JPH10304682A - Piezoelectric transformer drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep high power conversion efficiency by connecting the secondary side of a boosting element to a pair of input terminals provided at a piezoelectric transformer, and connecting a capacitor element in parallel to these input terminals. SOLUTION: A piezoelectric transformer 12 is equipped with a pair of input terminals 12a and 12b being the counter electrodes provided at the front and rear, and an output terminal 12c being the electrode provided at the side end face far from these input terminals 12a and 12b. Then, the secondary terminal 3c of a boosting element 13 is connected to one input terminal 12a, and the secondary terminal 14c of the boosting element 14 is connected to the other input terminal 12b, Furthermore, a capacitance element 20 is connected in parallel with one pair of those input terminals 12a and 12b. Hereby, high power conversion efficiency can be kept by tuning it with the drive frequency of the switching elements 15 and 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a piezoelectric transformer driving circuit for driving a piezoelectric transformer at a high frequency.

[0002]

2. Description of the Related Art Japanese Patent Laid-Open No. 8-275 is an example of the prior art.
No. 553 describes a drive circuit in which an inductance element or a transformer element is added to an input terminal of a piezoelectric transformer.

FIG. 6 is a circuit diagram showing an example of a conventional piezoelectric transformer drive circuit. The piezoelectric transformer 2 has a pair of input terminals 2a and 2b and an output terminal 2c. The cold cathode tube 1 is connected between the output terminal 2c and the ground. The secondary terminal 3c of the booster 3 is connected to the input terminal 2a, and the secondary terminal 4c of the booster 4 is connected to the input terminal 2b.

The boosting elements 3 and 4 are composed of an auto-transformer whose primary side is drawn from the middle of one coil, and increases the voltage at a boosting ratio n.

[0005] The common terminals 3b and 4b are connected to each other. The positive terminal of the DC power supply 7 is connected to the common terminals 3b and 4b, and the negative terminal is grounded.
The primary terminals 3a and 4a are grounded via the switching elements 5 and 6, and the switching elements 5 and 6 alternately open and close, so that a pulse-like current flows through the primary circuit and the secondary circuit has When a sinusoidal alternating voltage is applied to the input terminals 2a and 2b by resonance and a high sinusoidal voltage is output from the output terminal 2c by the piezoelectric effect, the cold cathode tube 1 starts discharging.

[0006]

A cold cathode tube is widely used as a backlight light source for a liquid crystal display panel. As the size of the liquid crystal display panel increases, the power consumption of the light source tends to increase. It is necessary to increase the volume of the piezoelectric transformer itself for driving the cathode tube.

If it is desired to reduce the mounting area of the piezoelectric transformer as much as possible, the thickness can be increased. However, since the distance between the input terminals is long, the capacitance between the input terminals is reduced.

On the other hand, in order to keep the power conversion efficiency of the piezoelectric transformer high, it is necessary to make the voltage waveform between the input terminals as close to a sine wave as possible. For this reason, it is necessary to increase the inductance component of the boosting element by an amount corresponding to the decrease in the capacitance between the input terminals, but the component shape of the boosting element becomes large.

As a technique for increasing the capacitance of the piezoelectric transformer, it is conceivable to alternately laminate the piezoelectric material and the electrodes. However, the structure becomes complicated as compared with the single-plate type, and the manufacturing cost increases significantly. Would.

Further, when the cold cathode tube is installed close to the liquid crystal display panel, a leak current may flow from the cold cathode tube to the metal support structure of the liquid crystal display panel. Such a leak current lowers the power conversion efficiency of the CCFL lighting circuit and affects the luminance distribution of the CCFL. Since the leak current increases in proportion to the frequency of the current flowing through the cold-cathode tube, the drive frequency can be reduced as a countermeasure. Therefore, in order to maintain high power conversion efficiency even when the driving frequency is low, it is necessary to make the voltage waveform applied to the piezoelectric transformer as close to a sine wave as possible.

In order to compensate for the decrease in capacitance due to the increase in the volume of the piezoelectric transformer, or to reduce the drive frequency as a measure against leakage current, it is necessary to arbitrarily set the resonance frequency by some method.

An object of the present invention is to provide a piezoelectric transformer drive circuit that can reduce the size of a boosting element while maintaining high power conversion efficiency and that can set the resonance frequency of the drive circuit arbitrarily.

[0013]

According to the present invention, there is provided a booster comprising an inductance element or a transformer, and a switching element for opening and closing a current flowing through a primary side of the booster at a predetermined frequency. A piezoelectric transformer driving circuit, wherein a secondary side is connected to a pair of input terminals provided in the piezoelectric transformer, wherein a capacitance element is connected in parallel to each input terminal. According to the present invention, by connecting a capacitance element to the input terminal of the piezoelectric transformer in parallel, the capacitance component of the secondary circuit increases as a combined value of the capacitance of the piezoelectric transformer itself and the capacitance element. Therefore, as a measure against the increase in the volume of the piezoelectric transformer and the leakage current, the resonance frequency on the secondary side can be set arbitrarily, and the voltage waveform applied to the piezoelectric transformer is approximated to a sine wave by tuning to the drive frequency of the switching element. And high power conversion efficiency can be maintained. In addition, since it is not necessary to increase the inductance of the boosting element or to stack the piezoelectric transformers, the size and cost of the circuit can be reduced.

Further, the present invention comprises a boosting element comprising an inductance element or a transformer element, and a switching element for opening and closing a current flowing through a primary side of the boosting element at a predetermined frequency. A piezoelectric transformer driving circuit connected to a pair of input terminals provided in a transformer, wherein a capacitance element is connected in parallel to both ends of the switching element. According to the present invention, by connecting the capacitance element in parallel to both ends of the switching element, the capacitance component of the secondary circuit increases as the combined value of the capacitance of the piezoelectric transformer itself and the capacitance element. Therefore, as a measure against the increase in the volume of the piezoelectric transformer and the leakage current, the resonance frequency on the secondary side can be set arbitrarily, and the voltage waveform applied to the piezoelectric transformer is approximated to a sine wave by tuning to the drive frequency of the switching element. And high power conversion efficiency can be maintained. Moreover, since it is not necessary to increase the inductance of the boosting element or to stack the piezoelectric transformers,
Circuit size and cost can be reduced.

Further, the present invention comprises a pair of boosting elements composed of an inductance element or a transformer element, and a pair of switching elements for opening and closing a current flowing on the primary side of each boosting element at a predetermined frequency. Wherein the secondary side of the piezoelectric transformer is connected to a pair of input terminals provided on the piezoelectric transformer, wherein a capacitance element is connected to each primary side of the booster. It is. According to the present invention, a pair of boosting elements and a pair of switching elements are arranged symmetrically with respect to the piezoelectric transformer, and a capacitance element is connected to each primary side of the boosting element, thereby forming a capacitance component of the secondary circuit. Increases as a combined value of the capacitance of the piezoelectric transformer itself and the capacitance element. Therefore, as a measure against the increase in the volume of the piezoelectric transformer and the leakage current, the resonance frequency on the secondary side can be set arbitrarily, and the voltage waveform applied to the piezoelectric transformer is approximated to a sine wave by tuning to the drive frequency of the switching element. And high power conversion efficiency can be maintained. In addition, since it is not necessary to increase the inductance of the boosting element or to stack the piezoelectric transformers, the size and cost of the circuit can be reduced.

Further, according to the present invention, the boosting element is a transformer element, and the capacitance Cx of the capacitance element (combined capacitance when there are a plurality of capacitance elements);
The capacitance Cd between the input terminals of the piezoelectric transformer and the step-up ratio n of the step-up element satisfy the following relationship: 0.1 ≦ Cx / (n ² · Cd) ≦ 10 According to the present invention, Cx /
When the value of (n ² · Cd) is in the range of 0.1 to 10, the power conversion efficiency of the piezoelectric transformer can be kept high.
When this numerical value is smaller than 0.1, the effect of providing the capacitance element is reduced. On the other hand, if this value is larger than 10, the power entering and exiting the capacitance element increases, the secondary circuit becomes capacitive, and the reactive power increases. The reactive power circulates to the power supply side, but loss occurs due to the resistance of wiring and elements on the way, so that the overall power conversion efficiency is reduced.

Further, according to the present invention, the boosting element is an inductance element and the capacitance Cx of the capacitance element is
(The combined capacitance when there are a plurality of capacitance elements) and the capacitance Cd between the input terminals of the piezoelectric transformer satisfy the following relationship: 0.1 ≦ Cx / Cd ≦ 10. According to the present invention, Cx / C
When the value of d is in the range of 0.1 to 10, the power conversion efficiency of the piezoelectric transformer can be maintained high as described above.

[0018]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention. The piezoelectric transformer 12 is formed of an elongated plate-shaped piezoelectric material, and has a pair of input terminals 12a and 12b, which are counter electrodes provided on the front and back surfaces, and an electrode provided on an end face far from the input terminals 12a and 12b. And an output terminal 12c. One of a pair of terminals provided on the cold cathode tube 11 is connected to the output terminal 12c,
The other is grounded. The secondary terminal 3c of the booster 13 is connected to the input terminal 12a, and the twelfth terminal 14c of the booster 14 is connected to the input terminal 12b.

In this embodiment, the capacitance Cx
Is connected to each of the input terminals 12a and 12b.
Are connected in parallel. As a result, the capacitance component of the secondary side circuit becomes the capacitance C of the piezoelectric transformer 12.
d and a combined value of the capacitance Cx of the capacitance element 20 and increase. Therefore, the resonance frequency of the secondary circuit is determined by the combined capacitance value and the combined inductance value of boosting elements 13 and 14.

The boosting elements 13 and 14 are constituted by auto-transformers in which the primary side is drawn from the middle of one coil.
A turn ratio n1: n2 between the number of turns n1 between the common terminals 13b, 14b and the primary terminals 13a, 14a and the number of turns n2 between the common terminals 13b, 14b and the secondary terminals 13c, 14c.
= 1: (n-1), and the voltage is increased at the boost ratio n.

The common terminals 13b and 14b are connected to each other. The positive terminal of the DC power supply 17 is connected to the common terminals 13b and 14b, and the negative terminal is grounded. The primary terminals 13a and 14a are grounded via switching elements 15 and 16 such as transistors, and a pulsed current flows through the primary circuit by opening and closing the switching elements 15 and 16.

In this manner, the pair of boosting elements 13 and 14 and the pair of switching elements 15 and 16
Are symmetrically arranged with respect to.

The operation will be described. Switching elements 15 and 16 are alternately opened and closed synchronously by an external oscillation circuit (not shown). When the switching element 15 is turned on, power is accumulated in the primary circuit of the booster element 13 during the on-period of the pulse, and power on the primary side is released to the secondary circuit during the off-period. Next, when the switching element 16 is turned on, electric power is accumulated in the primary circuit of the booster element 14 during the ON period of the pulse, and the electric power on the primary side is released to the secondary circuit during the OFF period.

As the boosting elements 13 and 14 alternately store and release power, an AC voltage is generated in the secondary circuit, and the inductance components of the boosting elements 13 and 14 and the input terminals 12a and 1
Resonance occurs in a resonance circuit formed by a capacitance component between 2b. Then, a sine wave AC voltage determined by the resonance frequency of the resonance circuit is applied to the input terminals 12a and 12b, mechanical vibration occurs in the piezoelectric transformer 12, and a high sine wave voltage due to the piezoelectric effect is generated from the output terminal 12c. Then, the discharge of the cold cathode tube 11 starts. The voltage applied to the cold cathode tube 11 can be controlled by the switching frequency of the switching elements 15 and 16.

At this time, by selecting the capacitance Cx of the capacitance element 20, the resonance frequency of the secondary circuit can be arbitrarily set.
By tuning to the driving frequencies of 5 and 16, the voltage waveform applied to the piezoelectric transformer 2 can be approximated to a sine wave, and high power conversion efficiency can be maintained.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention. Although the overall circuit configuration and operation are the same as those of FIG. 1, in this embodiment, the capacitance elements 20a and 20b having the capacitance Cx are connected to the respective switching elements 1a and 1b.
5 and 16 are respectively connected in parallel to both ends. As a result, the capacitance component of the secondary circuit is changed by the capacitance Cd of the piezoelectric transformer 12 and the capacitance element 20.
a and 20b increase as a combined value with the capacitance Cx.

Therefore, the capacitance elements 20a,
By selecting a capacitance Cx of 20b,
The resonance frequency of the secondary side circuit can be arbitrarily set, and the voltage applied to the piezoelectric transformer 2 can be approximated to a sine wave by tuning to the drive frequency of the switching elements 15 and 16, thereby achieving high power conversion efficiency. Can be maintained.

FIG. 3 is a circuit diagram showing a third embodiment of the present invention. Although the overall circuit configuration and operation are the same as those in FIG. 1, in the present embodiment, the primary terminals 13a and 14a of the booster elements 13 and 14 symmetrically arranged with respect to the piezoelectric transformer 12 are connected to the capacitance Cx of the capacitance Cx. The elements are connected respectively. Thus, the capacitance component of the secondary circuit increases as a composite value of the capacitance Cd of the piezoelectric transformer 12 and the capacitance Cx of the capacitance elements 20a and 20b.

Therefore, the capacitance elements 20a,
By selecting a capacitance Cx of 20b,
The resonance frequency of the secondary side circuit can be arbitrarily set, and the voltage applied to the piezoelectric transformer 2 can be approximated to a sine wave by tuning to the drive frequency of the switching elements 15 and 16, thereby achieving high power conversion efficiency. Can be maintained.

In each of the above embodiments, the booster 1
Although an example in which an automatic transformer is used as 3 and 14 is shown, it is also possible to use a transformer in which a primary coil and a secondary coil are separated.

FIG. 4 is a perspective view of the piezoelectric transformer 12. The piezoelectric transformer 12 is formed of piezoelectric ceramics,
For example, it is formed in a rectangular shape having a length L = 39.6 mm, a width w = 7.6 mm, and a thickness t = 2.2 mm. Input terminal 1
2a and 12b are formed on the front and back surfaces in a range from the end face to about half the length L, and the output terminal 12c is formed on the opposite end face. With such a shape, the driving frequency is 80
９０90 kHz, the maximum output power is 10 W, and the input capacitance Cd is 820 pF.

FIG. 5 is a graph showing changes in the combined inductance and the power conversion efficiency when the capacitance Cx is changed. The horizontal axis (logarithmic representation) is the ratio Cx / Cd between the capacitance Cx of the capacitance element (combined capacitance when there are a plurality of capacitance elements) and the capacitance Cd of the piezoelectric transformer 12. The solid line is the combined inductance (mH) of the boosting elements 13 and 14, and the left vertical axis (logarithmic display) is referred to. This shows the relationship between the combined inductance and Cx / Cd when the driving frequency of the piezoelectric transformer is matched with the resonance frequency of this circuit in order to increase the efficiency. The broken line is the power conversion efficiency (%) (actually measured value) of the piezoelectric transformer, and the right vertical axis (linear display) is referred to. The cold-cathode tubes used had a diameter of 3 mm and a length of 30 mm and were connected in series.
This corresponds to an output of 8W. The boosting ratios n of the boosting elements 13 and 14 used were 1.

Looking at the graph, the power conversion efficiency shows a hill-shaped curve, with an efficiency of about 90% at the maximum value, and the ratio Cx /
When Cd exceeds 10, it decreases rapidly. The combined inductance decreases as the ratio Cx / Cd increases, and sharply decreases when the ratio Cx / Cd exceeds 0.1. That is, the size of the booster used is reduced.

As described above, the ratio Cx / Cd is preferably in the range of 0.1 to 10, and it can be seen that the power conversion efficiency of the piezoelectric transformer can be kept high. Although the case where the boosting ratio n of the boosting elements 13 and 14 is 1 is shown here, when the boosting ratio n is larger than 1, Cx / Cd is changed to Cx / (n
^A similar theory can be established by substituting ² * Cd).

[0035]

As described above in detail, according to the present invention, by connecting a capacitance element in parallel to the input terminal of the piezoelectric transformer, both ends of the switching element, or each primary side of the boosting element, the circuit of the secondary side circuit is formed. The capacitance component increases as a combined value of the capacitance of the piezoelectric transformer itself and the capacitance element. Therefore, the resonance frequency on the secondary side can be set arbitrarily, and by synchronizing with the drive frequency of the switching element, the combined inductance can be reduced with high power conversion efficiency, and the size of the boosting element can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a third embodiment of the present invention.

4 is a perspective view of the piezoelectric transformer 12. FIG.

FIG. 5 is a graph showing a change in a combined inductance and a power conversion efficiency when a capacitance Cx is changed.

FIG. 6 is a circuit diagram showing an example of a conventional piezoelectric transformer drive circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Cold-cathode tube 12 Piezoelectric transformer 13, 14 Boost element 15, 16 Switching element 17 DC power supply 20, 20a, 20b Capacitance element

Claims (5)

[Claims] 1. A booster element comprising an inductance element or a transformer element, and a switching element for opening and closing a current flowing through a primary side of the booster element at a predetermined frequency, wherein a secondary side of the booster element is provided in a piezoelectric transformer. A piezoelectric transformer driving circuit connected to a pair of input terminals, wherein a capacitance element is connected in parallel to each input terminal. 2. A booster comprising an inductance element or a transformer, and a switching element for opening and closing a current flowing through a primary side of the booster at a predetermined frequency, wherein a secondary side of the booster is provided in a piezoelectric transformer. A piezoelectric transformer driving circuit connected to the pair of input terminals, wherein a capacitance element is connected in parallel to both ends of the switching element. 3. A booster comprising an inductor or a transformer, and a pair of switching elements for opening and closing a current flowing through a primary side of each booster at a predetermined frequency. A piezoelectric transformer driving circuit, the sides of which are connected to a pair of input terminals provided in the piezoelectric transformer, wherein a capacitance element is connected to each primary side of the boosting element. 4. The boosting element is a transformer element, and the capacitance Cx of the capacitance element (combined capacitance when there are a plurality of capacitance elements), the capacitance Cd between the input terminals of the piezoelectric transformer, and the boosting ratio n of the boosting element are: The piezoelectric transformer drive circuit according to any one of claims 1 to 3, wherein the following relationship is satisfied: 0.1 ≦ Cx / (n ² · Cd) ≦ 10. 5. The step-up element is an inductance element, a capacitance Cx of the capacitance element (combined capacitance when there are a plurality of capacitance elements), and a capacitance Cd between input terminals of the piezoelectric transformer.
Satisfies the following relationship: 0.1 ≦ Cx / Cd ≦ 10. The piezoelectric transformer drive circuit according to claim 1, wherein