JP2002017090A - Driving method and driving device for piezoelectric transformer - Google Patents

Abstract

(57) [Summary] [Problem] To keep the emission luminance of a cold cathode tube constant by controlling the drive of a small and high-efficiency piezoelectric transformer that eliminates the effect of leakage current due to stray capacitance between the cold cathode tube and a reflector. I do. SOLUTION: A phase difference detection circuit 119 detects a phase difference between an output current and a voltage of a piezoelectric transformer 110, and based on the phase difference, an effective current detection circuit 115 generates a cold cathode tube 11
The piezoelectric transformer is driven and controlled via the oscillation control circuit 114, the variable oscillation circuit 113, and the drive circuit 112 such that the effective current flowing through the circuit 8 is detected and the effective current becomes equal to a predetermined set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a device for driving a piezoelectric transformer used in various high-voltage generators.

[0002]

2. Description of the Related Art FIG. 14 shows the structure of a Rosen type piezoelectric transformer which is a typical structure of a conventional piezoelectric transformer. This piezoelectric transformer has advantages such as miniaturization, non-combustibility, and no generation of noise due to electromagnetic induction as compared with the electromagnetic transformer.

In FIG. 14, a portion indicated by 1 is a low impedance portion of the piezoelectric transformer, and serves as an input portion when the piezoelectric transformer is used for boosting. The low impedance section 1 is polarized in the thickness direction (PD), and the electrodes 3U and 3D are arranged on the main surface in the thickness direction. on the other hand,
The portion indicated by 2 is a high impedance portion, and becomes an output portion when the piezoelectric transformer is used for boosting. The high impedance portion 2 is polarized in the longitudinal direction (PL), and the electrode 4 is disposed on the end face in the longitudinal direction.

A piezoelectric transformer as shown in FIG. 14 can obtain a very high boost ratio when the load is infinite.
In addition, they have recently been used as power supplies for cold-cathode tubes because of the characteristic that the step-up ratio decreases as the load decreases. An inverter using a piezoelectric transformer can efficiently generate a high voltage.

FIG. 15 is a block diagram showing the configuration of a conventional self-excited oscillation type driving device for a piezoelectric transformer. In FIG. 15, reference numeral 13 denotes a variable oscillation circuit that generates a voltage signal of a variable frequency. The voltage signal output from the variable oscillation circuit 13 generally has a pulse waveform, and the waveform shaping circuit 11 removes the high-frequency component thereof and converts the signal into a nearly sinusoidal AC signal. The output signal of the waveform shaping circuit 11 is voltage-amplified by the drive circuit 12 to a level sufficient to drive the piezoelectric transformer 10, and is input to one primary electrode 3U of the piezoelectric transformer 10. The piezoelectric transformer 10
The other primary electrode 3D is connected to the ground potential.
The output voltage boosted by the piezoelectric effect of the piezoelectric transformer 10 is extracted from the secondary electrode 4.

The high voltage output from the secondary electrode is applied to a series circuit of a cold cathode tube 17 and a feedback resistor 18 and to an overvoltage protection circuit section 20. The overvoltage protection circuit unit 20
The comparison circuit 15 includes resistors 19a and 19b and a comparison circuit 15. The comparison circuit 15 compares the voltage divided by the resistors 19a and 19b with the reference voltage Vref1, and compares the voltage divided by the reference voltage Vref1.
The high voltage output from the secondary electrode 4 is equal to the reference voltage Vre.
A signal is output to the oscillation control circuit 14 so as to prevent the voltage from becoming higher than the set voltage determined by f1. The overvoltage protection circuit section 20 does not operate when the cold cathode tube 17 is turned on.

A voltage generated at both ends of the feedback resistor 18 by a current flowing through a series circuit including the cold cathode tube 17 and the feedback resistor 18 is applied to one input terminal of the comparison circuit 16 as a feedback voltage. Then, the feedback voltage is compared with a reference voltage Vref2 applied to the other input terminal, and a signal is sent to the oscillation control circuit 14 so that a substantially constant current flows through the cold-cathode tube 17.

[0008] The oscillation control circuit 14 outputs a signal to the variable oscillation circuit 13 so as to oscillate at a frequency corresponding to the output signal from the comparison circuit 16. Note that this comparison circuit 16
It does not operate before the cold cathode tube 17 starts lighting.

Thus, the cold-cathode tube 17 is stably turned on. When the piezoelectric transformer is driven by the self-excited oscillation method, the driving frequency automatically tracks the resonance frequency even if the resonance frequency of the piezoelectric transformer changes according to the temperature.

As described above, by configuring the inverter using the piezoelectric transformer, it is possible to control the current flowing through the cold cathode tube 17 to be constant.

As shown in FIG. 9, two piezoelectric transformers 22 and 23 are provided to prevent uneven brightness of the cold cathode fluorescent lamp.
Are driven in parallel to drive the cold-cathode tubes 21 with two AC voltage signals V1 and V2 having a phase difference of 180 °, and a piezoelectric transformer 61 having the structure shown in FIG. As shown, a driving method has been proposed in which two output electrodes 4L and 4R of a piezoelectric transformer 61 are connected to two input terminals 641 and 642 of a cold cathode tube 64, respectively.

In these driving devices as well, it is necessary to perform frequency control and voltage control by feeding back the current flowing through the cold-cathode tube, as in the operation performed by the driving device shown in FIG. Alternatively, the luminance of the cold-cathode tube is detected and feedback is performed.

In order to keep the brightness of the cold cathode tube constant, the output current and output voltage of the piezoelectric transformer are detected (for example,
The output current detection circuit 24 and the output voltage comparison circuit 2 of FIG.
5) In addition, current control of the cold-cathode tube is performed by detecting a current flowing through the reflector.

In the conventional piezoelectric transformer driving device described above, current control of the cold cathode tube is performed by feeding back a voltage detected by a feedback resistor 18 (FIG. 15) connected to the cold cathode tube. I have.

[0015]

However, the cold cathode tube and the reflector (the reflector 26 in FIG. 9 and the reflector 65 in FIG. 11).
9 and 11 causes a current to flow from the cold-cathode tube to the reflector. As a result, there is a problem that the brightness of the cold cathode tube is uneven.

In order to solve this problem, Japanese Patent Application Laid-Open No.
As described in JP-A-8087, one end of each cold cathode tube is
Means for inputting voltages having different phases of 80 ° have been proposed. However, when a voltage is applied to one cold cathode tube 51 as shown in FIG. 12A or to two cold cathode tubes 51 and 52 connected in series as shown in FIG. Voltage V on application side, period tb
(2 application side), current flows out of the cold cathode tube to the reflector (current Ixa on the (+) side in FIG. 12B, current Ixb on the (+) side in FIG. 12C). On the low voltage side (voltage V1 application side in period tb, voltage V2 application side in period ta), current flows from the reflector into the cold cathode tube (current Ixa on the (-) side in FIG. 12B, FIG. 12C). Ix on the (-) side of
b).

Therefore, the output current of the piezoelectric transformer is
It includes both the current Ia flowing only through the cold cathode tubes and the leak currents Ixa and Ixb (Ix) flowing through the stray capacitance Cx. When the cold-cathode tube is turned on, the cold-cathode tube can be treated as a resistive load. Therefore, the current related to the brightness of the cold-cathode tube is the effective current I of the current (I) output from the piezoelectric transformer.
a = Icos θ (θ is the phase difference between the output voltage and the output current of the piezoelectric transformer as shown in FIG. 16). That is, the leak current I flowing through the reflector via the stray capacitance Cx
x becomes a reactive current and does not contribute to the brightness of the cold-cathode tube.

For this reason, in the driving device for the piezoelectric transformer configured as shown in FIGS. 9 and 11, the output current detection circuits 24 and 62 determine the current Ia flowing through the cold-cathode tube,
The leakage current Ix due to the stray capacitance Cx formed by the cold-cathode tube and the reflector is also detected at the same time. If the stray capacitance Cx due to the reflector is constant, it is possible to control the current flowing through the cold-cathode tube to a constant value by performing control taking that into account. However, since the stray capacitance Cx varies, it is difficult to control the current Ia flowing through the cold-cathode tube at a constant level, which causes luminance variation between the inverters. Also, in the case of a drive device using two piezoelectric transformers, similarly, it is difficult to control the tube current.

Therefore, Japanese Unexamined Patent Publication No. 11-27955 discloses that a leakage current is detected by a stray capacitance current detection circuit.
The tube current is detected by a tube current detection circuit to control the tube current. However, in the method disclosed herein, in a piezoelectric transformer that controls the driving frequency to keep the output voltage constant, the leakage current due to the stray capacitance changes when the frequency changes, and the impedance due to the stray capacitance changes. Change. As a result, the control circuit becomes complicated in order to obtain a circuit configuration taking into account the influence of the frequency.

Therefore, an object of the present invention is to eliminate the effect of a reactive current, which is a leakage current due to a stray capacitance between a cold cathode tube and a reflector, and to accurately control the tube current to be constant. Another object of the present invention is to provide a small and highly efficient driving method and a driving apparatus for a piezoelectric transformer in which the luminance of the cold cathode fluorescent lamp is stable.

[0021]

In order to achieve the above object, a first driving method of a piezoelectric transformer according to the present invention is to increase a voltage input from a primary terminal of the piezoelectric transformer by a piezoelectric effect and to increase two voltages. Output from the secondary terminal to the two terminals of the cold-cathode tube, detect the phase difference between the voltage applied to the cold-cathode tube and the current flowing through the cold-cathode tube, based on the detected phase difference, Detecting an effective current flowing through the cold-cathode tube, comparing the detected effective current with a predetermined set value, and controlling the piezoelectric transformer so that the effective current flowing through the cold-cathode tube becomes equal to the predetermined set value. It is characterized by driving control.

In order to achieve the above object, a second driving method of a piezoelectric transformer according to the present invention is characterized in that a voltage input from a primary terminal of a first piezoelectric transformer is boosted by a piezoelectric effect and the voltage is cooled from a secondary terminal. The voltage is output to one terminal of a cathode tube, the voltage input from the primary terminal of the second piezoelectric transformer is boosted by the piezoelectric effect, and the voltage is output from the secondary terminal to the other terminal of the cold cathode tube. Detecting the phase difference between the voltage applied to the current and the current flowing through the cold-cathode tube, detecting the effective current flowing through the cold-cathode tube based on the detected phase difference, and converting the detected effective current to a predetermined value. The driving of the first and second piezoelectric transformers is controlled such that an effective current flowing through the cold-cathode tube becomes equal to the predetermined set value as compared with a set value.

In the first and second driving methods, it is preferable that the cold-cathode tube is one, or two or more, connected in series.

In order to achieve the above object, a first driving device for a piezoelectric transformer according to the present invention comprises a piezoelectric transformer which boosts a voltage input from a primary terminal by a piezoelectric effect and outputs the boosted voltage from two secondary terminals. A driving circuit for driving the piezoelectric transformer, a variable oscillation circuit for outputting a voltage of a variable frequency to the driving circuit, and a cold cathode tube to which an output voltage from a secondary terminal of the piezoelectric transformer is applied to two terminals A current detection circuit that detects a current flowing through the cold-cathode tube, a voltage detection circuit that detects a voltage applied to the cold-cathode tube, and a current signal output from the current detection circuit and the voltage detection circuit. A phase difference detection circuit for detecting a phase difference between the output voltage signal and an effective current flowing through the cold-cathode tube based on a current signal from the current detection circuit and a phase difference from the phase difference detection circuit; Comparing the active current detected by the active current detection circuit with a predetermined set value, and oscillating the variable oscillation circuit so that the active current becomes equal to the predetermined set value. An oscillation control circuit for controlling the frequency.

In order to achieve the above object, a second driving device for a piezoelectric transformer according to the present invention comprises a first driving device for boosting a voltage input from a primary terminal by a piezoelectric effect and outputting the boosted voltage from a secondary terminal. A transformer, a second piezoelectric transformer that boosts a voltage input from a primary terminal by a piezoelectric effect and outputs the boosted voltage from a secondary terminal, and drives the first and second piezoelectric transformers with signals whose phases are different from each other by 180 °. A driving circuit, a variable oscillating circuit that outputs a voltage of a variable frequency to the driving circuit, and an output voltage from a secondary terminal of the first piezoelectric transformer is applied to one terminal. A cold cathode tube to which an output voltage from a secondary terminal is applied to the other terminal; a current detection circuit for detecting a current flowing through the cold cathode tube; and a voltage detection circuit for detecting a voltage applied to the cold cathode tube When, A phase difference detection circuit that detects a phase difference between a current signal output from the current detection circuit and a voltage signal output from the voltage detection circuit, and a current signal from the current detection circuit and a signal from the phase difference detection circuit. Based on the phase difference, an effective current detection circuit that detects an effective current flowing through the cold-cathode tube, and compares the effective current detected by the effective current detection circuit with a predetermined set value. And an oscillation control circuit for controlling an oscillation frequency of the variable oscillation circuit so as to be equal to the set value of the variable oscillation circuit.

In order to achieve the above object, a third driving method of a piezoelectric transformer according to the present invention is characterized in that a voltage input from a primary terminal of a first piezoelectric transformer is boosted by a piezoelectric effect and cooled from a secondary terminal. The voltage is output to one terminal of the cathode tube, and the same voltage as the input voltage of the first piezoelectric transformer, which is input from the primary terminal of the second piezoelectric transformer, is boosted by a piezoelectric effect. A voltage different from the output voltage of the first piezoelectric transformer by 180 ° in phase is output to the other terminal of the cold-cathode tube to turn on the cold-cathode tube.

In the third driving method, a phase difference between a voltage applied to the cold-cathode tube and a current flowing through the cold-cathode tube is detected, and based on the detected phase difference, the cold-cathode tube is detected. Detecting the effective current flowing therethrough, comparing the detected effective current with a predetermined set value, and setting the first current so that the effective current flowing through the cold-cathode tube becomes equal to the predetermined set value.
It is preferable to drive and control the second piezoelectric transformer.

In order to achieve the above object, a third driving device for a piezoelectric transformer according to the present invention comprises a first driving device for boosting a voltage input from a primary terminal by a piezoelectric effect and outputting the boosted voltage from a secondary terminal. A transformer and a voltage having the same phase as the input voltage of the first piezoelectric transformer, which is input from a primary terminal, is boosted by a piezoelectric effect and has a phase different from the output voltage of the first piezoelectric transformer by 180 ° A second piezoelectric transformer that outputs from a secondary terminal, a drive circuit that drives the first and second piezoelectric transformers with signals of the same phase, and a variable oscillator circuit that outputs a variable frequency voltage to the drive circuit. The output voltage from the secondary terminal of the first piezoelectric transformer is applied to one terminal, and the output voltage of the second piezoelectric transformer
A cold cathode tube in which an output voltage from the next terminal is applied to the other terminal, a current detection circuit for detecting a current flowing through the cold cathode tube, and a voltage detection circuit for detecting a voltage applied to the cold cathode tube A phase difference detection circuit that detects a phase difference between a current signal output from the current detection circuit and a voltage signal output from the voltage detection circuit; and a current signal from the current detection circuit and the phase difference detection circuit. Based on the phase difference of
Comparing an effective current detected by the effective current detection circuit with a predetermined set value, wherein the effective current is equal to the predetermined set value. And an oscillation control circuit for controlling an oscillation frequency of the variable oscillation circuit.

In order to achieve the above object, a fourth driving device for a piezoelectric transformer according to the present invention comprises: a pair of current amplifying circuits for current amplifying AC voltages having phases different from each other by 180 °; A pair of step-up transformers for amplifying each output signal of the pair of current amplifying circuits and outputting voltage signals having a phase difference of 180 °, and a primary electrode and a secondary electrode formed on the piezoelectric body, And a pair of piezoelectric transformers for boosting the voltage signal input to the primary electrode from each of the pair of boost transformers and outputting the boosted voltage signal from the secondary electrode.

According to the above configuration, only the effective current flowing through the cold cathode tube is detected based on the phase difference between the output current and the voltage of the piezoelectric transformer, and is formed between the cold cathode tube and the reflector. By eliminating the reactive current due to the stray capacitance and accurately controlling the tube current to be constant, the brightness of the cold-cathode tube is stabilized, and a small and highly efficient piezoelectric transformer driving method and device are realized. be able to.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
It is a block diagram showing an example of 1 composition of a drive device of a piezoelectric transformer by an embodiment.

FIG. 2 shows the connection state between the piezoelectric transformer 110 of FIG. 1 and the cold cathode tube 118 as a load and the stray capacitance C.
It is a figure which shows the reactive currents Ixa and Ixb by x typically.

First, as shown in FIG.
Reference numeral 10 denotes an input-side electrode 11 formed by processing a piezoelectric material such as lead zircon titanate (PZT) into a rectangular shape and near the center of the rectangular plate.
00U and 1100D are formed, and output electrodes 1100L and 1100R are provided on both end surfaces of the rectangular plate, respectively. The drive unit of the piezoelectric transformer 110 is the input side electrode 1
Polarization is performed in the thickness direction by 100U and 1100D,
The power generation section is polarized by the input side electrodes 1100U and 1100D and the respective output side electrodes 1100L and 1100R. The piezoelectric transformer 110 includes an input electrode 110
When an AC voltage in the half-wavelength oscillation mode is applied between 0U and 1100D, the input-side electrodes 1100U and 1100D
The voltage applied to D is extracted as two voltages 180 ° out of phase by the two output electrodes 1100L and 1100R formed on the end faces.

As shown in FIG. 2, the output voltage obtained by multiplying the input voltage by the boosting ratio by 180 ° from the piezoelectric transformer is applied to each input terminal of the cold cathode tube 118. The cold-cathode tube 118 usually has a stray capacitance Cx due to the reflection plate 120 or the like. In such a case, when positive and negative voltages are applied to both ends of the cold cathode tube 118, respectively, the high voltage side (period ta) of the cold cathode tube 118
(The voltage V2 application side at the time and the voltage V1 application side at the time tb), the current Ixa2 from the cold cathode tube 118 to the reflection plate 120,
Ixb1 flows out, and on the low voltage side (voltage V1 application side during period ta, voltage V2 application side during period tb), the reflection plate 12
From 0, currents Ixa1 and Ixb2 flow into the cold cathode tube 118. Therefore, the output current of the piezoelectric transformer 110 includes a current Ia (effective current) contributing to light emission of the cold cathode tube 118,
Floating capacitance C formed between cold-cathode tube 118 and reflector
x (reactive current Ix).

As a result, to keep the brightness of the cold-cathode tube 118 constant, it is necessary to detect only the effective current Ia that contributes to the light emission of the cold-cathode tube 118 and feed it back.

In FIG. 1, reference numeral 113 denotes a variable oscillation circuit for generating a voltage signal of a variable frequency. Variable oscillation circuit 11
The output signal of No. 3 is usually a voltage signal having a pulse waveform,
The driving circuit 112 removes high-frequency components and converts the signal into an AC signal close to a sine wave. The output signal of the variable oscillation circuit 113 is input to the drive circuit 112. Drive circuit 1
12 is voltage amplified to a level sufficient to drive the piezoelectric transformer 110,
To the primary side electrode 1100U. Here, as the piezoelectric transformer 110, a piezoelectric transformer having the structure shown in FIG. 2 is used.

The output voltage boosted by the piezoelectric effect of the piezoelectric transformer 110 is applied to the secondary electrodes 1100L, 1100R.
Taken out of Secondary electrode 1100L, 1100R
Are output to the two input terminals of the cold cathode fluorescent lamp 118,
The cold cathode tube 118 lights up.

When the cold-cathode tube 118 is turned on, positive and negative voltages whose phases differ by 180 ° are alternately applied from the two input terminals. The output signal of the current detection circuit 116 for detecting the current flowing through the cold-cathode tube 118 and the output signal of the voltage detection circuit 117 for detecting the voltage applied to both ends of the cold-cathode tube 118 are the voltage and current of the cold-cathode tube 118 Is supplied to a phase difference detection circuit 119 which detects a phase difference between the first and second signals. Output signal of phase difference detection circuit 119 and current detection circuit 1
The output signal of 16 is supplied to an effective current detection circuit 115, and an effective current flowing through the cold cathode tube 118 is detected.

The output signal of the effective current detection circuit 115 is supplied to one input terminal of the oscillation control circuit 114, and is supplied to the other input terminal of the oscillation control circuit 114 so that a constant effective current flows through the cold cathode tube 118. Reference voltage Vre supplied
and variable oscillation circuit 11 according to the comparison result.
3 is controlled.

When the effective current flowing through the cold-cathode tube 118 becomes larger than the set value determined by the reference voltage Vref, the oscillation control circuit 114 changes the oscillation frequency to the piezoelectric transformer 110.
The variable oscillation circuit 113 is controlled to move away from the resonance frequency of the piezoelectric transformer 110. On the other hand, if the effective current becomes smaller than the set value, the variable oscillation circuit 113 is controlled so that the oscillation frequency approaches the resonance frequency of the piezoelectric transformer 110. As described above, by controlling the driving of the piezoelectric transformer 110 in a self-excited manner to keep the effective current flowing through the cold cathode tube 118 constant, the load of the cold cathode tube 118 fluctuates and the piezoelectric transformer 1
The cold cathode tube 118 can be stably turned on even if the characteristics of the 10 change.

FIG. 3 is a circuit diagram showing a specific example of the configuration around the drive circuit 112 in FIG. In FIG. 3, the current detection circuit 116 has one end of the primary winding connected to the piezoelectric transformer 11.
The current transformer CT includes a current transformer CT having the other end connected to the cold cathode tube 118, and a resistor R1 connected between the secondary windings of the current transformer CT as a load for current detection.

The current signal detected by the secondary winding of the current transformer CT is used to generate a phase difference
It is supplied to one input terminal of the D gate. The other input terminal of the AND gate is supplied with a voltage signal divided by the resistors R2 and R3 constituting the voltage detection circuit 117. Here, since the output voltage of the piezoelectric transformer 110 is used to detect a phase difference from the output current, its absolute value is not necessary, and the voltage is divided by the resistors R2 and R3 to the input threshold level of the AND gate. .

The active current detection circuit 115 includes a peak hold circuit including a diode D1, a capacitor C1, and a resistor R5, a switching element Q1, and a resistor R5.
4 The current signal detected by the secondary winding of the current transformer CT is supplied to a peak hold circuit for detecting the absolute value of the current.

The output signal of the AND gate is input to the switching element Q1, and the switching element Q1 is switched according to the input level of the voltage signal and the current signal, that is, the phase difference.
1 is turned on / off, and only the effective current component of the current signal is detected by the peak hold circuit.

In this embodiment, the piezoelectric transformer is P
Although formed using a piezoelectric ceramic such as ZT, an output voltage having a 180 ° phase difference can be obtained with a single crystal material such as LiNbO ₃ as long as the material exhibits piezoelectricity.

The piezoelectric transformer is not limited to the half-wavelength vibration mode as shown in FIG. 2, but outputs voltages having phases different from each other by 180 °. A similar effect can be obtained with other piezoelectric transformers.

Further, even when two cold cathode tubes are connected as a load of the piezoelectric transformer, the voltage applied to the two cold cathode tubes and the current flowing through the cold cathode tubes are detected, and the phase difference between the two tubes is detected. The same effect can be obtained by detecting only the effective current component included in the output current and using it for luminance control.

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention.
It is a block diagram showing an example of 1 composition of a drive device of a piezoelectric transformer by an embodiment.

In FIG. 4, the piezoelectric transformers 315, 31
Numeral 6 is made of a piezoelectric material such as PZT, which exhibits piezoelectricity, and the voltage applied to the primary electrode is multiplied by a step-up ratio and taken out from the secondary electrode.

The two piezoelectric transformers 31 shown in FIG.
5 and 316 are 1 by the phase inversion circuit 317, respectively.
Input voltages having phases different by 80 ° are applied. As a result, each of the piezoelectric transformers 315 and 316 has 18
Output voltages different in phase by 0 ° are taken out and input to respective input terminals of the cold cathode tube 118. This embodiment is different from the first embodiment in that the cold cathode tube 118 is driven by using two piezoelectric transformers, and the other controls are the same.

In the case of driving according to the present embodiment,
Since the cold-cathode tube 118 usually has a stray capacitance Cx due to the reflection plate 120 or the like, if a positive and a negative voltage are alternately applied to both ends of the cold-cathode tube 118, the cold-cathode tube A current flows from the reflector 118 to the reflector 120, and a current flows from the reflector 120 to the cold cathode tube 118 on the low voltage side.

Therefore, the piezoelectric transformers 315, 316
Is the current I that contributes to the light emission of the cold-cathode tube 118.
a (effective current) and a current (a reactive current I) flowing through a stray capacitance Cx formed between the cold-cathode tube 118 and the reflector 120.
x).

As a result, to keep the brightness of the cold-cathode tube 118 constant, it is necessary to detect only the effective current Ia contributing to the light emission of the cold-cathode tube 118 and feed it back.

In FIG. 4, reference numerals 311 and 312 denote variable oscillation circuits for generating a voltage signal of a variable frequency. The output signals of the variable oscillation circuits 311 and 312 are usually voltage signals having a pulse waveform, and the driving circuits 313 and 314 remove high-frequency components and convert the signals into an AC signal close to a sine wave. The output signals of the driving circuits 314 and 313 are voltage-amplified to a level sufficient to drive the piezoelectric transformers 315 and 316, respectively, and the piezoelectric transformers 315 and 31 are amplified.
6 is inputted to the primary side electrode. Here, the voltages input to the two piezoelectric transformers 315 and 316 respectively have the same amplitude and a phase difference of 180 °.

The output voltage boosted by the piezoelectric effect of the piezoelectric transformers 315 and 316 is taken out from the secondary electrode. By the input voltages having different phases of 180 °, output voltages having different phases of 180 ° are taken out from the piezoelectric transformer having the same structure. The two high-voltages output from the secondary electrodes are applied to two input terminals of the cold-cathode tube 118, and the cold-cathode tube 118 is turned on.

When the cold-cathode tube 118 is turned on, positive and negative voltages whose phases differ by 180 ° are alternately applied from the two input terminals. The output signal of the current detection circuit 116 for detecting the current flowing through the cold-cathode tube 118 and the output signal of the voltage detection circuit 117 for detecting the voltage applied to both ends of the cold-cathode tube 118 are the voltage and current of the cold-cathode tube 118 Is supplied to a phase difference detection circuit 119 which detects a phase difference between the first and second signals. Output signal of phase difference detection circuit 119 and current detection circuit 1
The output signal of 16 is supplied to an effective current detection circuit 115, and an effective current flowing through the cold cathode tube 118 is detected.

The output signal of the effective current detection circuit 115 is supplied to one input terminal of the oscillation control circuit 114, and is applied to the other input terminal of the oscillation control circuit 114 so that a constant effective current flows through the cold cathode tube 118. Reference voltage Vre supplied
f, and according to the comparison result, the variable oscillation circuit 3
11 and 312 are controlled. Note that an output signal of the oscillation control circuit 114 is input to the phase inversion circuit 317, and output signals having a 180 ° phase difference are input from the phase inversion circuit 317 to the variable oscillation circuits 311 and 312, respectively.

When the effective current flowing through the cold-cathode tube 118 becomes larger than the set value determined by the reference voltage Vref, the oscillation control circuit 114 changes the oscillation frequency to the piezoelectric transformer 31.
The variable oscillation circuits 312 and 311 are controlled in a direction away from the resonance frequencies 5 and 316. On the other hand, when the effective current is smaller than the set value, the oscillation frequencies are reduced.
The variable oscillation circuits 312 and 312
11 is controlled. As described above, the piezoelectric transformer 315 is provided in a self-excited manner in which the effective current flowing through the cold cathode tube 118 is kept constant.
By controlling the drive of the piezoelectric transformers 316, the load of the cold cathode tubes 118 fluctuates, and the piezoelectric transformers 315, 3
The cold cathode tube 118 can be stably turned on even if the characteristics of the 16 change.

In the above description, since two piezoelectric transformers having the same structure were used, voltages having phases different from each other by 180 ° were applied to the input terminals of the piezoelectric transformer. However, as a modification of the present embodiment, a piezoelectric transformer having a different polarization structure can be used. In this case, as shown in FIG. 5, one piezoelectric transformer 416 having input side electrodes 4161U and 4161D and an output side electrode 4162, and input side electrodes 4151U and 4151D and an output side electrode 415
The polarization directions of the other piezoelectric transformer 415 having the same direction 2 are the same direction PD in the thickness direction, PL1 in the longitudinal direction, and PL2 in the opposite direction to PL1. You can get the output.

As a result, as in the piezoelectric transformer driving device shown in FIG. 6, the phase inverting circuit 317 can be eliminated, and the two variable oscillation circuits 311 and 312 are also provided with two piezoelectric transformers 41.
One variable oscillation circuit 113 common to 5 and 416 can be used.

In this embodiment, the piezoelectric transformer is P
Although formed using a piezoelectric ceramic such as ZT, an output voltage having a 180 ° phase difference can be obtained with a single crystal material such as LiNbO ₃ as long as the material exhibits piezoelectricity.

Further, the same effect can be obtained with other piezoelectric transformers as long as inputs are made from both ends of the cold cathode tube.

Further, even when two cold cathode tubes are connected as a load of the piezoelectric transformer, the voltage applied to the two cold cathode tubes and the current flowing through the cold cathode tubes are detected, and the phase difference between them is detected. The same effect can be obtained by detecting the effective current among the output currents and controlling the luminance to be constant.

(Third Embodiment) FIG. 7 shows a third embodiment of the present invention.
FIG. 4 is a configuration diagram of a drive circuit and a peripheral portion thereof in the piezoelectric transformer drive device according to the embodiment.

In FIG. 7, reference numerals 605 and 606 denote piezoelectric transformers. The piezoelectric transformers 605 and 606 exhibit resonance characteristics similarly to other piezoelectric elements. When an AC voltage near the resonance frequency is input to the primary electrode, The voltage output multiplied by the boosting ratio by the piezoelectric effect is taken out from the secondary electrode. However, frequency components other than the resonance frequency are
5, 606 loss or conversion to heat,
This causes unnecessary stress and lowers reliability. Therefore, it is desirable to drive the piezoelectric transformers 605 and 606 with a sine wave near the resonance frequency as much as possible.

However, as shown in the second embodiment, in an apparatus using two piezoelectric transformers,
A drive circuit is required for each of the two piezoelectric transformers.

Therefore, as shown in FIG.
603, 604 and a pair of step-up transformers 601, 602
Thus, the drive circuit 600 is configured.

In the drive circuit 600 shown in FIG. 7, an intermediate tap is provided at the center of the secondary winding of the step-up transformers 601 and 602 and the ground is dropped to the ground, and the phases differ from the two terminals of the secondary winding by 180 °. The generated voltage is applied to each of the piezoelectric transformers 605 and 606. Here, the inductance of the secondary winding of the step-up transformers 601 and 602 is set so that voltage resonance can be performed at a desired frequency in consideration of the primary-side capacitance of the piezoelectric transformers 605 and 606.

The pair of FETs 603 and 604 include:
Rectangular wave signals CLK and / CLK having opposite phases are input to the gate terminals. When the FET 603 is on,
The FET 604 is off, and when each FET is on, current flows from the power supply Vd to the primary winding of the step-up transformers 601 and 602, and energy is stored. When the FET in the ON state is turned OFF, the energy stored in the inductor is converted into a voltage from the secondary winding and output to the piezoelectric transformers 605 and 606.

Thus, the piezoelectric transformers 605 and 6
06 is driven by a pair of FETs 603 and 604 and a pair of step-up transformers 601 and 602 with a sinusoidal voltage. From the output terminals of the piezoelectric transformers 605 and 606, voltages whose phases differ by 180 ° are output,
The cold cathode tube 607 is driven by the voltage signal.

When the driving circuit 600 having the above-described configuration is used, the driving circuit 6 is driven like the driving device of the piezoelectric transformer shown in FIG.
00 can be shared by the two piezoelectric transformers 605 and 606, and a voltage having the same drive waveform can be applied to the two piezoelectric transformers 605 and 606 in driving the piezoelectric transformers. The output voltages can be made substantially equal, and the voltages applied to the cold cathode tubes 607 can be made substantially equal. Further, there are effects such as downsizing of the driving device of the piezoelectric transformer and reduction of the number of parts.

[0073]

As described above, according to the present invention,
Based on the phase difference between the output current and voltage of the piezoelectric transformer,
By detecting only the effective current flowing through the cold-cathode tube, eliminating the reactive current due to the stray capacitance formed between the cold-cathode tube and the reflector, and accurately controlling the tube current to be constant,
This has the special effect that the brightness of the cold-cathode tube is stable and a small and highly efficient driving method and a driving device for a piezoelectric transformer can be realized.

[Brief description of the drawings]

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

FIG. 2 is a diagram schematically showing reactive currents Ixa and Ixb during periods ta and tb due to a stray capacitance Cx when a voltage is applied to a cold cathode tube from a piezoelectric transformer.

FIG. 3 is a circuit diagram showing a specific configuration example of a peripheral portion of a driving circuit 112 in FIG. 1;

FIG. 4 is a block diagram showing a configuration example of a driving device for a piezoelectric transformer according to a second embodiment of the present invention.

FIG. 5 is a perspective view of a piezoelectric transformer used in another driving device according to the second embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration example of another driving device using the piezoelectric transformer of FIG. 5;

FIG. 7 is a configuration diagram of a driving circuit 600 and its peripheral portion in a driving device for a piezoelectric transformer according to a third embodiment of the present invention.

8 is a block diagram showing a configuration example of a driving device for a piezoelectric transformer using the driving circuit configuration of FIG. 7;

FIG. 9 is a block diagram showing a configuration of a conventional piezoelectric transformer driving device.

FIG. 10 is a perspective view showing the structure of a conventional piezoelectric transformer.

11 is a block diagram showing a conventional piezoelectric transformer drive circuit using the piezoelectric transformer of FIG.

FIGS. 12A and 12B schematically show leak currents Ixa and Ixb due to stray capacitance Cx when voltages having 180 ° out of phase are applied to both ends of one cold cathode tube. FIGS.
FIG. 12B shows the magnitude and direction of the leakage current Ixa with respect to the length direction of the cold-cathode tube in the period ta in FIG. 12A, and the leakage current Ixb in the length direction of the cold-cathode tube in the period tb in FIG. (C) showing the size and direction of

FIG. 13 shows one end of each of two cold cathode tubes connected in series.
Stray capacitance Cx when voltages different in 80 ° phase are applied
(A) schematically showing the leakage currents Ixa and Ixb due to the above, FIG. 13 (b) showing the magnitude and direction of the leakage current Ixa with respect to the length direction of the cold-cathode tube during the period ta in FIG. FIG. 7C showing the magnitude and direction of the leak current Ixb with respect to the length direction of the cold cathode tube during the period tb in FIG.

FIG. 14 is a perspective view showing the structure of a conventional piezoelectric transformer.

FIG. 15 is a block diagram showing a configuration of a conventional piezoelectric transformer driving device.

FIG. 16 is a waveform diagram of an output voltage and an output current of a piezoelectric transformer.

[Explanation of symbols]

110, 315, 316, 415, 416, 605, 6
06 Piezoelectric transformer 1100U, 1100D, 4151U, 4151D, 4
161U, 4161D Input side (primary side) electrode 1100L, 1100R, 4152, 4162 Output side (secondary side) electrode 111, 700 Power supply 112, 313, 314, 600 Drive circuit 113, 311, 312, 704 Variable oscillation circuit 114 , 705 Oscillation control circuit 115, 706 Effective current detection circuit 116, 709 Current detection circuit 117, 710 Voltage detection circuit 118, 607 Cold cathode fluorescent lamp 119, 707 Phase difference detection circuit 120, 720 Reflector 317 Phase inversion circuit 601, 602 Boost Transformer 603, 604 FET

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiyuki Asahi 1006 Kazuma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. 72) Inventor Kawa Osamu Osamu 1006 Kadoma, Kadoma-shi, Osaka F-term (reference) in Matsushita Electric Industrial Co., Ltd. DA05 DA06 DC02 DC04 DC05 EA09 5H730 AA14 AS11 BB57 BB80 BB82 BB85 DD04 EE48 EE74 EE79 FD01 FD31 FD61 FG07 ZZ19

Claims (9)

[Claims] 1. A voltage input from a primary terminal of a piezoelectric transformer is boosted by a piezoelectric effect and output from two secondary terminals to two terminals of a cold cathode tube, and a voltage applied to the cold cathode tube; Detecting a phase difference with a current flowing through the cold-cathode tube; detecting an effective current flowing through the cold-cathode tube based on the detected phase difference; comparing the detected effective current with a predetermined set value; A driving method of the piezoelectric transformer, wherein the driving of the piezoelectric transformer is controlled such that an effective current flowing through the cold cathode tube becomes equal to the predetermined set value. 2. A voltage input from a primary terminal of a first piezoelectric transformer is boosted by a piezoelectric effect and output from a secondary terminal to one terminal of a cold cathode tube. The input voltage is boosted by the piezoelectric effect and output from the secondary terminal to the other terminal of the cold-cathode tube, and the phase difference between the voltage applied to the cold-cathode tube and the current flowing through the cold-cathode tube is detected. Detecting an effective current flowing through the cold-cathode tube based on the detected phase difference; comparing the detected effective current with a predetermined set value; The driving method of the piezoelectric transformer, wherein the driving of the first and second piezoelectric transformers is controlled so as to be equal to: 3. The cold cathode tube according to claim 1, wherein one or two or more cold cathode tubes are connected in series.
A driving method of the piezoelectric transformer according to the above. 4. A piezoelectric transformer that boosts a voltage input from a primary terminal by a piezoelectric effect and outputs the boosted voltage from two secondary terminals, a driving circuit for driving the piezoelectric transformer, and a variable frequency voltage for the driving circuit. A variable oscillation circuit that outputs an output voltage from a secondary terminal of the piezoelectric transformer, a cold cathode tube that applies an output voltage to two terminals, a current detection circuit that detects a current flowing through the cold cathode tube, and the cold cathode A voltage detection circuit that detects a voltage applied to the tube; a phase difference detection circuit that detects a phase difference between a current signal output from the current detection circuit and a voltage signal output from the voltage detection circuit; An effective current detection circuit for detecting an effective current flowing through the cold-cathode tube based on a current signal from a detection circuit and a phase difference from the phase difference detection circuit; An oscillation control circuit that compares an electric current with a predetermined set value and controls an oscillation frequency of the variable oscillation circuit so that the effective current becomes equal to the predetermined set value. Drive. 5. A first piezoelectric transformer which boosts a voltage input from a primary terminal by a piezoelectric effect and outputs the boosted voltage from a secondary terminal, and boosts a voltage input from the primary terminal by a piezoelectric effect to a secondary A second piezoelectric transformer that outputs from a terminal, and the first and second piezoelectric transformers each have a phase of 1
A driving circuit that drives with a signal different by 80 °, a variable oscillation circuit that outputs a voltage of a variable frequency to the driving circuit, an output voltage from a secondary terminal of the first piezoelectric transformer is applied to one terminal, A cold cathode tube to which an output voltage from a secondary terminal of the second piezoelectric transformer is applied to the other terminal, a current detection circuit for detecting a current flowing through the cold cathode tube, and a voltage applied to the cold cathode tube , A phase difference detection circuit that detects a phase difference between a current signal output from the current detection circuit and a voltage signal output from the voltage detection circuit, and a current signal from the current detection circuit. And an effective current detection circuit that detects an effective current flowing through the cold-cathode tube based on a phase difference from the phase difference detection circuit, and compares an effective current detected by the effective current detection circuit with a predetermined set value. I The way the active current is equal to the predetermined setting value, the piezoelectric transformer driving apparatus characterized by comprising an oscillation control circuit for controlling the oscillation frequency of said variable oscillating circuit. 6. A voltage input from a primary terminal of a first piezoelectric transformer is boosted by a piezoelectric effect and output from a secondary terminal to one terminal of a cold cathode tube. Entered the first
A voltage having the same phase as the input voltage of the piezoelectric transformer is boosted by a piezoelectric effect, and a voltage having a phase different from the output voltage of the first piezoelectric transformer by 180 ° from the secondary terminal is applied to the other terminal of the cold cathode tube. And driving the cold-cathode tube. 7. A phase difference between a voltage applied to the cold-cathode tube and a current flowing through the cold-cathode tube, and an effective current flowing through the cold-cathode tube is detected based on the detected phase difference. Comparing the detected effective current with a predetermined set value, and controlling the driving of the first and second piezoelectric transformers so that the effective current flowing through the cold cathode tube becomes equal to the predetermined set value. 7. The driving method of a piezoelectric transformer according to claim 6, wherein: 8. A first piezoelectric transformer that boosts a voltage input from a primary terminal by a piezoelectric effect and outputs the boosted voltage from a secondary terminal, and an input voltage of the first piezoelectric transformer input from a primary terminal. A second piezoelectric transformer, which boosts a voltage having the same phase as that of the first piezoelectric transformer by a piezoelectric effect and outputs the voltage from the secondary terminal as a voltage having a phase different from that of the output voltage of the first piezoelectric transformer by 180 °; A driving circuit that drives the piezoelectric transformer with a signal having the same phase; a variable oscillation circuit that outputs a variable frequency voltage to the driving circuit; and an output voltage from a secondary terminal of the first piezoelectric transformer. A cold-cathode tube to which an output voltage from a secondary terminal of the second piezoelectric transformer is applied to the other terminal; a current detection circuit for detecting a current flowing through the cold-cathode tube; Applied voltage A voltage detection circuit for detecting, a phase difference detection circuit for detecting a phase difference between a current signal output from the current detection circuit and a voltage signal output from the voltage detection circuit, and a current signal from the current detection circuit. Based on the phase difference from the phase difference detection circuit, an effective current detection circuit that detects an effective current flowing through the cold-cathode tube, and compares the effective current detected by the effective current detection circuit with a predetermined set value. An oscillation control circuit for controlling an oscillation frequency of the variable oscillation circuit so that the effective current is equal to the predetermined set value. 9. A pair of current amplifying circuits for current-amplifying AC voltages having phases different from each other by 180 °, and voltage-amplifying respective output signals of the pair of current amplifying circuits, each having a phase of 180 °. A pair of step-up transformers for outputting different voltage signals; a primary electrode and a secondary electrode formed on the piezoelectric body;
And a pair of piezoelectric transformers for boosting the voltage signal input to the primary electrode from each of the pair of boost transformers and outputting the boosted voltage signal from the secondary electrode. .