode, and applying a drive signal to an input electrode of the piezoelectric transformer,
A piezoelectric transformer driving device for boosting a drive signal and extracting it from an output electrode of a piezoelectric transformer to drive a load, comprising: a current detection circuit (7) for detecting a current flowing into the load; Set the drive signal to a frequency higher than the resonance frequency at the time of drive, start the drive frequency sweep from the drive signal, stop the drive frequency sweep when the load current by the current detection circuit reaches the set value, and after the sweep is stopped Has a frequency control circuit (8) for controlling the load current within a set frequency range that is in a fixed relationship with the resonance frequency of the piezoelectric transformer at the time of actual driving. Set the amplitude of the drive signal within a factor of 2 and perform control to gradually increase the voltage amplitude of the drive signal until the drive frequency sweep is stopped when the load current by the current detection circuit reaches the set value. Characterized by having a width control circuit (9), third embodiment of the present invention in order to achieve the above object, a variable oscillation circuit for generating an AC signal (1),
A drive circuit (3) that amplifies the AC signal to generate a drive signal;
It has a piezoelectric transformer (4) in which an input electrode and an output electrode are formed on a piezoelectric body, and a load (5) connected to the output electrode, and applies a drive signal to the input electrode of the piezoelectric transformer to boost the drive signal. Take out from the output electrode of the piezoelectric transformer,
A piezoelectric transformer driving device for driving a load, comprising: a current detection circuit (7) for detecting a current flowing into the load; and setting the drive signal to a frequency higher than a resonance frequency at the time of actual driving of the piezoelectric transformer at the start of driving. Then, the driving frequency is started to be swept by the driving signal, and when the load current by the current detection circuit reaches a set value, the driving frequency is stopped to be swept. A frequency control circuit (8) for controlling a load current within a set frequency range having a relationship, and an FET element (31) as a drive circuit
By connecting a parallel circuit of a resistance element (10) and a diode (11) to the gate of the FET element, connecting a capacitance element (12) between the gate and the drain of the FET element,
In order to achieve the above object, a fourth embodiment of the present invention is characterized in that at least one of connecting a capacitive element (13) between the drain of the FET element and the ground is employed. Variable oscillator circuit (1), a drive circuit (3) that amplifies an AC signal to generate a drive signal, and a piezoelectric transformer (4) in which an input electrode and an output electrode are formed on a piezoelectric body.
And a load (5) connected to an output electrode, the drive signal being applied to the input electrode of the piezoelectric transformer, the drive signal being boosted and taken out from the output electrode of the piezoelectric transformer to drive the load. A transformer drive device, comprising: a current detection circuit (7) for detecting a current flowing into a load; and setting a drive signal to a frequency higher than a resonance frequency at the time of actual driving of the piezoelectric transformer at the start of drive, and driving the drive by the drive signal. Starts the frequency sweep, stops the drive frequency sweep when the load current by the current detection circuit reaches the set value, and after the stop, within the set frequency range that has a fixed relationship with the resonance frequency of the piezoelectric transformer during actual driving A frequency control circuit (8) for controlling the load current, and a FET element (31) as a drive circuit.
And the inductance (1) is connected to the drain of the FET element.
4) and a series circuit of the capacitor (15)
Connect only the inductance (14) to the drain of the ET element, or connect the step-up transformer (1) to the drain of the FET element.
The piezoelectric transformer driving device is characterized in that at least one of connecting the primary side of 6) and connecting a capacitor (17) to the secondary side of the step-up transformer is adopted.

In order to achieve the above object, a fifth embodiment of the present invention comprises a primary (input) electrode (21) and a secondary (input) electrode made of a piezoelectric ceramic material or a piezoelectric single crystal.
In a piezoelectric transformer (20) configured to include a secondary (output) electrode (22) and convert a drive AC voltage input to the primary electrode into a voltage and output the secondary AC from the secondary electrode, 10MΩ to several hundreds in resistance value on the secondary side electrode
In order to achieve the above object, a sixth embodiment of the present invention is characterized in that a high resistance conductive film (22) of the order of MΩ is formed. Side) electrode (25) and a secondary (output side) electrode (26), and the piezoelectric transformer () converts the driving AC voltage input to the primary electrode into a voltage and outputs the converted AC voltage from the secondary electrode. In 24), the boundary between the primary side electrode formed on the piezoelectric transformer and the secondary side electrode part (the center part of the piezoelectric transformer) is a straight line, and the other end side is the two corners of the primary side electrode. A radius of about 2 to 7 mm, or 2
A piezoelectric transformer characterized in that a chamfer of about 5 mm is provided.

In the present invention, according to the first aspect, the sweep start frequency of the drive signal at the start of driving of the piezoelectric transformer is not set to be much higher than the resonance frequency of the piezoelectric transformer, that is, the drive start signal frequency Is set to within 1.1 times (more certainly, within 1.07 times) the resonance frequency of the piezoelectric transformer at the time of actual driving to reduce the high frequency component of the driving voltage, thereby deteriorating the characteristics and mechanical destruction. , A highly reliable piezoelectric transformer driving device with high driving efficiency can be provided.

According to the second aspect of the invention, the amplitude of the drive signal at the start of driving the piezoelectric transformer is gradually increased, that is, the hysteresis region indicating the drive signal frequency at the start of driving when the piezoelectric transformer is actually driven. Higher than the voltage amplitude during normal driving, and within 0.7 times (0.5 times when a remarkable effect is desired) the drive frequency is reduced until the load current reaches the set value. By gradually increasing the voltage, it is possible to provide a highly reliable piezoelectric transformer drive device with extremely low probability of characteristic deterioration and mechanical destruction, and high drive efficiency.

According to the third aspect of the invention, the response of the drive circuit (drive circuit) for outputting the drive signal of the piezoelectric transformer is delayed, that is, in the experiment, the response of the drive circuit is reduced by the resonance frequency of the use mode of the piezoelectric transformer. If the frequency is reduced at a frequency of 5 to 7 times or more, it is possible to provide a highly reliable piezoelectric transformer driving device having a very small probability of characteristic deterioration and mechanical destruction and high driving efficiency.

According to the fourth aspect of the invention, the high frequency component of the output of the drive circuit (drive circuit) for outputting the drive signal of the piezoelectric transformer is reduced, that is, in the experiment, the series resonance frequency of the series resonance circuit is reduced. By setting the resonance frequency within the range of 0.7 to 1.3 times the resonance frequency of the piezoelectric transformer, it is possible to provide a highly reliable piezoelectric transformer driving device having a very small probability of characteristic deterioration and mechanical destruction and a high driving efficiency. be able to.

In the present invention, according to the fifth aspect of the present invention, a high-resistance conductive film or the like is used to form a resistance of about 10 MΩ to several hundred MΩ on the secondary side electrode portion of the piezoelectric transformer, so that a high frequency generated at the start of driving is obtained. By suppressing the amplitude value of the high voltage generated on the secondary side of the piezoelectric transformer by the voltage, it is possible to provide a piezoelectric transformer having all of the conditions of high reliability, long life, and high drive efficiency.

According to the sixth aspect of the present invention, the high-frequency vibration mode, which is a high frequency, is made extremely weak by the primary electrode structure of the piezoelectric transformer. It is possible to provide a piezoelectric transformer that satisfies all the conditions of high performance, long life, and high drive efficiency.

Further, a liquid crystal display device utilizing the invention can be provided.

[0026]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a block diagram of a piezoelectric transformer driving apparatus according to an embodiment of the present invention.
In the figure, the piezoelectric transformer 4 may be of a Rosen type or of another type, and may be any piezoelectric transformer.
FIG. 2 shows the admittance frequency characteristics as viewed from the drive terminals of the piezoelectric transformer 4. The operation of the piezoelectric transformer driving device according to one embodiment of the present invention will be described in detail with reference to both drawings.

When the start signal of the driving device is output to the frequency control circuit 8 and the driving of the piezoelectric transformer 4 is started, the variable oscillation circuit 1, which is an oscillation circuit whose oscillation frequency can be varied by an external terminal, comprises a piezoelectric transformer. An oscillation frequency fs, which is higher than the hysteresis region indicated by hatching in FIG. The frequency control circuit 8 controls the oscillation frequency by an external terminal. The output signal of the variable oscillation circuit 1 is shaped into a clear pulse waveform by the waveform shaping circuit 2. Waveform shaping circuit 2
Is amplified by a drive circuit 3 comprising an FET 31 and a boosting coil 32 to a level sufficient to drive the piezoelectric transformer 4. The piezoelectric transformer 4 boosts the input voltage by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5 as a load. The load current is detected by the current detecting circuit 7 based on the voltage between both ends of the feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5. If the load current has not reached the set value, the frequency control circuit 8 Decrease the output frequency. Then, the output signal of the variable oscillation circuit 1 is again shaped into a clear pulse waveform by the waveform shaping circuit 2, voltage-amplified by the drive circuit 3, and input to the piezoelectric transformer 4. The input voltage is boosted by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5. Then, a current detection circuit 7 is provided by a feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5.
Detects the load current. This operation is repeated until the load current reaches the set value.

That is, at the time of startup, the drive frequency is swept to a frequency lower than the frequency fs higher than the hysteresis region near the resonance frequency of the piezoelectric transformer 4, and when the load current reaches the set value, the sweep of the drive frequency is stopped. During normal operation, the driving frequency of the piezoelectric transformer 4 is set so as to be in a frequency range f1 to f2 higher than the hysteresis region in FIG. 2, and the driving frequency is varied so that the load current is constant. When the frequency range f1 to f2 is set, the operation becomes unstable in the hysteresis region near the resonance frequency of the piezoelectric transformer. Therefore, the low frequency f1 is set higher than the hysteresis region, and the boost ratio (efficiency) of the piezoelectric transformer is set.
Decreases as the drive frequency moves away from the resonance frequency, so a high frequency f2 is within the range where the efficiency can be reduced,
Then, it is determined from the width of the control range of the load current. Therefore, in the steady operation, the load current is kept constant in the frequency range f1 to f2 having a fixed relation from the resonance frequency, and the cold cathode fluorescent lamp 5 is stably turned on. It is also the role of the frequency control circuit 8 to control the drive frequency to be within the preset frequency range f1 to f2.

Here, when the piezoelectric transformer 4 is driven by a drive signal having a large high-frequency component, all high-frequency components other than components near the resonance frequency are lost as heat in the piezoelectric transformer 4, and the characteristics of the piezoelectric transformer 4 deteriorate. When the high-frequency component is particularly large, the high-frequency component is mechanically broken. Therefore, from the viewpoint of the reliability of the piezoelectric transformer 4 and the conversion efficiency, it is necessary to minimize the high-frequency component of the drive waveform. However, if the driving frequency is swept from a frequency higher than the hysteresis region near the resonance frequency at the start of driving, a high-frequency voltage is applied to the piezoelectric transformer, thereby deteriorating the material constant of the piezoelectric transformer and causing damage such as mechanical destruction.
Also, the input high-frequency voltage is boosted by the piezoelectric transformer, and a high-frequency voltage is applied to the load of the piezoelectric transformer, thereby damaging the load. In particular, when the load is light, or when the secondary side of the piezoelectric transformer is open or close to open, there is a problem that the piezoelectric transformer is mechanically broken by the boosted high-frequency voltage. FIG. 3 shows an input voltage waveform of the piezoelectric transformer 4 at the start of driving. FIG. 3A shows a driving waveform when the driving frequency at the start of driving is set to be much higher than the resonance frequency of the piezoelectric transformer 4. It can be seen that the transformer 4 becomes a pure capacitive element at a frequency considerably higher than the resonance frequency, and a large high-frequency component is superimposed on the driving voltage waveform at the start of driving. At this time, the characteristics of the piezoelectric transformer 4 are often deteriorated or mechanically broken. FIG. 3B shows a waveform when the driving frequency at the start of driving is set slightly higher than the resonance frequency of the piezoelectric transformer 4. Since the piezoelectric transformer 4 does not become a pure capacitive element, the driving voltage at the start of driving is obtained. High frequency components are not superimposed on the waveform. At this time, there was no deterioration of the characteristics of the piezoelectric transformer 4 and no mechanical destruction. In an experiment, it was recognized that setting the driving frequency at the start of driving to within 1.1 times the resonance frequency at the time of actual driving of the piezoelectric transformer 4 had a remarkable effect. Further, it was recognized that the effect would be more certain if the driving frequency at the start of driving was set within 1.07 times the resonance frequency at the time of actual driving of the piezoelectric transformer 4.

According to the embodiment of the present invention, the driving start signal frequency is set to be within 1.1 times (more certainly, within 1.07 times) the resonance frequency of the piezoelectric transformer 4 when it is actually driven. By reducing the high-frequency component of the driving voltage, it is possible to provide a highly reliable piezoelectric transformer driving device having a very small probability of characteristic deterioration and mechanical destruction and high driving efficiency.

(Embodiment 2) FIG. 4 is a block diagram of a piezoelectric transformer driving apparatus according to a second embodiment of the present invention. In the figure, the piezoelectric transformer 4 may be of a Rosen type or of another type, and may be any piezoelectric transformer. Also, in this figure, blocks denoted by the same reference numerals 1 to 8 as those in FIG. 1 have the same functions. FIG. 5 shows the admittance frequency characteristics as viewed from the drive terminals of the piezoelectric transformer 4. FIG. 6 is an envelope waveform of the drive voltage applied to the drive terminal of the piezoelectric transformer 4. The operation of the piezoelectric transformer driving device according to the second embodiment of the present invention will be described in detail with reference to these drawings.

When the driving start signal is output to the frequency control circuit 8 and the amplitude control circuit 9 and the driving of the piezoelectric transformer 4 is started, the variable oscillation circuit 1 capable of changing the oscillation frequency by an external terminal is shown in FIG. Piezoelectric transformer 4 indicated by oblique lines
An oscillation frequency fs, which is a drive AC signal having a frequency higher than the hysteresis region near the resonance frequency of the above, is generated. The frequency control circuit 8 controls this oscillation frequency.
The output signal of the variable oscillation circuit 1 is shaped into a clear pulse waveform by a waveform shaping circuit 2 capable of changing the pulse width or pulse height by an external control terminal. The output of the waveform shaping circuit 2 is amplified by a drive circuit 3 comprising an FET 31 and a step-up coil 32 to a level sufficient to drive the piezoelectric transformer 4. The piezoelectric transformer 4 boosts the input voltage by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5 as a load. Here, at the start of driving, the amplitude control circuit 9 issues a command to the waveform shaping circuit 2 to reduce the pulse width or the pulse height. And the cold cathode fluorescent lamp 5
A load current is detected by a feedback resistor 6 connected in series to a current detection circuit 7 via a current detection circuit 7. If the load current has not reached a set value, the frequency control circuit 8 lowers the output frequency of the variable oscillation circuit 1 to control the amplitude. The circuit 9 issues a command to the waveform shaping circuit 2 to slightly widen the pulse width or increase the pulse height.
Then, the output signal of the variable oscillation circuit 1 is again shaped into a clear pulse waveform by the waveform shaping circuit 2, voltage-amplified by the drive circuit 3, and input to the piezoelectric transformer 4.
The input voltage is boosted by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5. Then, a load current is detected via a current detection circuit 7 by a feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5. This operation is repeated until the load current reaches the set value.

That is, at the time of startup, the drive frequency is swept to a frequency fs lower than the hysteresis region near the resonance frequency of the piezoelectric transformer 4, and when the load current reaches the set value, the sweep of the drive frequency is stopped. In addition, at the time of starting, the drive voltage of the piezoelectric transformer 4 is gradually lowered and gradually increased until the load current reaches a set value. During normal operation, the driving frequency of the piezoelectric transformer 4 is set to be in a frequency range f1 to f2 higher than the hysteresis region in FIG. 5, and the driving frequency is set in the frequency range f1 to f2 so that the load current is constant. Is variable. When the frequency range f1 to f2 is set, the operation becomes unstable in the hysteresis region near the resonance frequency of the piezoelectric transformer. Therefore, the low frequency f1 is set higher than the hysteresis region, and the efficiency (step-up ratio) of the piezoelectric transformer is Decreases as the distance from the resonance frequency increases, so that the high frequency f2 is determined within a range where the efficiency is reduced and from the control range of the load current. Therefore, at the time of steady operation, the load current is kept constant in the frequency range f1 to f2 having a fixed relationship with the resonance frequency of the piezoelectric transformer 4, and the cold cathode fluorescent lamp 5 is stably turned on. The frequency control circuit 8 also controls the drive frequency so as to be within a preset frequency range f1 to f2.

Here, when the piezoelectric transformer 4 is driven by a drive signal having a large high-frequency component, all high-frequency components other than the components near the resonance frequency are lost in the piezoelectric transformer 4 and converted into heat. Since the piezoelectric transformer 4 is deteriorated or mechanically broken, it is necessary to reduce the high-frequency component of the driving waveform from the viewpoint of the reliability of the piezoelectric transformer 4 and the conversion efficiency. At the start of driving, the driving frequency is swept from a frequency higher than the hysteresis region near the resonance frequency.The piezoelectric transformer becomes a pure capacitive element at a frequency considerably higher than the resonance frequency. At this time, a high-frequency voltage is applied to the piezoelectric transformer and the material constant of the piezoelectric transformer Deteriorates and causes damage such as mechanical destruction. Also, the input high-frequency voltage is boosted by the piezoelectric transformer, and a high-frequency voltage is applied to the load of the piezoelectric transformer to damage the load. When the load is light or the secondary side of the piezoelectric transformer is open or almost open, the boost is performed. There is a problem that the piezoelectric transformer is mechanically broken by the applied high-frequency voltage. FIG. 6 shows an output voltage waveform of the drive circuit 3 at the start of driving. FIG. 6A shows an envelope waveform of the driving voltage when the amplitude of the driving voltage is kept constant from the start of driving. Is an envelope waveform of the drive voltage when the amplitude value of the drive voltage at the start of the drive is reduced, and the voltage value is gradually increased so as to be constant at the end of the sweep of the drive frequency.

FIG. 7 shows the input current waveform of the piezoelectric transformer 4 at the start of driving. FIG. 7A shows the waveform when the piezoelectric transformer 4 is driven at a constant voltage from the start of driving. It can be seen that the high frequency component is superimposed. At this time, the characteristics of the piezoelectric transformer 4 were degraded or mechanically broken. (B) shows a drive waveform of the piezoelectric transformer 4 when the drive voltage is gradually reduced and the drive voltage is gradually increased at the start of driving, and a high frequency component is not superimposed on the drive voltage waveform at the start of drive. At this time, there was no deterioration of the characteristics of the piezoelectric transformer 4 and no mechanical destruction. In experiments, it was recognized that setting the driving voltage at the start of driving to within 0.7 times that of the normal driving had a remarkable effect.

As described above, the driving signal frequency at the start of driving is higher than the hysteresis region shown when the piezoelectric transformer 4 is actually driven, and the voltage amplitude during normal driving is 0.7.
Set within x (0.5x if you want a noticeable effect)
By lowering the drive frequency and gradually increasing the drive voltage until the load current reaches the set value, a highly reliable and highly efficient piezoelectric actuator with very low probability of characteristic degradation and mechanical destruction A transformer drive can be provided.

(Embodiment 3) FIG. 8 is a block diagram of a piezoelectric transformer driving apparatus according to a third embodiment of the present invention. In the figure, the piezoelectric transformer 4 may be of a Rosen type or of another type, and may be any piezoelectric transformer. Also, in this figure, blocks denoted by the same reference numerals 1 to 8 as those in FIG. 1 have almost the same functions. The operation of the piezoelectric transformer driving device according to the third embodiment of the present invention will be described in detail with reference to FIG.

When a drive start signal is output to the frequency control circuit 8 and the driving of the piezoelectric transformer 4 is started, the frequency control circuit 8 issues an instruction to the variable oscillation circuit 1 through an external terminal.
The variable oscillation circuit 1, which is an oscillation circuit capable of changing the oscillation frequency, generates an oscillation frequency fs which is near the resonance frequency of the piezoelectric transformer 4 and is higher than the hysteresis region shown by hatching in FIG. That is, the frequency control circuit 8 controls the oscillation frequency. The output signal of the variable oscillation circuit 1 is shaped into a clear pulse waveform by the waveform shaping circuit 2. The output of the waveform shaping circuit 2 is amplified by a drive circuit 3 comprising an FET 31 and a step-up coil 32 to a level sufficient to drive the piezoelectric transformer 4. Here, the resistor 10 and the diode 11 are connected to the gate of the FET 31 constituting the drive circuit 3, and the response of the drive circuit 3 is slightly delayed.

The piezoelectric transformer 4 raises the input voltage by the piezoelectric effect, and its output signal is applied to the cold cathode fluorescent lamp 5 as a load. A load current is detected by a feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5 via a current detection circuit 7. If the load current has not reached a set value, the frequency control circuit 8 outputs the output of the variable oscillation circuit 1. Decrease frequency. Then, the output signal of the variable oscillation circuit 1 is again
Is shaped into a clearer pulse waveform, and the voltage is amplified by the drive circuit 3 and input to the piezoelectric transformer 4. The input voltage is boosted by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5. Then, a load current is detected via a current detection circuit 7 by a feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5. This operation is repeated until the load current reaches the set value. That is, at the time of startup, the drive frequency is swept to a frequency lower than the frequency fs higher than the hysteresis region near the resonance frequency of the piezoelectric transformer 4, and when the load current reaches the set value, the sweep of the drive frequency is stopped. In a normal operation, the driving frequency of the piezoelectric transformer 4 is higher than the hysteresis region in FIG.
The drive frequency is controlled to be within f2, and the drive frequency is varied so that the load current is constant within this frequency range. When the frequency range f1 to f2 is set, the operation becomes unstable in the hysteresis region near the resonance frequency of the piezoelectric transformer. Therefore, the low frequency f1 is set higher than the hysteresis region, and the efficiency (step-up ratio) of the piezoelectric transformer is Decreases as the distance from the resonance frequency of the piezoelectric transformer increases, so that the high frequency f2 is determined within a range in which the efficiency is allowed to decrease, and from the width of the control range of the load current. Therefore,
During a steady operation, the load current is kept constant in the drive frequency range f1 to f2, and the cold cathode fluorescent lamp 5 is stably turned on. The frequency control circuit 8 also controls the drive frequency so as to be within a preset frequency range f1 to f2.

Here, when the piezoelectric transformer 4 is driven by a drive signal having a large high-frequency component, all high-frequency components other than components near the resonance frequency are converted into heat in the piezoelectric transformer 4, and
Since the characteristics of the piezoelectric transformer 4 are degraded or mechanically broken, it is necessary to reduce the high-frequency component of the drive waveform from the viewpoint of the reliability of the piezoelectric transformer 4 and the conversion efficiency. At the start of driving, the driving frequency is swept from a frequency higher than the hysteresis region near the resonance frequency.The piezoelectric transformer becomes a pure capacitive element at a frequency much higher than the resonance frequency, and when driven by a normal drive circuit, the current with a large amount of high frequency components Flows. FIG. 9 shows an input current waveform of the piezoelectric transformer 4 at the start of driving, and FIG.
Is a waveform when the drive frequency at the start of driving is set higher than the resonance frequency of the piezoelectric transformer 4 and driven by a normal drive circuit. It can be seen that a large high-frequency component is superimposed on the drive current waveform at the start of drive. . At this time, the characteristics of the piezoelectric transformer 4 were degraded or mechanically broken.

However, according to the present invention, the resistor 10 and the diode 1 are connected to the gate of the FET 31 constituting the drive circuit 3.
1 is connected and the response of the drive circuit 3 is delayed, so that no high-frequency current flows into the piezoelectric transformer at this time. Therefore, as shown in FIG. 9B, the high-frequency component is not superimposed on the input current waveform of the piezoelectric transformer 4 at the start of driving. At this time, there was no deterioration of the characteristics of the piezoelectric transformer 4 and no mechanical destruction.

As described above, by slightly delaying the response of the drive circuit 3, it is possible to provide a highly reliable piezoelectric transformer drive device having a very small probability of characteristic deterioration and mechanical destruction and a high drive efficiency. can do.
In an experiment, it was confirmed that it was effective to reduce the response of the drive circuit at a frequency 5 to 7 times or more the resonance frequency of the operation mode of the piezoelectric transformer.

In order to slightly delay the response of the drive circuit 3, in addition to the circuit shown in FIG.
The same effect can be obtained by inserting a capacitor 12 between the drain and ground of the FET 31 as shown in FIG.
Similar effects can be obtained by using a combination of the methods shown in FIGS.

(Embodiment 4) FIG. 12 is a block diagram of a piezoelectric transformer driving apparatus according to a fourth embodiment of the present invention. In the figure, the piezoelectric transformer 4 may be of a Rosen type or of another type, and may be any piezoelectric transformer. Also, in this figure, blocks denoted by the same reference numerals 1 to 8 as those in FIG. 1 have almost the same functions. The operation of the piezoelectric transformer driving device according to the fourth embodiment of the present invention will be described in detail with reference to FIG.

When a driving start signal is output to the frequency control circuit 8 and the driving of the piezoelectric transformer 4 is started, the frequency control circuit 8 issues an instruction to the variable oscillation circuit 1 through an external terminal.
The variable oscillation circuit 1, which is an oscillation circuit capable of changing the oscillation frequency, generates an oscillation frequency fs which is near the resonance frequency of the piezoelectric transformer 4 and is higher than the hysteresis region shown by hatching in FIG. That is, the frequency control circuit 8 controls the oscillation frequency. The output signal of the variable oscillation circuit 1 is shaped into a clear pulse waveform by the waveform shaping circuit 2. The output of the waveform shaping circuit 2 is voltage-amplified to a level sufficient to drive the piezoelectric transformer 4 by a drive circuit 3 including an FET 31 and a resistance element 33. The piezoelectric transformer 4 boosts the input voltage by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5 as a load. A load current is detected by a feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5 via a current detection circuit 7. If the load current has not reached a set value, the frequency control circuit 8 outputs the output of the variable oscillation circuit 1. Decrease frequency. Then, the output signal of the variable oscillation circuit 1 is again shaped into a clear pulse waveform by the waveform shaping circuit 2, voltage-amplified by the drive circuit 3, and input to the piezoelectric transformer 4. The input voltage is boosted by the piezoelectric effect, and the output signal is applied to the cold cathode fluorescent lamp 5. Then, a load current is detected via a current detection circuit 7 by a feedback resistor 6 connected in series to the cold cathode fluorescent lamp 5. This operation is repeated until the load current reaches the set value.

That is, at the time of startup, the drive frequency is swept to a frequency lower than the frequency fs higher than the hysteresis region near the resonance frequency of the piezoelectric transformer 4, and when the load current reaches the set value, the sweep of the drive frequency is stopped. In a normal operation, the driving frequency of the piezoelectric transformer 4 is controlled so as to be higher than the hysteresis region in FIG. 2 and to be within a frequency range f1 to f2 having a fixed relationship with the resonance frequency of the piezoelectric transformer 4. The drive frequency is varied so that the load current is constant within the range. When the frequency range f1 to f2 is set, the operation becomes unstable in the hysteresis region near the resonance frequency of the piezoelectric transformer. Therefore, the low frequency f1 is set higher than the hysteresis region, and the efficiency (step-up ratio) of the piezoelectric transformer is Decreases as the distance from the resonance frequency of the piezoelectric transformer increases, so that the high frequency f2 is determined within a range in which the efficiency is allowed to decrease, and from the width of the control range of the load current. Therefore, during the steady operation, the load current is kept constant in the drive frequency range f1 to f2, and the cold cathode fluorescent lamp 5 is stably turned on. The frequency control circuit 8 also controls the drive frequency so as to be within a preset frequency range f1 to f2.

Here, when the piezoelectric transformer 4 is driven by a drive signal having a large high-frequency component, all high-frequency components other than components near the resonance frequency are converted into heat in the piezoelectric transformer 4, and
Since the characteristics of the piezoelectric transformer 4 are degraded or mechanically broken, it is necessary to reduce the high-frequency component of the drive waveform from the viewpoint of the reliability of the piezoelectric transformer 4 and the conversion efficiency. At the start of driving, the driving frequency is swept from a frequency higher than the hysteresis region near the resonance frequency.The piezoelectric transformer becomes a pure capacitive element at a frequency much higher than the resonance frequency, and when driven by a normal drive circuit, the current with a large amount of high frequency components Flows.

However, according to the present invention, since the inductance element (coil) 14 and the capacitor 15 are connected to the drain, which is the output of the FET 31 constituting the drive circuit 3, the high-frequency output of the drive circuit 3 is reduced. High frequency current does not flow into the transformer. Here, if the series resonance frequency of the inductance element (coil) 14, the capacitor 15, and the input capacitance of the piezoelectric transformer 4 is set to be substantially equal to the resonance frequency of the piezoelectric transformer 4, the effect of reducing the high frequency is great. Further, the series resonance frequency of the inductance element (coil) 14 and the input capacitance of the piezoelectric transformer 4 can be set so as to be substantially equal to the resonance frequency of the piezoelectric transformer 4. Accordingly, there was no deterioration of the characteristics of the piezoelectric transformer 4 and no mechanical destruction.

As described above, by reducing the high-frequency component of the output of the drive circuit 3, the piezoelectric transformer driving device with high reliability and high driving efficiency with extremely small probability of characteristic deterioration and mechanical destruction. Can be provided. In an experiment, it is particularly effective to set the series resonance frequency of the series resonance circuit within a range of 0.7 to 1.3 times the resonance frequency of the piezoelectric transformer. It was confirmed that it was effective in eliminating destruction.

In order to reduce the high-frequency component of the output of the drive circuit 3, in addition to the circuit shown in FIG. 12, the primary side of the step-up transformer 16 is connected to the drain of the FET 31 as shown in FIG. A similar effect can be obtained even if a capacitor 17 is inserted into the capacitor.

(Fifth Embodiment) FIG. 14 is a schematic view of a Rosen-type piezoelectric transformer 20 according to a fifth embodiment of the present invention, which is made of a rectangular material made of a piezoelectric ceramic material such as lead zirconate titanate (PZT). Primary (input) electrode 2 on plate
It is configured by attaching primary and secondary (output) electrodes 23. As shown by arrows and P in the figure, the primary side electrode portion is polarized in the thickness direction of the rectangular plate, and the secondary side electrode portion is polarized in the length direction. A high-resistance conductive film 22 having a resistance value of about 10 MΩ to several 100 MΩ is formed on the secondary electrode portion of the piezoelectric transformer 20.

The conventional piezoelectric transformer driving device sweeps the driving frequency from a frequency higher than a hysteresis region indicated by the piezoelectric transformer at the start of driving to a lower frequency, and drives when a predetermined frequency range higher than the hysteresis region is reached. After the frequency sweep is stopped, the output current of the piezoelectric transformer is controlled by varying the drive frequency of the piezoelectric transformer in a frequency region higher than the hysteresis region in order to keep the output current of the piezoelectric transformer substantially constant. However, at the start of driving, a high-frequency voltage is applied to the piezoelectric transformer, thereby deteriorating the material constant of the piezoelectric transformer and causing damage such as mechanical destruction. Also, the input high-frequency voltage is boosted by the piezoelectric transformer and a high-frequency voltage is applied to the load of the piezoelectric transformer to damage the load. When the secondary side of the piezoelectric transformer is open or nearly open, the output of the secondary electrode is output. The voltage increases and the damage to the piezoelectric transformer increases.

However, in the present invention, the resistance of about 10 MΩ to several hundreds MΩ becomes a load of the output of the piezoelectric transformer 20 due to the high resistance conductive film 22, and even if the load is opened, the secondary electrode 23 of the piezoelectric transformer 20 is opened. Since the output voltage of the piezoelectric transformer 20 does not become excessively high, the piezoelectric transformer 20 can be prevented from being mechanically broken by the high-frequency voltage. Further, since a protection circuit is not formed using a resistance element on the circuit board, even if an output lead wire taken out of the piezoelectric
Can be prevented from being destroyed.

An object of the present invention is to suppress the amplitude value of the high voltage generated on the secondary side of the piezoelectric transformer by the high-frequency voltage generated at the start of driving, thereby achieving high reliability, long life, and high driving efficiency. It is an object of the present invention to provide a piezoelectric transformer having all of the above conditions. Here, a description has been given of a piezoelectric transformer made of a piezoelectric ceramic. However, a resistance of, for example, about 10 MΩ to several hundred MΩ is formed on the secondary-side electrode portion by using a high-resistance conductive film even for a piezoelectric single-crystal piezoelectric transformer. Needless to say, the same effect can be obtained.

(Embodiment 6) FIG. 15 is a schematic view of a Rosen-type piezoelectric transformer 24 according to a sixth embodiment of the present invention, which is a rectangle made of a piezoelectric ceramic material such as lead zirconate titanate (PZT). Primary (input) electrode 2 on plate
5 and a secondary (output) electrode 26 are provided. As shown by arrows and P in the figure, the primary electrode portion is polarized in the thickness direction of the rectangular plate, and the secondary electrode portion is polarized in the length direction. The boundary between the primary side electrode 25 formed on the piezoelectric transformer 24 and the secondary side electrode part (the center part of the piezoelectric transformer 24) is a straight line, and the end side of the other piezoelectric transformer 24 has a radius at two corners. A roundness of about 2 to 7 mm or a chamfer of about 2 to 5 mm is provided. Due to the electrode structure of the primary electrode 25, the piezoelectric transformer 24 can extremely weaken high-order resonance.

The piezoelectric transformer driving device sweeps the driving frequency from a frequency higher than the hysteresis region indicated by the piezoelectric transformer to a lower frequency at the start of driving, and stops the sweeping of the driving frequency when the frequency reaches a predetermined frequency higher than the hysteresis region. Thereafter, in order to make the output current of the piezoelectric transformer substantially constant, the drive current of the piezoelectric transformer is varied in a frequency region higher than the hysteresis region to control the output current. However, at the start of driving, a high-frequency current flows into the piezoelectric transformer, and is boosted to a high voltage by the piezoelectric transformer 24 to damage the piezoelectric transformer 24. However, in the present invention, the higher-order resonance mode of the piezoelectric transformer 24 can be extremely weakened, so that even if a high-frequency current is input, the voltage cannot be boosted, so that the load is not damaged and the piezoelectric transformer 24 itself is not damaged. .

An object of the present invention is to make the high-frequency vibration mode, which is a high frequency, extremely weak by the primary electrode structure of the piezoelectric transformer, so that a high-frequency current generated at the start of driving can be input to the piezoelectric transformer. Another object of the present invention is to provide a piezoelectric transformer having all of the requirements of high reliability, long life, and high drive efficiency. Here, a description has been given of a piezoelectric transformer made of a piezoelectric ceramic. However, needless to say, a similar effect can be obtained by forming a similar secondary-side electrode also for a piezoelectric single-crystal piezoelectric transformer.

The piezoelectric transformer and the piezoelectric transformer driving device of the present invention can be applied to, for example, a liquid crystal display device.

[0060]

As is apparent from the above description,
According to the first aspect of the present invention, the sweep start frequency of the drive signal at the start of driving of the piezoelectric transformer is not set to be much higher than the resonance frequency of the piezoelectric transformer, that is, the drive start signal frequency is reduced. By setting the resonance frequency within 1.1 times (more certainly within 1.07 times) the resonance frequency at the time of actual driving and reducing the high frequency component of the driving voltage, the probability of characteristic deterioration and mechanical destruction is extremely high. A small and highly reliable piezoelectric transformer driving device with high driving efficiency can be provided.

According to the second aspect of the invention, by gradually increasing the amplitude value of the drive signal at the start of driving the piezoelectric transformer, that is, the hysteresis area indicating the drive signal frequency at the start of drive when the piezoelectric transformer is actually driven. Higher than the voltage amplitude during normal driving, and within 0.7 times (0.5 times when a remarkable effect is desired) the drive frequency is reduced until the load current reaches the set value. By gradually increasing the voltage, it is possible to provide a highly reliable piezoelectric transformer drive device with extremely low probability of characteristic deterioration and mechanical destruction, and high drive efficiency.

According to the third aspect of the invention, the response of the drive circuit (drive circuit) for outputting the drive signal of the piezoelectric transformer is delayed, that is, in the experiment, the response of the drive circuit is reduced by the resonance frequency of the use mode of the piezoelectric transformer. If the frequency is reduced at a frequency of 5 to 7 times or more, it is possible to provide a highly reliable piezoelectric transformer driving device having a very small probability of characteristic deterioration and mechanical destruction and high driving efficiency.

According to the fourth aspect of the invention, the high frequency component of the output of the drive circuit (drive circuit) for outputting the drive signal of the piezoelectric transformer is reduced, that is, in the experiment, the series resonance frequency of the series resonance circuit is reduced by the piezoelectric transformer. By setting the resonance frequency within the range of 0.7 to 1.3 times the resonance frequency of the piezoelectric transformer, it is possible to provide a highly reliable piezoelectric transformer driving device having a very small probability of characteristic deterioration and mechanical destruction and a high driving efficiency. be able to.

Further, according to the fifth aspect of the present invention, by forming a resistor of a predetermined magnitude on the secondary electrode portion of the piezoelectric transformer with a high-resistance conductive film or the like, a high-frequency voltage generated at the start of driving can be obtained. By suppressing the amplitude value of the high voltage generated on the secondary side of the piezoelectric transformer, it is possible to provide a piezoelectric transformer having all the conditions of high reliability, long life, and high drive efficiency.

According to the sixth aspect of the present invention, the high-frequency vibration mode, which is a high frequency, is extremely weakened by the primary electrode structure of the piezoelectric transformer. It is possible to provide a piezoelectric transformer that satisfies all the conditions of high performance, long life, and high drive efficiency.

According to the seventh and eighth aspects, a stable liquid crystal display can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of a piezoelectric transformer driving device according to the present invention.

FIG. 2 is a frequency characteristic diagram of admittance of the piezoelectric transformer used in the first embodiment.

FIG. 3 is a drive current waveform diagram of the piezoelectric transformer by sweeping the drive frequency according to the first embodiment.

FIG. 4 is a block diagram of a piezoelectric transformer driving device according to a second embodiment of the present invention;

FIG. 5 is a frequency characteristic diagram of admittance of the piezoelectric transformer used in the second embodiment.

FIG. 6 is a waveform diagram of a voltage applied to a piezoelectric transformer when a drive frequency is swept according to the second embodiment.

FIG. 7 is a drive current waveform diagram of a piezoelectric transformer by sweeping the drive frequency according to the second embodiment.

FIG. 8 is a block diagram of Embodiment 3-1 of a piezoelectric transformer driving device according to the present invention.

FIG. 9 is a drive current waveform diagram of a piezoelectric transformer by sweeping the drive frequency according to the embodiment 3-1.

FIG. 10 is a block diagram of Embodiment 3-2 of the piezoelectric transformer driving device according to the present invention.

FIG. 11 is a block diagram of Embodiment 3-3 of the piezoelectric transformer driving device according to the present invention.

FIG. 12 is a block diagram of Embodiment 4-1 of a piezoelectric transformer driving device according to the present invention.

FIG. 13 is a block diagram of Embodiment 4-2 of a piezoelectric transformer driving device according to the present invention.

FIG. 14 is a structural diagram of Embodiment 5 of a piezoelectric transformer according to the present invention.

FIG. 15 is a structural diagram of a piezoelectric transformer according to a sixth embodiment of the present invention.

FIG. 16 is a structural diagram of a conventional Rosen-type piezoelectric transformer.

FIG. 17 is a block diagram of a conventional piezoelectric transformer driving device.

FIG. 18 is a block diagram of another conventional piezoelectric transformer driving device.

FIG. 19 is a block diagram of another conventional piezoelectric transformer driving device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Variable oscillation circuit 2 Waveform shaping circuit 3 Drive circuit (drive circuit) 31 FET 32 Boost coil 4 Piezoelectric transformer 5 Cold cathode fluorescent lamp 6 Feedback resistance 7 Current detection circuit 8 Frequency control circuit 9 Amplitude control circuit 10 Resistance 11 Diode 12 Capacitor 13 Capacitor 14 Coil (inductance) 16 Step-up transformer 17 Capacitor 20 Piezoelectric transformer 21 Primary electrode 22 High-resistance conductive film 23 Secondary electrode 24 Piezoelectric transformer 25 Primary electrode 26 Secondary electrode

Claims (8)

[Claims] 1. A variable oscillation circuit for generating an AC signal, a drive circuit for amplifying the AC signal to generate a drive signal, a piezoelectric transformer having a piezoelectric body having an input electrode and an output electrode, and being connected to the output electrode. Having a load, applying the drive signal to the input electrode of the piezoelectric transformer, boosting the drive signal and extracting it from the output electrode of the piezoelectric transformer,
What is claimed is: 1. A piezoelectric transformer driving device for driving a load, comprising: a current detection circuit for detecting a current flowing into the load; and at the start of driving, at least 1.1 times the resonance frequency of the piezoelectric transformer at the time of actual driving. Set the drive signal to a high frequency, start the sweep of the drive frequency from the drive signal, stop the sweep of the drive frequency when the load current by the current detection circuit reaches a set value, after the stop of the sweep A frequency control circuit for controlling the load current within a set frequency range having a fixed relationship with a resonance frequency of the piezoelectric transformer during actual driving. 2. A variable oscillation circuit for generating an AC signal, a drive circuit for amplifying the AC signal to generate a drive signal, a piezoelectric transformer having a piezoelectric body having an input electrode and an output electrode, and being connected to the output electrode. Having a load, applying the drive signal to the input electrode of the piezoelectric transformer, boosting the drive signal and extracting it from the output electrode of the piezoelectric transformer,
A piezoelectric transformer driving device that drives the load, comprising: a current detection circuit that detects a current flowing into the load; and setting the drive signal to a frequency higher than a resonance frequency of the piezoelectric transformer when the piezoelectric transformer is actually driven at a start of driving. The driving frequency starts to be swept from the driving signal, and when the load current by the current detection circuit reaches a set value, the driving frequency is stopped to be swept. A frequency control circuit for controlling the load current within a set frequency range having a fixed relationship with a frequency; and at the start of driving, setting the amplitude of the driving signal within 0.7 times the driving signal during normal driving; And an amplitude control circuit for performing control to gradually increase the voltage amplitude of the drive signal until the sweep of the drive frequency is stopped when the load current by the current detection circuit reaches a set value. Piezoelectric transformer driving apparatus characterized by. 3. A variable oscillation circuit for generating an AC signal, a drive circuit for amplifying the AC signal to generate a drive signal, a piezoelectric transformer having a piezoelectric body having an input electrode and an output electrode, and being connected to the output electrode. Having a load, applying the drive signal to the input electrode of the piezoelectric transformer, boosting the drive signal and extracting it from the output electrode of the piezoelectric transformer,
A piezoelectric transformer driving device that drives the load, comprising: a current detection circuit that detects a current flowing into the load; and setting the drive signal to a frequency higher than a resonance frequency of the piezoelectric transformer when the piezoelectric transformer is actually driven at a start of driving. The driving frequency starts to be swept from the driving signal, and when the load current by the current detection circuit reaches a set value, the driving frequency is stopped to be swept. A frequency control circuit that controls the load current within a set frequency range that is in a fixed relationship with the frequency, wherein an FET element is used as the drive circuit, and a parallel circuit of a resistance element and a diode is connected to the gate of the FET element. Or connecting a capacitor between the gate and drain of the FET element, or connecting a capacitor between the drain and ground of the FET element. Piezoelectric transformer drive circuit, characterized in that it employs one. 4. A variable oscillation circuit for generating an AC signal, a drive circuit for amplifying the AC signal to generate a drive signal, a piezoelectric transformer having a piezoelectric body having an input electrode and an output electrode, and being connected to the output electrode. Having a load, applying the drive signal to the input electrode of the piezoelectric transformer, boosting the drive signal and extracting it from the output electrode of the piezoelectric transformer,
A piezoelectric transformer driving device that drives the load, comprising: a current detection circuit that detects a current flowing into the load; and setting the drive signal to a frequency higher than a resonance frequency of the piezoelectric transformer when the piezoelectric transformer is actually driven at a start of driving. The driving frequency starts to be swept from the driving signal, and when the load current by the current detection circuit reaches a set value, the driving frequency is stopped to be swept. A frequency control circuit for controlling the load current within a set frequency range that is in a fixed relationship with the frequency, wherein an FET element is used as the drive circuit, and a series circuit of an inductance and a capacitor is connected to a drain of the FET element. Connecting only the inductance to the drain of the FET element, or connecting the primary side of the step-up transformer to the drain of the FET element, Piezoelectric transformer drive circuit, characterized in that it employs at least one of either a capacitor on the secondary side of the step-up transformer. 5. A piezoelectric device comprising a piezoelectric ceramic material or a piezoelectric single crystal, a primary side (input side) electrode and a secondary side (output side) electrode. In a piezoelectric transformer which converts and outputs from the secondary side electrode, a high resistance conductive film having a predetermined resistance value is formed on a secondary side electrode portion of the piezoelectric transformer. Trance. 6. A primary AC (input side) electrode and a secondary side (output side) electrode which are made of a piezoelectric ceramic material or a piezoelectric single crystal, and drive AC voltage inputted to said primary side electrode is applied to a voltage. In the piezoelectric transformer which converts and outputs from the secondary side electrode, 2 of the primary side electrodes formed in the piezoelectric transformer
The boundary with the secondary electrode portion, that is, the central portion of the piezoelectric transformer is a straight line, and the other end is rounded at a radius of a predetermined size at two corners of the primary electrode, or a predetermined size. A piezoelectric transformer characterized by having a chamfered shape. 7. A liquid crystal display device wherein at least one piezoelectric transformer driving device according to claim 1 is used as a backlight driving device of a liquid crystal display device. 8. A liquid crystal display device, wherein the piezoelectric transformer according to claim 5 is used as a piezoelectric transformer of a backlight driving device of a liquid crystal display device.