Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-05-31
2003-01-21
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C315S2090PZ
Reexamination Certificate
active
06509671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving method and a driving circuit for a piezoelectric transformer, a cold cathode tube emission device using a cold cathode tube as a load for a piezoelectric transformer, a liquid crystal panel whose luminance is controlled by the cold cathode tube emission device incorporated therein and an apparatus provided with the liquid crystal panel such as a mobile telephone, a communication terminal etc.
2. Description of the Related Art
In the following, a driving circuit using a conventional piezoelectric transformer will be explained.
Generally in a piezoelectric transformer, due to the impedance of a load connected to the secondary side, a voltage step-up ratio indicating a voltage to be output to the secondary side relative to a voltage entering the primary side is changed, and the driving efficiency, shown by the electric power sent to the secondary side relative to the electric power entering the primary side, also is changed, so that a driving frequency for obtaining a maximum voltage step-up ratio and maximum driving efficiency also is changed. In other words, in order to drive a piezoelectric transformer efficiently at a predetermined voltage step-up ratio, the driving frequency must be set according to the impedance of the load to be connected.
For example, in the case of using a cold cathode tube as a load for a piezoelectric transformer, since a cold cathode tube generally shows a high impedance of not less than several hundreds of M &OHgr; until it starts to light and an abrupt impedance decrease from several hundreds of &OHgr; to several tens of &OHgr; after lighting, in order to light the cold cathode tube efficiently by using the piezoelectric transformer, the frequency and the voltage level of an AC voltage applied to the primary side of the piezoelectric transformer should be changed before and after lighting.
As a conventional technique to achieve this purpose, cold cathode tube driving devices disclosed in JP 6(1994)-167694A and others are known, and
FIG. 6
shows a block diagram of a driving device disclosed in this publication.
In
FIG. 6
, an output signal from a free-running multivibrator
106
is amplified by a current amplification circuit
107
, and the output signal whose voltage is stepped up further by a wire-wound transformer
108
, if necessary, is applied to the primary side of a piezoelectric transformer
101
. A cold cathode tube
102
is connected as a load to the secondary side output of the piezoelectric transformer
101
, and an electric current flowing in the cold cathode tube
102
is detected by a load current detection circuit
109
. The detected current level is converted into a voltage and is input to one of the input terminals of an integration circuit
104
via an AC voltage rectification circuit
110
. To the other input terminal, a signal from a variable voltage device
103
is supplied, so that an oscillating frequency of the free-running multivibrator
106
is controlled from the integration circuit
104
via a voltage level shift circuit
105
.
In order to light the cold cathode tube
102
serving as the load of the piezoelectric transformer
101
, the voltage applied to the piezoelectric transformer
101
is set by the variable voltage device
103
and the voltage level shift circuit
105
etc., and a driving frequency of the piezoelectric transformer
101
is swept so as to light the cold cathode tube
102
. After lighting, the driving frequency of the piezoelectric transformer
101
is swept further, and furthermore, according to the current level detected by the load current detection circuit
109
etc., the voltage applied to the piezoelectric transformer
101
is controlled by the variable voltage device
103
and the voltage level shift circuit
105
etc., so that the emission luminance of the cold cathode tube
102
is adjusted.
When a load with a variable impedance such as a cold cathode tube is connected to a piezoelectric transformer, according to a conventional driving method, before the cold cathode tube starts to light, an AC voltage having a large amplitude corresponding to the load in a high impedance state is applied to the secondary side of the piezoelectric transformer at a frequency that is higher than a resonance frequency in an open state. The impedance of the cold cathode tube is reduced according to a change in the lighting state, and the flowing electric current is increased. The cold cathode tube can be lit steadily by detecting this electric current flowing in the cold cathode tube and sweeping the frequency to its low frequency side, and also by changing the amplitude of the applied voltage to become smaller. As a result, there is a problem that the piezoelectric transformer must be driven in the state of low driving efficiency.
Furthermore, corresponding to the high impedance of the cold cathode tube before it starts to light, a high voltage is applied to the piezoelectric transformer. However, corresponding to the reduction of the impedance due to the lighting of the cold cathode tube, a control operation to reduce the applied voltage is performed, so that there is a possibility of momentarily applying a high voltage to the cold cathode tube in a low impedance state. At this time, a strong distortion occurs in the piezoelectric transformer due to a large electric current flowing in the piezoelectric transformer. In particular, when the electric power per volume of the piezoelectric transformer is large, the distortion effected on the piezoelectric transformer may destroy the piezoelectric transformer itself or cause mechanical damage leading to the destruction.
Furthermore, a piezoelectric transformer has a characteristic variance arising from the shape or the material property etc. with respect to frequency characteristics of impedance or an admittance seen from the primary side, or a resonance frequency etc.
FIG. 7
is a graph showing the relationship of a voltage step-up ratio and driving efficiency relative to a driving frequency of a piezoelectric transformer. In
FIG. 7
, the horizontal axis shows driving frequency of the piezoelectric transformer, and the vertical axis on the left side shows a voltage step-up ratio indicating a ratio of a voltage output from the secondary side relative to the voltage applied to the primary side of the piezoelectric transformer, and furthermore, the vertical axis on the right side shows driving efficiency, indicating a ratio of electric power output from the secondary side relative to the electric power applied to the primary side of the piezoelectric transformer.
FIG. 7
shows that the voltage step-up ratio and the frequency efficiency of the piezoelectric transformer are variable within a certain form tolerance.
In the case where frequency characteristics of the voltage step-up ratio or the driving efficiency relative to the driving frequency are different as shown in
FIG. 7
, for example, a piezoelectric transformer achieving a voltage step-up ratio of a maximum value &ggr;
max
with a driving frequency f
&ggr;2
can be driven with maximum driving efficiency &eegr;
max
by driving at a frequency f
&eegr;2
. However, when it is driven at a frequency f
&eegr;1
or a frequency f
&eegr;3
, the driving efficiency becomes lower than &eegr;
max
.
In this way, due to the characteristic variance arising from the shape or the material property etc. of the piezoelectric transformer, there is a problem that the piezoelectric transformer cannot be driven with the maximum driving efficiency only by using the driving frequency or the driving voltage predetermined by the driving circuit.
Furthermore, when a liquid crystal panel with a built-in cold cathode tube driving device is incorporated into an apparatus such as a mobile telephone or communication equipment etc., a sweep of the driving frequency becomes a problem. In other words, when a sweep of the driving frequency is performed before starting to light over to the lighting state not continuously but switching the frequency
Moritoki Katsunori
Nakatsuka Hiroshi
Takeda Katsu
Yamaguchi Takeshi
Budd Mark O.
Matsushita Electric - Industrial Co., Ltd.
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