JPH10285947A - Piezoelectric-transformer driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the peak value of the primary-side input current of a piezoelectric transformer at the time of switching start. SOLUTION: The piezoelectric-transformer driving device 1 is composed of a DC power 2, an inductor 3 for resonance, a MOS FET 4 and a high-frequency choke inductor 5. The high-frequency choke inductor 5 is connected to a source power supply S for the MOS FET 4. An input electrode 11 for a piezoelectric transformer 6 driven by the piezoelectric-transformer driving device 1 is joied with a drain electrode D for the MOS FET 4, and a common electrode 12 is connected with the ground side end of the high-frequency choke inductor 5. That is, the piezoelectric transformer 6 is connected in parallel with a series circuit consisting of the MOS FET 4 and the high-frequency choke inductor 5.

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, and more particularly to a piezoelectric transformer driving device incorporated in a cold cathode tube lighting device used as a backlight of a liquid crystal display.

[0002]

2. Description of the Related Art As a piezoelectric transformer driving device of this kind,
Conventionally, the one shown in FIG. 14 is known. The piezoelectric transformer driving device 79 includes a DC power supply 80, a resonance inductor 81, and a MOS field effect transistor (hereinafter, referred to as a MOS FET) 82 as a switching element. The input electrodes 89 and 90 of the piezoelectric transformer 83 are connected to both ends of the MOS FET 82, respectively. Further, a load 84 such as a cold cathode tube is connected between the output electrode 91 and the input electrode 90 of the piezoelectric transformer 83.

In the piezoelectric transformer driving device 79 having the above configuration, when a rectangular-wave switching element control signal is applied to the gate electrode G of the MOS FET 82, the MO
The SFET 82 repeats ON / OFF. MOS FE
When T82 is ON, a current flows through the resonance inductor 81, and energy is stored in the resonance inductor 81. At this time, the voltage of the drain electrode D of the MOS FET 82 is maintained at substantially zero volt.

The MOS FET 82 is turned off from the on state.
In the state, the input current I is applied to the primary side of the piezoelectric transformer 83.
Flows, and the voltage of the drain electrode D rises. Since a resonance circuit is formed by the resonance inductor 81 and the input capacitance of the piezoelectric transformer 83, the voltage of the drain electrode D becomes zero volt again after a predetermined time since the MOSFET 82 is turned off. When the MOS FET 82 is turned on, a current flows through the resonance inductor 81 again, and energy is accumulated in the resonance inductor 81. Thus, a half-sine input voltage is applied to the primary side of the piezoelectric transformer 83.

[0005]

Generally, in a piezoelectric transformer driving device, the switching element is turned on.
/ OFF for a while (usually several hundred ms)
ec) because the mechanical vibration of the piezoelectric transformer is unstable,
There is a phenomenon that the voltage waveform of the switching element connected to the primary side of the piezoelectric transformer is distorted.

Therefore, the piezoelectric transformer driving device 79 is
For a while after ON / OFF of the OS FET 82 is started, switching is not performed at zero volt, and a pulse-like primary-side input current I having a high peak value is applied to the piezoelectric transformer 83.
Flows through the MOS FET 82 to the common electrode (ground electrode). The primary-side input current I at the start of the switching generates a large pulse-like stress in the piezoelectric transformer 83, and the pulse-like stress is repeatedly applied to the piezoelectric transformer 83. Was causing it.

Accordingly, an object of the present invention is to provide a piezoelectric transformer driving device capable of reducing the peak value of the primary input current of the piezoelectric transformer at the start of switching and preventing the piezoelectric transformer from cracking or chipping. It is in.

[0008]

In order to achieve the above object, a piezoelectric transformer driving apparatus according to the present invention comprises: (a) a DC power supply; and (b) a resonance power supply having one end electrically connected to the DC power supply. A switching element having one end electrically connected to the other end of the resonance inductor;
(D) high-frequency current blocking means electrically connected to the other end of the switching element for blocking a current having a frequency higher than the driving frequency of the piezoelectric transformer.

Here, as the high-frequency current blocking means, for example, a high-frequency choke inductor is used. In this case, the inductance value of the high-frequency choke inductor is preferably 0.1 μH or more and 1.5 μH or less.

[0010]

With the above arrangement, the frequency components higher than the driving frequency of the piezoelectric transformer in the primary input current of the piezoelectric transformer at the start of switching are stored in the high-frequency current blocking means in the form of electromagnetic energy or the like, and then the heat is transferred. It is converted into heat energy or the like as loss. Therefore, while the piezoelectric transformer driving frequency component of the primary side input current is maintained, the frequency component higher than the piezoelectric transformer driving frequency is reduced. As a result, the peak value of the primary-side input current decreases.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a piezoelectric transformer driving device according to the present invention will be described below with reference to the accompanying drawings.

As shown in FIG. 1, the piezoelectric transformer driving device 1 includes a DC power supply 2, a resonance inductor 3, a switching element 4, and a high-frequency choke inductor 5. In the case of the present embodiment, N
A channel enhancement type MOS FET was used.

The high-frequency choke inductor 5 serving as high-frequency current blocking means blocks a primary-side input current I of a piezoelectric transformer 6 described later that has a frequency higher than the piezoelectric transformer driving frequency. However, the high-frequency current blocking means has an impedance value of MOSF for the driving frequency component of the piezoelectric transformer 6 in the primary-side input current I of the piezoelectric transformer 6.
Any circuit may be used as long as it can obtain a large impedance value with respect to a frequency component higher than the driving frequency of the piezoelectric transformer 6 in the primary-side input current I, which is approximately equal to or less than the resistance value in the ON state of the ET4. However, the present invention is not limited to high-frequency choke inductors.

One end of the DC power supply 2 is connected to one end of the resonance inductor 3, and the other end is connected to one end of the high-frequency choke inductor 5 and is grounded. MOS
The drain electrode D of the FET 4 is connected to the other end of the resonance inductor 3, and the source electrode S is connected to the other end of the high-frequency choke inductor 5. That is, the resonance inductor 3, the MOS FET 4, and the high-frequency choke inductor 5
Form a series circuit.

The piezoelectric transformer 6 is connected to the piezoelectric transformer driving device 1 having the above configuration. That is, MOS
The drain electrode D of the FET 4 is connected to the input electrode 11 of the piezoelectric transformer 6, and the ground-side end of the high-frequency choke inductor 5 is connected to the common electrode 12 of the piezoelectric transformer 6.
Therefore, a series circuit including the MOS FET 4 and the high-frequency choke inductor 5 is connected in parallel with the piezoelectric transformer.

The piezoelectric transformer 6 is, for example, a Rosen type piezoelectric transformer having a laminated structure as shown in FIGS.
This Rosen type piezoelectric transformer 6 was manufactured by C.U. A. This is a piezoelectric transformer with a structure announced by Rosen. Hereinafter, the Rosen type piezoelectric transformer will be specifically described. The Rosen type piezoelectric transformer 6 is formed by manufacturing a ceramic green sheet 15 such as lead zirconate titanate (PZT) by a doctor blade method, and forming an internal electrode 16 on the green sheet 15 by a screen printing method or the like.
a and 16b are provided, and the green sheets 15 are laminated and pressed and sintered.

Next, after the sintered laminate is cut and polished, input electrodes 11 are provided on the upper surface and the front end surface of the piezoelectric transformer 6 substantially in the left half in FIG. A common electrode (ground electrode) 12 is provided, and an output electrode 13 is provided on the right end face. These electrodes 11 to 13
Is provided by a method such as silver baking. Input electrode 11
Are electrically connected to the internal electrode 16b, and the common electrode 12 is electrically connected to the internal electrode 16a.

Next, a predetermined bias voltage is applied to the electrodes 11 to 13, and as shown in FIGS. 3 and 4, the input portion of the substantially left half of the piezoelectric transformer 6 is in a direction parallel to the laminating direction. The polarization processing is performed, and the polarization processing is performed on the output portion in the substantially right half in a direction perpendicular to the stacking direction. 3 and 4, the direction of the arrow indicates the direction of polarization.

In this piezoelectric transformer 6, the input electrode 1
When an AC voltage having a frequency substantially equal to the natural resonance frequency of the piezoelectric transformer 6 in the longitudinal direction is applied between the piezoelectric transformer 6 and the common electrode 12, the piezoelectric transformer 6 generates strong mechanical vibration in the longitudinal direction. In this case, charges are generated by the piezoelectric effect, and an output voltage is generated between the output electrode 13 and the common electrode 12. In particular, since the piezoelectric transformer 6 having the laminated structure has a large boosting ratio, it can cope with a low input voltage.

Further, the output electrode 1 of the piezoelectric transformer 6
One end of a load 8 such as a cold cathode tube is connected to
The other end of the load 8 is connected to 2.

Next, the operation and effect of the piezoelectric transformer driving device 1 having the above configuration will be described. (In case of steady state) The gate electrode G of the MOSFET 4
When a rectangular wave switching element control signal shown in FIG. 5A is given, the MOS FET 4 repeats ON / OFF. When the MOS FET 4 is in the ON state, a current flows through the resonance inductor 3 and energy is transferred to the resonance inductor 3.
Is accumulated in At this time, the voltage of the drain electrode D of the MOS FET 4 is kept at substantially zero volt (see FIG. 5B).

When the MOS FET 4 changes from the ON state to the OFF state, the input current I flows to the primary side of the piezoelectric transformer 6, and the voltage of the drain electrode D increases. Since the resonance circuit is formed by the resonance inductor 3 and the input capacitance of the piezoelectric transformer 6, the voltage of the drain electrode D becomes zero again about π (LC) ^1/2 after the MOS FET 4 is turned off. Become a bolt. At this time, the MOS FET 4 is
If the state is changed from the FF state to the ON state, the loss due to switching can be reduced, and the efficiency of the piezoelectric transformer 6 is improved.

When the MOS FET 4 is turned on, a current flows through the resonance inductor 3 again, and energy is stored in the resonance inductor 3. The ON / OFF repetition frequency of the switching element control signal is set near the natural resonance frequency in the longitudinal direction of the piezoelectric transformer 6, and the ON time and O
Preferably, the ratio of the FF time is set to approximately 1: 1. Thus, a half-sine input voltage is applied to the primary side of the piezoelectric transformer 6.

When a voltage is applied to the input portion of the piezoelectric transformer 6, the piezoelectric transformer 6 generates a strong mechanical vibration in the longitudinal direction, and charges are generated at the output portion by the piezoelectric effect. And
An AC high voltage (output voltage) of several hundred volts or more is generated between the output electrode 13 of the piezoelectric transformer 6 and the common electrode 12.

(At the start of switching)
In this piezoelectric transformer driving device 1, a MOS FET
The mechanical vibration of the piezoelectric transformer 6 is unstable for a while after the ON / OFF of the piezoelectric transformer 4 is started, and the drain electrode D of the MOS FET 4 connected to the primary side of the piezoelectric transformer 6 is unstable.
Is distorted (see FIG. 5C). Therefore, MOS
For a while after ON / OFF of the FET 4 is started, switching is not performed when the voltage of the drain electrode D is zero volt. Therefore, on the primary side of the piezoelectric transformer 6, a pulse-like input current I having a high peak value flows through the piezoelectric transformer 6, the MOS FET 4, and the high-frequency choke inductor 5.

However, since the high-frequency choke inductor 5 is connected to the source electrode S of the MOS FET 4,
Of the primary-side input current I at the start of switching, a frequency component higher than the driving frequency of the piezoelectric transformer 6 is stored in the high-frequency choke inductor 5 in the form of electromagnetic energy, and then converted to heat energy or the like as heat loss. . Accordingly, a frequency component higher than the driving frequency of the piezoelectric transformer 6 in the primary input current I is reduced. As a result, the peak value of the input current I flowing on the primary side of the piezoelectric transformer 6 at the start of switching can be reduced, and the piezoelectric transformer driving device 1 with high reliability against cracking or chipping of the piezoelectric transformer 6 can be obtained. Can be.

This will be described more specifically. 6 to 12
The length of the piezoelectric transformer 6 is 30 mm and the thickness is 1.9 m.
m, a width of 6 mm, a laminated Rosen type piezoelectric transformer having a total of 15 layers including internal electrodes 16a and 16b, and a primary input current I of the piezoelectric transformer 6 when the inductance value of the resonance inductor 3 is 62 μH. Is a result obtained by measuring the waveform of (1) with the inductance value L of the high-frequency choke inductor 5 being variously changed. For comparison,
FIG. 6 also shows the measurement results of the conventional piezoelectric transformer driving device (when the inductance value L of the high-frequency choke inductor 5 is 0 μH). The voltage of the DC power supply 2 was set to 20V. 6 to 12, (A) shows the waveform of the primary input current I at the start of switching, and (B) shows the waveform of the primary input current I in a steady state.

FIGS. 6A to 12A show that the inductance value L of the high-frequency choke inductor 5 is 0.1 μH.
Even with the degree, the effect of suppressing the peak value of the primary-side input current I at the start of switching is recognized (see FIG. 7A), and as the inductance value L increases, the peak value of the primary-side input current I is suppressed. The effect increases. Then, the peak value of the primary-side input current I, which was 20 A or more in the conventional piezoelectric transformer driving device, is the inductance value L of 1.5.
When it becomes about μH, it is halved to about 10 A, and a substantially maximum effect can be obtained (see FIGS. 6A and 12A).

6 (B) to 12 (B), the influence of the insertion of the high-frequency choke inductor 5 on the primary input current I in the steady state is almost recognized. And always keeps a peak value of about 1A. Therefore, it is understood that the high-frequency choke inductor 5 effectively acts only on suppressing the peak value of the primary-side input current I at the start of switching without affecting the primary-side input current I in the steady state.

FIG. 13 shows the relationship between the peak value of the primary side input current I at the start of switching and the inductance value L of the high-frequency choke inductor 5 based on FIGS. 6 (A) to 12 (A). It is a graph.

The piezoelectric transformer driving device according to the present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the gist. The switching element is not limited to a MOS FET, but may be, for example, a bipolar transistor or the like.

Further, the piezoelectric transformer 6 of the above embodiment is
Although the electrode 12 is a common electrode, the present invention is not limited to this. The electrode 11 may be a common electrode and the electrode 12 may be used as an input electrode. Further, a piezoelectric transformer in which the polarization direction of the output portion of the right half of the piezoelectric transformer 6 is reversed. Further, the piezoelectric transformer is not limited to the one having a laminated structure, and may be a single-plate Rosen-type piezoelectric transformer or a piezoelectric transformer in which input and output electrodes are independent from each other.

[0033]

As is apparent from the above description, according to the present invention, the high-frequency current blocking means for blocking the current having a frequency higher than the driving frequency of the piezoelectric transformer is electrically connected to the switching element. The high frequency component of the primary input current of the piezoelectric transformer at the start which is higher than the driving frequency of the piezoelectric transformer can be reduced by the high frequency current blocking means. As a result, the peak value of the primary-side input current at the start of switching can be reduced, and a piezoelectric transformer driving device with high reliability against cracking or chipping of the piezoelectric transformer can be obtained.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of a piezoelectric transformer driving device according to the present invention.

FIG. 2 is an external perspective view of the piezoelectric transformer shown in FIG.

FIG. 3 is a structural view seen from III-III in FIG. 2;

FIG. 4 is a structural view seen from IV-IV in FIG. 2;

FIG. 5 is a timing chart of the switching element shown in FIG. 1;

FIG. 6 is a graph showing a primary-side input current waveform of a piezoelectric transformer when an inductance value L of a high-frequency choke inductor is zero.

FIG. 7 is a graph showing a primary-side input current waveform of a piezoelectric transformer when an inductance value L of a high-frequency choke inductor is 0.1 μH.

FIG. 8 is a graph showing a primary-side input current waveform of a piezoelectric transformer when an inductance value L of a high-frequency choke inductor is 0.275 μH.

FIG. 9 is a graph showing a primary-side input current waveform of the piezoelectric transformer when the inductance value L of the high-frequency choke inductor is 0.505 μH.

FIG. 10 is a graph showing a primary-side input current waveform of a piezoelectric transformer when an inductance value L of a high-frequency choke inductor is 0.778 μH.

FIG. 11 is a graph showing a primary-side input current waveform of the piezoelectric transformer when an inductance value L of the high-frequency choke inductor is 1.14 μH.

FIG. 12 is a graph showing a primary-side input current waveform of a piezoelectric transformer when an inductance value L of a high-frequency choke inductor is 1.54 μH.

FIG. 13 is a graph showing a relationship between an inductance value L of a high-frequency choke inductor and a peak value of a primary-side input current of a piezoelectric transformer.

FIG. 14 is an electric circuit diagram showing a conventional piezoelectric transformer driving device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1: Piezoelectric transformer drive device 2: DC power supply 3: Resonance inductor 4: Switching element (MOS FET) 5: High frequency choke inductor

Claims (3)

[Claims] A DC power supply; a resonance inductor having one end electrically connected to the DC power supply; a switching element having one end electrically connected to the other end of the resonance inductor; And a high frequency current blocking means electrically connected to the other end for blocking a current having a frequency higher than the piezoelectric transformer driving frequency. 2. The piezoelectric transformer driving device according to claim 1, wherein said high-frequency current blocking means is a high-frequency choke inductor. 3. The piezoelectric transformer driving device according to claim 2, wherein the inductance value of the high-frequency choke inductor is 0.1 μH or more and 1.5 μH or less.