JP2001273995A - Piezoelectric transformer inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a piezoelectric transformer inverter which enables reducing a cost by omitting a level shift circuit and by downsizing a circuit scale and a shape. SOLUTION: A voltage V101 to which a load current is outputted by a load current value setting circuit 101 is set at an integral circuit 104, and a load current of a cold-cathode tube 102 is detected by a load current detection circuit 103, and a differential voltage to deduce a detection voltage V103 from the load current control set voltage V101 by the integral circuit 104 is integrated, and a voltage oscillator 105 is oscillated at a frequency f105 proportional to an output voltage of the integral circuit 104, and a ceramic piezoelectric transformer driving circuit 106 drives the ceramic piezoelectric transformer 107, based on this frequency signal, and the ceramic piezoelectric transformer 107 will turn on a light of the cold-cathode ray tube 102.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer inverter, and more particularly, to a piezoelectric transformer inverter for driving a discharge lamp for driving a cold cathode tube for a liquid crystal backlight using a ceramic piezoelectric transformer.

[0002]

2. Description of the Related Art A piezoelectric transformer is a voltage conversion element utilizing the vibration of a ceramic element, and is smaller and more efficient than an inverter using a conventional winding transformer. An alternative method is being considered. However, compared to an inverter using a winding transformer, a piezoelectric transformer inverter is more expensive, which is a factor preventing its spread.

As a known example of a cold cathode tube lighting device using a piezoelectric transformer, Japanese Patent No. 2767730 is known.

FIG. 7 is a block diagram of a cold-cathode tube lighting device disclosed in Japanese Patent No. 2767730. In FIG. 7, a variable voltage device 1 determines a set voltage V1 for determining a load current, and supplies the set voltage V1 to an integration circuit 5. The cold-cathode tube 2 serves as a backlight source for liquid crystal. A load current detection circuit 3 detects a load current I2 flowing through the cold-cathode tube 2, and the load current I2
To the AC voltage rectifier circuit 4. The AC voltage rectifier circuit 4 rectifies an AC voltage proportional to the detected load current, and supplies an output voltage E4 to the integrating circuit 5.

The integrating circuit 5 integrates the difference voltage between the set voltage V1 from the variable voltage device 1 and the output voltage E4 from the AC voltage rectifying circuit 4, and converts the output voltage V5 to a voltage level shift circuit 6.
Give to. The voltage level shift circuit 6 receives the input voltage V
5 is shifted by V06 to output a voltage V6.
This voltage V6 is supplied to the astable multivibrator 7, and the astable multivibrator 7 receives the supplied voltage V6.
Oscillates and supplies the current signal to the current amplification circuit 8. When the voltage V8 obtained by amplifying the frequency signal f7 input from the astable multivibrator 7 is applied to the winding transformer 9, the output voltage V8 of the current amplification circuit 8 is boosted to V9 by the winding transformer 9. The ceramic transformer 10 is driven. The winding transformer 9 is inserted as needed.

In FIG. 7, a set voltage V of the variable voltage device 1, a magnitude I of a load current of the cold cathode tube 2, an output voltage V of the load current detection circuit 3, an output voltage E of the AC voltage rectification circuit 4, an integration circuit. 5, output voltage V5, output voltage V6 of voltage level shift circuit 6, shift voltage V06, oscillation frequency f7 of astable multivibrator 7, output voltage V8 of current amplifying circuit 8, step-up ratio N of winding transformer 9, output voltage V
9. The frequency characteristic of the ceramic transformer 10 is represented by F (j2π
f), assuming that the output voltage V is 10, each is represented by the following equation.

V3 = k1 · I2 (1) E4 = k2 · V3 (2) V5 (t) = k3 · ∫ (E4-V1) dt (3) V6 = V5 + V06 (4) f7 = k4 · V6 (5) V8 = Vpsin2πf7t = Vpsin2πk4 · V6t (6) V9 = N · V8 (7) V10 = | F (j2πf7) | V9 = N | F (j2πk4 · V6) | Vpsin2πk4 · V6t (8) I2 = N | F (j2πk4 · V6) | Vp / ZL (9) where Vp is the output amplitude of the current amplifier circuit 8 and ZL
Is the impedance of the cold cathode tube 2. When the integrated value of the integrating circuit 2 is 0, that is, E4-V1 = 0 (10), the output does not change and becomes constant.

In the cold-cathode tube lighting device shown in FIG. 7, when the cold-cathode tube lighting is started, the load current I2 does not flow, so that E4-V1 <0 (11).

[0009]

However, E4-
When V1 <0, the output of the integrating circuit 2 does not become negative, becomes constant at the GND level, the output frequency f7 of the astable multivibrator 7 does not change, and the cold cathode tube 2 cannot be turned on. Therefore, in the method of Japanese Patent No. 2776730, by providing the level shift circuit 6 and level-shifting the output voltage of the integrating circuit 5, the output frequency f7 of the astable multivibrator 7 changes even when E4-V1 <0. The cold cathode tube 2 is configured to be turned on.

Therefore, in the system of Japanese Patent No. 2776730, the voltage level shift circuit 6 is necessarily required, and the circuit becomes complicated, the shape cannot be reduced, and the cost increases.

FIG. 8 shows a time change of the output voltage V5 of the integrating circuit 5 when the cold cathode tube driving device shown in FIG. 7 operates. As shown in FIG. 8, the output voltage V of the integration circuit 5
5 decreases with time from the level shift voltage V06,
The balance is achieved when E4 = V1, and the load current is kept constant.

SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide a piezoelectric transformer inverter which does not require a level shift circuit, can reduce the circuit size and shape, and can reduce the cost.

Another object of the present invention is to add a voltage clamp circuit so that the lower limit of the frequency setting of the voltage variable oscillation circuit does not change due to fluctuations in the power supply voltage, and to facilitate the design of the frequency setting.

[0014]

SUMMARY OF THE INVENTION The present invention relates to a piezoelectric transformer inverter for lighting a discharge tube with a high voltage obtained by using a step-up ratio of a ceramic piezoelectric transformer. A detection circuit, an integration circuit for integrating a difference voltage obtained by subtracting an output voltage of the load current detection circuit from a load current setting voltage for setting a set value of the load current, and a voltage for outputting a frequency signal proportional to the output voltage of the integration circuit A variable oscillation circuit, and a drive circuit for driving the ceramic piezoelectric transformer based on a frequency signal from the voltage variable oscillation circuit, and driving the ceramic piezoelectric transformer at a negative slope of the step-up ratio frequency characteristic of the ceramic piezoelectric transformer, thereby providing a load. It is characterized in that the current is controlled to be constant.

[0015] Preferably, the discharge tube is a cold cathode tube for a liquid crystal backlight. Still more preferably, a voltage clamp circuit is provided between the integration circuit and the variable voltage oscillation circuit so that the output voltage of the integration circuit does not exceed a certain voltage.

[0016]

FIG. 1 is a block diagram of one embodiment of the present invention. The piezoelectric transformer inverter 110 of FIG.
7 is different from the conventional example of FIG. 7 in that the voltage level shift circuit 6 is eliminated. Load current value setting circuit 10
1 supplies an output voltage 101 for setting a load current to an integrating circuit 104. The load current detection circuit 103 detects the load current I102 of the cold cathode tube 102, and supplies a voltage V103 proportional to the load current to the integration circuit 104. The integration circuit 104 outputs the output voltage V101 of the load current setting circuit 101.
From the load current detection circuit 103 to the voltage variable oscillation circuit 105.

The variable voltage oscillation circuit 105 includes the integration circuit 10
4 is output, a signal having an oscillation frequency f105 proportional to the output voltage V104 is output. When the output voltage V104 is low, the oscillation frequency f105 is set to a high frequency.
Is higher, the oscillation frequency f105 is set to oscillate at a lower frequency. The range of the oscillation frequency f105 can be arbitrarily set according to the boost frequency characteristics of the ceramic piezoelectric transformer. The ceramic piezoelectric transformer driving circuit 106 drives the ceramic piezoelectric transformer 107 with the driving voltage V106 based on the frequency signal given from the voltage variable oscillation circuit 105. The ceramic piezoelectric transformer 107 drives the cold cathode tube 102 with the output voltage V107.

In the embodiment shown in FIG. 1, the driving frequency of the ceramic piezoelectric transformer 107 is varied with a voltage obtained by integrating the difference between the load current and a reference voltage for determining the load current, so that the load current is controlled to be constant. The points are the same as in FIG. However, the integration circuit 104 subtracts the output voltage V103 of the load current detection circuit 103 for detecting the load current flowing to the cold cathode tube 102 from the output voltage V101 of the load current value setting circuit 101 for determining the load current flowing to the cold cathode tube 102. The point that the voltage level shift circuit 6 shown in FIG. 7 is omitted by outputting a value obtained by integrating the voltage is different from the conventional example.

FIG. 2 is a block diagram showing a specific example of the integration circuit 104 shown in FIG. As shown in FIG. 2, the integration circuit 104 includes a comparator 302 and a capacitor C301 connected between the output of the comparator 302 and the ground. Piezoelectric transformer inverter 110 shown in FIG.
When the cold-cathode tube 102 is turned on, the load current does not flow before the cold-cathode tube 102 is turned on. Therefore, only the + side load current value setting voltage V101 is input to the comparator 302 in the integrating circuit 104. , The comparator 302 starts charging the capacitor C301. Therefore, the voltage variable oscillation circuit 1
The voltage of the integration circuit 104 is input to 05 with a time constant for charging the capacitor C301, and the frequency f105 starts to sweep from a high frequency to a low frequency.

FIG. 3 shows the output voltage V of the integrating circuit shown in FIG.
Oscillation frequency f105 of 104 versus voltage variable oscillation circuit 105
FIG. 4 shows an example of a time change of the voltage V104 output from the integration circuit 104.

The oscillation frequency range of the voltage variable oscillation circuit 105 is set so as to use the negative slope of the step-up ratio / frequency characteristic of the ceramic piezoelectric transformer 107. When the frequency f105 decreases, the ceramic piezoelectric transformer 1
07 is increased, and the ceramic piezoelectric transformer 107
Output voltage V107 also increases. This output voltage V10
7 exceeds the lighting voltage of the cold cathode tube 102,
02 lights up, the load current I102 flows, and the voltage V proportional to the load current I102 is detected by the load current detection circuit 103.
103 is output. The integration circuit 104 outputs a voltage value V104 obtained by integrating a voltage obtained by subtracting the load current detection voltage V103 from the load current setting voltage V101.

By repeating this, load current setting voltage V1
When 01 becomes equal to the load current detection voltage V103, the cold-cathode tube 102 continues to emit light at the load current determined by the load current setting voltage.

As described above, according to this embodiment, the load current detecting circuit for detecting the load current flowing through the cold cathode tube 102 from the output voltage V101 of the load current value setting circuit 101 which determines the load current flowing through the cold cathode tube 102 By setting the integration circuit 104 so as to output a value obtained by integrating the voltage obtained by subtracting the output voltage V103 from the output voltage V103, the lighting of the cold-cathode tube 102 can be performed without providing the voltage level shift circuit 6 shown in FIG. Dimming can be facilitated.

FIG. 5 is a block diagram showing a second embodiment of the present invention. The piezoelectric transformer inverter 210 according to the embodiment shown in FIG. 5 is different from the integrating circuit 104 shown in FIG.
Clamp circuit 1 between the power supply and the variable voltage oscillation circuit 105
08, and the other configuration is the same as that of FIG. The voltage clamp circuit 108 clamps the voltage so that the output voltage V104 of the integration circuit 104 does not exceed a certain voltage. That is, the normal integration circuit 10
4 changes from the GND level to the power supply voltage. Since the oscillation frequency of the voltage variable oscillation circuit 105 is determined by the output voltage range of the integration circuit 104, in the case of the embodiment shown in FIG. 1, the set lower limit frequency also changes due to the fluctuation of the power supply voltage. In some cases, the step-up ratio of the ceramic piezoelectric transformer 107 does not reach the lighting voltage of the discharge tube and lighting cannot be performed, and it is necessary to set a lower limit frequency in anticipation of fluctuations in the power supply voltage.

FIG. 6 shows the frequency versus boost ratio characteristics of the ceramic piezoelectric transformer. As is clear from FIG. 6, if the set value of the lower limit frequency shifts from A to B, a necessary boosting ratio cannot be obtained, and the cold cathode tube 102 cannot be turned on. Therefore, a voltage clamp circuit 108 is connected to the output of the integration circuit 104, and the voltage 105 of the integration circuit 104 is clamped at a voltage lower than the power supply voltage so that the lower limit of the frequency does not change due to power supply voltage fluctuation. be able to.

The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0027]

As described above, according to the present invention, the load current becomes constant by eliminating the level shift circuit and driving the ceramic piezoelectric transformer in the negative slope of the step-up ratio frequency characteristic of the ceramic piezoelectric transformer. With such control, the circuit scale and shape can be reduced, and the cost can be reduced.

Further, if the voltage clamp circuit is provided, the frequency setting lower limit value of the voltage variable oscillation circuit does not change due to the power supply voltage fluctuation, and the design of the frequency setting can be simplified.

[Brief description of the drawings]

FIG. 1 is a block diagram of a piezoelectric transformer inverter according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a specific example of the integration circuit shown in FIG.

FIG. 3 is a diagram illustrating an output voltage of an integration circuit versus frequency characteristics of a voltage variable oscillation circuit in the embodiment illustrated in FIG. 1;

FIG. 4 is a diagram illustrating an example of a time change of a voltage output from an integration circuit.

FIG. 5 is a block diagram showing a piezoelectric transformer inverter according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a piezoelectric transformer frequency versus boost ratio characteristic of the embodiment shown in FIG. 5;

FIG. 7 is a block diagram of a piezoelectric transformer inverter described in Japanese Patent No. 2767730.

8 is a diagram showing an example of a time change of an output voltage of an integration circuit in the piezoelectric transformer inverter shown in FIG.

[Explanation of symbols]

101 load current value setting circuit, 102 cold cathode tube, 10
3 Load current detection circuit, 104 integration circuit, 105 variable voltage oscillation circuit, 106 ceramic piezoelectric transformer drive circuit, 107 ceramic piezoelectric transformer, 108 voltage clamp circuit, 110, 210 piezoelectric transformer inverter.

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2H093 NC06 NC42 NC49 NC59 ND49 ND54 NE06 3K072 AA01 CA16 DD03 DE02 EB05 EB07 GB01 HA06 3K083 AA91 BA05 BC35 5H007 BB03 DA03 DB02 DC02 5H730 AS02 AS11 BB57 BB61 FF0131

Claims (3)

[Claims] 1. A piezoelectric transformer inverter for lighting a discharge tube with a high voltage obtained by using a step-up ratio of a ceramic piezoelectric transformer, wherein: a load current detection circuit for detecting a load current flowing through the discharge tube; An integration circuit for integrating a difference voltage obtained by subtracting an output voltage of the load current detection circuit from a load current setting voltage for setting a set value of the voltage; and a voltage variable oscillation circuit for outputting a frequency signal proportional to the output voltage of the integration circuit. A driving circuit for driving the ceramic piezoelectric transformer based on a frequency signal from the voltage variable oscillation circuit, wherein the ceramic piezoelectric transformer is driven at a negative slope of a step-up ratio frequency characteristic of the ceramic piezoelectric transformer. A piezoelectric transformer inverter that controls the load current to be constant. 2. The piezoelectric transformer inverter according to claim 1, wherein the discharge tube is a cold cathode tube for a liquid crystal backlight. 3. A voltage clamp circuit is provided between the integration circuit and the variable voltage oscillation circuit so that an output voltage of the integration circuit does not exceed a predetermined voltage. Item 2. The piezoelectric transformer inverter according to item 1.