Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-07-12
2002-06-18
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06407480
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a driving circuit for a piezoelectric transformer, widely used as a step-up transformer for driving the backlight of a liquid crystal display monitor, and the like.
BACKGROUND ART
In recent years, liquid crystal display monitors have been incorporated in portable compact video apparatuses, such as a VCR integrated with a camera and a digital camera. As a step-up transformer for driving a cold cathode tube widely used for the backlight etc. of a liquid crystal display monitor of compact video apparatus, a piezoelectric transformer which can be made thinner, has higher efficiency and can be free from the generation of magnetic line forces has begun to be used in place of an electromagnetic transformer having been used conventionally. The piezoelectric transformer is a voltage conversion device, wherein an input AC voltage is applied to the primary electrode of a piezoelectric device, mechanical vibration is generated by using a piezoelectric effect, and a voltage amplified depending on a voltage step-up ratio determined by the shape of the piezoelectric transformer is taken out from the secondary electrode thereof. The piezoelectric transformer does not cause any leak magnetic fluxes because it does not use a method wherein voltage is changed via magnetic energy by using windings. For this reason, it is advantageous in not causing noise outside its inverter. In addition, since the piezoelectric transformer selects and outputs only the frequency determined by its outer dimensions, its output waveform is close to a sine wave, thereby being advantageous in reducing the occurrence of high-frequency noise. Furthermore, since the piezoelectric transformer is a mineral formed by sintering a ceramic material, it has the advantage of causing no risk of smoking and ignition.
FIG. 61
is a graph showing the general characteristic of a piezoelectric transformer, the abscissa represents the frequency [Hz] of an input voltage, and the ordinate represents an output value [dB].
As shown in
FIG. 61
, the piezoelectric transformer has a resonance characteristic, and the output value obtained from the secondary electrode differs depending on the frequency of the AC voltage input to the primary electrode. Therefore, in the piezoelectric transformer, in order to control the brightness of the backlight at a constant level, the voltage output from the secondary electrode can be adjusted to a desired level by controlling the frequency of the AC voltage input to the piezoelectric transformer. As described above, the voltage having the desired level is output from the secondary electrode of the piezoelectric transformer, whereby a stable voltage is applied to a cold cathode tube. A typical technology for the driving circuit of such a piezoelectric transformer has been disclosed in a Japanese monthly magazine, NIKKEI ELECTRONICS, Nov. 7, 1994 (No. 621), pages 147 to 157.
The configuration of the piezoelectric transformer-driving circuit in accordance with the above-mentioned prior art will be described briefly below by referring to FIG.
62
.
FIG. 62
is a block diagram showing the configuration of the above-mentioned conventional piezoelectric transformer-driving circuit.
In
FIG. 62
, a piezoelectric transformer
101
is a voltage transformation device for obtaining an amplified voltage. A transformer with winding
102
disposed ahead of the piezoelectric transformer
101
and used as a pre-transformer is an electromagnetic transformer provided to supplement the insufficient voltage step-up of the piezoelectric transformer
101
. The voltage from the piezoelectric transformer
101
is applied to a cold cathode tube
103
. A current detector
104
detects the current flowing through the cold cathode tube
103
and converts it into a voltage signal. A rectifying circuit
105
rectifies the sine wave like AC voltage output from the current detector
104
, and converts it into a detection signal of a near DC voltage. A first adder
107
calculates the balance voltage between the detection signal output from the rectifying circuit
105
and a brightness setting voltage, that is, reference data (a reference voltage) which is input externally. An integrator
150
used as a filter circuit integrates the balance voltage output from the first adder
107
and converts it into a DC voltage.
A second adder
180
adds the DC voltage, which is the output of the integrator
150
, to an initial value for determining the oscillation frequency of the piezoelectric transformer
101
at the time of power-on, and outputs a frequency setting voltage. A V-F converter
190
oscillates at a frequency corresponding to the above-mentioned frequency setting voltage. The V-F converter
190
has been set so that the oscillation frequency becomes high when the frequency setting voltage is negative, and so that the oscillation frequency becomes low when the frequency setting voltage is positive. Furthermore, the V-F converter
190
has been set to oscillate at a frequency sufficiently higher than the resonance frequency of the piezoelectric transformer
101
at the time of power-on. A driving circuit
110
formed of a power transistor amplifies the signal output from the V-F converter
190
to drive the winding transformer
102
.
In the driving circuit of the piezoelectric transformer
101
configured as described above, at the time of power-on, the V-F converter
190
oscillates at a frequency higher than the resonance frequency of the piezoelectric transformer
101
, and a voltage lower than the voltage level of the resonance frequency is output from the secondary electrode of the piezoelectric transformer
101
. The voltage output from the secondary electrode of the piezoelectric transformer
101
is applied to the cold cathode tube
103
.
A current in proportion to the applied voltage flows through the cold cathode tube
103
, and the current flowing through the cold cathode tube
103
is converted into a voltage by the current detector
104
, and is further converted into a nearly DC voltage by the rectifying circuit
105
.
In the above-mentioned configuration, when the reference voltage to be supplied externally has a voltage level shown at point A of the characteristic curve shown in FIG.
61
and when the voltage obtained from the rectifying circuit
105
is the voltage at point B of the characteristic curve shown in
FIG. 61
, the balance voltage at the first adder
107
becomes positive. For this reason, the input voltage of the V-F converter
190
rises gradually, and the oscillation frequency output from the V-F converter
190
begins to lower. This operation raises the level of the voltage output from the piezoelectric transformer
101
and increases the current flowing through the cold cathode tube
103
.
On the other hand, when the voltage obtained from the rectifying circuit
105
is the voltage at point C of the characteristic curve shown in
FIG. 61
, the balance voltage at the first adder
107
becomes negative, and the oscillation frequency output from the V-F converter
190
rises. This operation lowers the voltage level output from the piezoelectric transformer
101
and reduces the current flowing through the cold cathode tube
103
.
As described above, the conventional piezoelectric transformer-driving circuit feeds back the current flowing through the cold cathode tube
103
and controls the oscillation frequency so that the current value is equal to the level of the reference data supplied externally thereby stabilizing the brightness of the backlight.
Furthermore, as another method of stabilizing the brightness of the backlight, methods have been disclosed in the Japanese published unexamined Utility Model Application No. Hei 4-58085 and the Japanese published unexamined Utility Model Application No. Hei 5-4779. In both of these methods, control is carried out at the resonance frequency at which the piezoelectric device has the highest efficiency. The principle thereof uses the fact that the phase difference between the voltage phase and
Kobayashi Takahiro
Nakanishi Hideyuki
Budd Mark O.
Pearne & Gordon LLP
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