JPH11299247A - Piezoelectric transformer type power converting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the heat generations of the auto-transformers of a piezoelectric transformer type power converting apparatus, by providing its first auto-transformer whose secondary side terminal is connected with one input electrode of its piezoelectric transformer and whose intermediate terminal is connected with its input power supply, and by providing its second auto-transformer whose secondary side terminal is connected with the other input electrode of its piezoelectric transformer and whose primary side terminal is connected with the primary side terminal of the first auto-transformer. SOLUTION: In this apparatus, using two auto-transformers, they are so connected in series with each other that the same current is made to flow in respective auto- transformers and they halve their applied voltage with each other. Also, the inductance per one auto-transformer is made small to make the magnetic core small. Then, by the resonance of the inductances of auto-transformers 40, 45 and the input electrostatic capacitance of a piezoelectric transformer 50, a DC input voltage V1 is converted into a half-wave-form sinusoidal voltage V2 , and further, the voltage V2 is changed increasingly into an excitation voltage V3 by the turn ratios of the auto-transforms 40, 45 to be applied across input electrodes 51, 52 of the piezoelectric transformer 50. Thereby, the heat generations of the auto-transformers 40, 45 can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric transformer type power converter used for an inverter for a liquid crystal backlight.

[0002]

2. Description of the Related Art Generally, in a liquid crystal display, a liquid crystal display does not emit light by itself, and thus a backlight system in which a cold cathode tube is arranged on the back and side surfaces of a liquid crystal display is mainly used. To drive this cold cathode tube, it is necessary to apply an AC high voltage of several hundred volts or more, depending on its own length and diameter. The power converter that generates the AC high voltage is generally called a backlight inverter. Japanese Patent Application Laid-Open No. 52-113578 discloses a method of generating an AC high voltage by using a piezoelectric transformer to fire and operate a discharge tube.

As this type of piezoelectric transformer, for example,
In 956, C.I. A. A Rosen-type piezoelectric transformer announced by Rosen is used. As shown in FIG. 5, this piezoelectric transformer has a pair of input electrodes 4 provided on the upper and lower surfaces of the left half of a plate-shaped piezoelectric ceramic element 2 made of, for example, lead zirconate titanate (PZT) by silver baking or the like. 5 and an output electrode 6 provided on the right half end face in the same manner. As shown by the arrow in FIG.
The driving portion of the left half of the ceramic element 2 is in the thickness direction,
The right half power generation section is subjected to polarization processing in the longitudinal direction.

In the Rosen-type piezoelectric transformer, when an AC voltage having a frequency substantially equal to the longitudinal resonance frequency of the ceramic element 2 is applied between the input electrodes 4 and 5 from the AC voltage source 8, the ceramic element 2 is moved in the longitudinal direction. A strong mechanical vibration is generated, and electric charges are generated by the piezoelectric effect in the right half of the power generation unit, so that an output voltage V ₀₁ is generated between the output electrode 6 and one input electrode, for example, the input electrode 5.

Devices using a liquid crystal display include a notebook personal computer, a mobile personal computer, and a personal digital assistant. These devices operate on a built-in battery or on a supply voltage from an AC adapter. There is. At this time, for example, the voltage value may drop to 7 V when the internal battery is operated, and may become 20 V when the AC adapter is supplied. In this case, 7V to 20V is the input voltage range of the backlight inverter.

A conventional example of a piezoelectric transformer type backlight inverter will be described with reference to FIG. In this inverter, an output current Io is detected by an output current detection unit 70, a frequency is changed by a frequency variable unit 80 based on the detected voltage, and a MOSFET 20 as a main switch is driven via a drive unit 10. Due to the resonance between the inductance of the autotransformer 30 and the input capacitance of the piezoelectric transformer 50, the DC input voltage V1 is converted into a half-wave sinusoidal voltage V2 as shown in FIG. Further, V2 is determined by the winding ratio of the single-turn transformer 30.
Is increased to the excitation voltage V3 and applied between the input electrodes 51 and 52 of the piezoelectric transformer 50. Here, the primary terminal 31
The number of windings between the secondary terminal 33 and the primary terminal 3 is N13.
Assuming that the number of turns between 1 and the intermediate terminal 32 is N12, the turn ratio of the single turn transformer 30 is N13 / N12. When the frequency of the excitation voltage V3 reaches near the resonance frequency of the piezoelectric transformer 50, the piezoelectric transformer 50 outputs a high voltage Vo, and the cold-cathode tube 60 is turned on. The voltage V2 is obtained by resonance between the inductance of the autotransformer 30 and the input capacitance of the piezoelectric transformer.
Is a half-wave sine wave, the transition loss of the MOSFET 20 is extremely small, and the input capacitance of the piezoelectric transformer 50 is also smoothly charged and discharged by resonance, so that a large dielectric loss caused by steep charging and discharging is drastically reduced. Single transformer 3
0 corresponds to the case where the boosting ratio of the piezoelectric transformer 50 is insufficient and the cold-cathode tube cannot be lit, and the lack of the piezoelectric transformer boosting ratio is compensated by the boosting ratio of the single-turn transformer.

[0007]

However, the input voltage of the backlight inverter fluctuates over a wide range as described above. Therefore, the loss of the single-turn transformer 30 shown in FIG. 3 is large when a low input is made because a large current flows through the winding, so that the winding loss (copper loss) is large. The core loss (iron loss) increases. As a result, the autotransformer generates a considerable amount of heat, which raises the ambient temperature, causing fluctuations in the resonance frequency, and the backlight inverter cannot maintain stable operation. there were.

Further, with the miniaturization of the notebook personal computer, there is a strong demand for downsizing of the backlight inverter, and each component mounted on the backlight inverter is necessarily reduced in size. Reducing the size of the autotransformer 30 to a small size only increases heat generation due to the above-mentioned loss, and cannot be put to practical use. For this reason, the shape and dimensions of the single-turn transformer have been an obstacle in downsizing, particularly, thinning the backlight inverter.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a piezoelectric transformer type power conversion device which suppresses heat generation of an autotransformer, is small in size and thin, and has stable operation, focusing on the above-mentioned problems.

[0010]

According to the present invention, there is provided a piezoelectric transformer for receiving an AC voltage from an input electrode and outputting the AC voltage from an output electrode by utilizing a piezoelectric effect, and a secondary terminal connected to one of the input electrodes of the piezoelectric transformer. A first autotransformer having an intermediate terminal connected to an input power supply, and a secondary terminal connected to the other input electrode of the piezoelectric transformer, and a primary terminal connected to a primary terminal of the first autotransformer. A piezoelectric unit having a second autotransformer connected thereto, switch means connected to an intermediate terminal of the second autotransformer, and a drive unit for generating a drive voltage of a predetermined frequency to drive the switch means. It is a transformer type power converter.

In the present invention, the number of single-wound transformers is reduced from two to one, thereby minimizing the size of the single-wound transformer and suppressing heat generation due to power loss. That is, since two autotransformers are connected and used in series, the same current flows through each autotransformer, and the applied voltage is divided into approximately two equal parts. Therefore, the power loss is equally divided into two and borne by each of the autotransformers. Further, since the inductance per single winding transformer can be smaller than before, the magnetic core can be reduced, and the winding of the winding can be reduced. For this reason, the power loss per autotransformer is significantly reduced as compared with the conventional one, and the autotransformer can be reduced in size and the heat generation can be suppressed.

Further, only the AC component is applied to the input section of the piezoelectric transformer. For this reason, even if the electrode material of the piezoelectric transformer element, such as silver, which is likely to cause migration, is used, no migration occurs because there is no DC component.
Therefore, the reliability of the piezoelectric transformer type power converter can be improved.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a schematic circuit diagram of a piezoelectric transformer type backlight inverter according to a first embodiment of the present invention. In the figure, the output current Io is detected by the output current detection unit 70, and the frequency variable unit 80
And the frequency is varied, and the MOSFET 20 as the main switch is driven via the drive unit 10. Single transformer 4
Due to the resonance between the inductances 0 and 45 and the input capacitance of the piezoelectric transformer 50, the DC input voltage V1 is converted into a half-wave sinusoidal voltage V2. In addition, single-winding transformers 40 and 45
V2 is boosted to the excitation voltage V3 by the turn ratio, and is applied between the input electrodes 51 and 52 of the piezoelectric transformer 50. When the frequency of the half-wave sine-shaped voltage V2 approaches the resonance frequency of the piezoelectric transformer 50, the piezoelectric transformer 50 outputs a high voltage Vo, and the cold-cathode tube 60 is turned on.

FIG. 2 is a schematic circuit diagram of a piezoelectric transformer type backlight inverter according to a second embodiment of the present invention. The difference from the first embodiment shown in FIG. 1 is that the MOSFET serving as the main switch is an N-channel type in FIG. 1, whereas a P-channel type in FIG.
The operation of the circuit is the same as in the first embodiment. The present invention can be applied irrespective of the structure of the piezoelectric transformer. In addition to the Rosen type shown in FIG. 5, a laminated structure type (see, for example, JP-A-8-69890), or a drive unit is disposed at the center. Structure (for example, refer to JP-A-9-74236)
It can also be applied to

[0015]

EXAMPLE A conventional piezoelectric transformer-type backlight inverter shown in FIG. 3 and a piezoelectric transformer-type backlight inverter of the present invention shown in FIG. 1 were fabricated, and the temperature rise of the single-turn transformer was measured and evaluated. Table 1 shows the results.

[0016]

[Table 1]

Example 1 in Table 1 is a conventional inverter,
One single-winding transformer having an outer dimension of 17 mm long × 8 mm wide × 5.5 mm high is used. Example 2 is an inverter according to the present invention, which is 13.5 mm long × 12 mm wide × 3 height. Two 5 mm single winding transformers were used. The external dimensions of the inverter according to the present invention are 10 mm longer than those of the conventional one due to the external dimensions of the single-winding transformer, but the height can be reduced and the thickness can be reduced. Also, the volume occupied by the inverter has been reduced to about 74% of the conventional one. Further, the temperature rise was drastically reduced from 59 ° C. to 23 ° C. due to a decrease in the heat generated by the single turn transformer.

[0018]

As described above, according to the present invention, it is possible to obtain a piezoelectric transformer type power conversion device which suppresses heat generation of the autotransformer, is small in size and thin, and has stable operation.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram of a first piezoelectric transformer type backlight inverter according to the present invention.

FIG. 2 is a schematic circuit diagram of a second piezoelectric transformer type backlight inverter according to the present invention.

FIG. 3 is a main circuit diagram showing a conventional piezoelectric transformer type backlight inverter.

FIG. 4 is a diagram showing a voltage waveform of a piezoelectric transformer type backlight inverter.

FIG. 5 is an explanatory diagram showing the operating principle of a Rosen-type piezoelectric transformer.

[Explanation of symbols]

10 drive unit, 20 MOSFET, 21 MOSFE
T 30 Single-winding transformer, 40 Single-winding transformer, 41 Primary-side terminal 42 Intermediate terminal, 43 Secondary-side terminal, 45 Single-winding transformer 46 Primary-side terminal, 47 Middle-side terminal, 48 Secondary-side terminal 11 Piezoelectric transformer, 51 Input electrode , 52 input electrode 60 cold-cathode tube, 70 output current detection unit, 80 frequency variable unit

Claims (1)

[Claims] 1. A piezoelectric transformer for receiving an AC voltage from an input electrode and outputting the AC voltage from an output electrode using a piezoelectric effect, a secondary terminal connected to one of the input electrodes of the piezoelectric transformer, and an intermediate terminal serving as an input power supply. A connected first autotransformer and a second autotransformer having a secondary terminal connected to the other input electrode of the piezoelectric transformer and a primary terminal connected to a primary terminal of the first autotransformer. And a switch unit connected to an intermediate terminal of the second autotransformer; and a drive unit for generating a drive voltage of a predetermined frequency to drive the switch unit. Conversion device.