Piezoelectric-transformer inverter

Details Piezoelectric-transformer inverter Piezoelectric-transformer inverter

1999-04-14

2001-11-20

Ramirez, Nestor (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S318000

Reexamination Certificate

active

06320301

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric-transformer inverter for converting DC voltage to AC voltage through use of a piezoelectric transformer.
2. Description of the Related Art
Recently, back lit liquid crystal displays have come into general use as display units for portable information processing apparatus such as laptop personal computers. A fluorescent tube, such as a cold-cathode tube, is used as a light source of such a back light. In order to light the fluorescent tube, a high AC voltage must be applied thereto. Because a battery and an AC adapter are generally used as an input power source of the portable information processing apparatus such as a laptop personal computer, a DC/AC inverter must be used to convert a low DC voltage supplied from the input power source to an AC voltage that is sufficiently high to power the fluorescent tube.
Recently, there has been developed a piezoelectric-transformer inverter which uses a piezoelectric transformer that is smaller than an electromagnetic transformer. In order to use such transformer to convert power for the fluorescent tube, a piezoelectric transformer is sometimes required to meet the following requirements:
(1) it must be operable at a low voltage; specifically, that output by one lithium-ion cell; and
(2) it must be small and thin.
In relation to Requirement (1): In general, a voltage of 700 V (peak-to-peak voltage) or higher is needed in a steady state to light a fluorescent tube such as a cold-cathode tube, although the specific value depends on the length of the fluorescent tube. However, the voltage of one lithium-ion cell is only about 2.5 to 4 V. To compensate for this difference, the inverter must provide a step-up ratio within the range of 175 (=700/4) to 300 (=700/2.5). Since the luminance efficiency (luminance/power consumption of the cold cathode tube) of the cold cathode tube, including peripheral panel parts, is considered to be highest at a frequency of about 50 to 100 kHz, the drive frequency must generally fall within the range of 50 to 100 kHz.
In relation to Requirement (2): A Rosen-type piezoelectric transformer, which is a basic piezoelectric transformer, will be described with reference to FIG.
1
. In a Rosen-type piezoelectric transformer
1
, primary electrodes
3
are formed in one half of a piezoelectric substrate
2
, formed of piezoelectric ceramics. The primary electrodes
3
are formed in the longitudinal direction, on top and bottom main faces of the piezoelectric substrate
2
. The piezoelectric substrate
2
is polarized in a direction perpendicular to the surfaces of primary electrodes
3
(in the thicknesswise direction of the piezoelectric substrate
2
) in this area. In the other half of the piezoelectric substrate, the piezoelectric substrate
2
is polarized in the longitudinal direction, and a secondary electrode
4
is formed on an end surface adjacent to the second half region. The directions of polarization of the piezoelectric substrate
2
are shown by arrows P (arrows P are also used to indicate directions of polarization in the drawings relating to the description that follows). In the Rosen-type piezoelectric transformer
1
, when an AC voltage output from an input power source
6
is applied to the primary electrodes
3
which face each other with the piezoelectric substrate
2
disposed therebetween the applied voltage is converted to a mechanical distortion. This distortion excites a mechanical vibration in the longitudinal direction of the Rosen-type piezoelectric transformer
1
, and the mechanical vibration is then converted to an electrical vibration. Thus, a transformer function is realized, so that a stepped-up voltage is applied to a fluorescent tube
5
serving as a load.
FIG. 2
shows possible vibration modes in the longitudinal direction of the Rosen-type piezoelectric transformer
1
. Since the opposite ends of the piezoelectric transformer
1
are open ended as shown in (a), the piezoelectric transformer
1
can vibrate in a &lgr;/2 (half wavelength) mode, a &lgr; mode, or a
3
&lgr;/2 mode, which are shown at (b), (c) and (d), respectively, of FIG.
2
and vibrates at a lowest frequency in the &lgr;/2 mode. When the vibration frequency of the &lgr;/2 mode is represented by fo, the vibration frequencies of the &lgr; mode and the
3
&lgr;/2 mode are represented by
2
fo and
3
fo, respectively. In the &lgr;/2 mode, the &lgr; mode, and the
3
&lgr;/2 mode, the length L of the piezoelectric transformer
1
and the wavelength &lgr; satisfy the equations L=&lgr;/2, L=&lgr;, and L=
3
&lgr;/2, respectively. In other words, when the piezoelectric transformer is driven at a specific frequency (for example, in the frequency range of 50 to 100 kHz), the size of the piezoelectric transformer can be advantageously decreased through employment of the &lgr;/2 mode.
However, in practice, &lgr;-mode piezoelectric transformers have conventionally been used more often than have &lgr;/2-mode piezoelectric transformers, because &lgr;/2-mode piezoelectric transformers have the following drawbacks. First, the step-up ratios (=output voltage/input voltage) of &lgr;/2-mode piezoelectric transformers are generally lower than those of &lgr;-mode piezoelectric transformers. In addition, when voltage input to a piezoelectric transformer is not sinusoidal, a &lgr;-mode vibration is also generated as a harmonic of a &lgr;/2-mode vibration. As a result, distortion of the output voltage or current increases. In general, in a piezoelectric-transformer inverter used for a back light of a liquid crystal display, current flowing through the fluorescent tube is monitored, and frequency control is performed in order to maintain the peak value of the monitored current at a constant level. However, when the distortion of waveform is large, the root-mean-square value of the current changes even if the peak value of the current is maintained constant. Therefore, the current-controlling performance is poor.
First Conventional Example
In order to solve the above-described problems and to meet the requirements of piezoelectric-transformer inverters, various improved piezoelectric-transformer inverters and improved piezoelectric transformers have been proposed, as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 9-107684, 9-56175, and 9-74236.
FIG. 3
shows a piezoelectric-transformer inverter disclosed in Japanese Patent Application Laid-Open No. 9-107684. This piezoelectric-transformer inverter
11
is formed of a piezoelectric transformer
13
, a frequency control circuit
14
, a step-up circuit (drive circuit)
15
, a drive voltage control circuit
16
, and a light adjustment circuit
17
. The piezoelectric transformer
13
applies a voltage to a fluorescent tube
12
. The frequency control circuit
14
detects current that is supplied from the secondary electrode of the piezoelectric transformer
13
to the fluorescent tube
12
, and controls the drive frequency of the piezoelectric transformer
13
in order to maintain the detected current at a predetermined level. The step-up circuit
15
divides the drive frequency, generates a drive voltage having a divided frequency, and supplies the generated drive voltage to the primary electrodes of the piezoelectric transformer
13
. The drive voltage control circuit
16
controls the drive voltage such that the drive voltage applied to the piezoelectric transformer
13
is maintained at a predetermined constant level even when the input power source voltage V
DD
changes. The light adjustment circuit
17
controls averaged load current (tube current) flowing through the fluorescent tube
12
, by means of PWM control.
In the piezoelectric-transformer inverter
11
as well, the drive frequency is controlled by the frequency control circuit
14
and the like such that the output current is maintained constant. The step-up circuit
15
is constructed such that push-pull operation (quasi-E-class operation) is realized through use

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