Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-06-05
2003-06-24
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S316010, C310S366000
Reexamination Certificate
active
06583534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a piezoelectric transformer, a piezoelectric transformer drive circuit and a piezoelectric transformer drive method used for various high-voltage generation apparatuses.
Furthermore, the present invention relates to a cold cathode tube drive apparatus using a piezoelectric transformer used for various high-voltage generation apparatuses, more particularly to a cold cathode tube drive apparatus using a piezoelectric transformer having sensor electrodes provided independently of primary and secondary electrodes.
2. Related Art of the Invention
FIG. B
18
shows the structure of a Rosen-type piezoelectric transformer, atypical structure of a conventional piezoelectric transformer. This piezoelectric transformer has the advantages that it can be made more compact than an electromagnetic transformer, is noncombustible and does not cause noise due to electromagnetic induction.
The portion designated by
1001
is the low impedance portion of the piezoelectric transformer and used as an input portion in the case when the transformer is used for voltage step-up. The low impedance portion
1001
is polarized in a thickness direction A. Primary electrodes
1003
U and
1003
D are disposed on the main faces of the low impedance portion in the direction the thickness thereof. On the other hand, the portion designated by
1002
is a high impedance portion and used as an output portion in the case when the transformer is used for voltage step-up. The high impedance portion
1002
is polarized in the longitudinal direction B. A secondary electrode
1004
is disposed at the end face in the longitudinal direction.
FIG. B
19
, detailed later, is a graph showing the characteristic of the above-mentioned piezoelectric transformer When the load of the piezoelectric transformer is infinite (indicated by a curve P
1
in FIG. B
19
), it is possible to obtain a very high step-up ratio in the case when the drive frequency of the piezoelectric transformer is equal to the resonance frequency thereof. On the other hand, when the load becomes small (indicated by a curve P
2
in FIG. B
19
), the step-up ratio lowers. Because of this characteristic, the piezoelectric transformer has been used as the power sources for cold cathode tubes in recent years. A cold cathode tube drive apparatus using a piezoelectric transformer can efficiently generate a high voltage. However, since it can easily generate a high voltage, if the piezoelectric transformer is controlled improperly, an overvoltage may generate from the piezoelectric transformer, resulting in the breakdown of the piezoelectric transformer and the like. To prevent this kind of breakdown and the like, it is proposed to provide an overvoltage protection circuit for the cold cathode tube drive apparatus.
FIG. B
20
is a block diagram showing the configuration of a cold cathode tube drive apparatus using a conventional piezoelectric transformer. In FIG. B
20
, numeral
1193
designates a variable oscillation circuit generating an AC drive signal for driving a piezoelectric transformer
1200
. The output of the variable oscillation circuit
1193
is usually a pulse waveform signal. The high-frequency components of the pulse waveform signal is eliminated by a waveform shaping circuit
1191
, whereby the pulse waveform signal is converted into an AC signal close to a sine wave signal. The output of the waveform shaping circuit
1191
is voltage-amplified to a level enough to drive the piezoelectric transformer
1200
by a drive circuit
1192
and input to the primary electrode (indicated by
1003
U in FIG. B
18
) of the piezoelectric transformer
1200
. The output voltage stepped up by the piezoelectric effect of the piezoelectric transformer
1200
is taken out from its secondary electrode (indicated by
1004
in FIG. B
18
)
The high voltage output from the secondary electrode is applied to a series circuit comprising a cold cathode tube
1197
and a feedback resistor
1198
and to an overvoltage protection circuit portion
1190
. In the over voltage protection portion
1190
, a voltage divider circuit comprising voltage division resistors
1199
a
and
1199
b
divides the high voltage output from the secondary electrode of the piezoelectric transformer
1200
. A comparison circuit
1195
compares the voltage divided by the voltage divider circuit with a set value Vref
1
and generates an error voltage. The error voltage output from the comparison circuit
1195
is applied to an oscillation control circuit
1194
. The oscillation control circuit
1194
controls the variable oscillation circuit
1193
so that the high voltage output from the secondary electrode of the piezoelectric transformer
1200
is equal to Vref
1
×(electric resistance value of the resistor
1199
a
+electric resistance value of the resistor
1199
b
)/electric resistance value of the resistor
1199
a
. The oscillation control circuit
1194
does not accept the output from the overvoltage protection circuit
1190
while the cold cathode tube
1197
is lit.
Furthermore, the voltage (current detection value) generated across the feedback resistor
1198
by the current flowing through the series circuit comprising the cold cathode tube
1197
and the feedback resistor
1198
is applied to a comparison circuit
1196
. The comparison circuit
1196
compares the current detection value with a set value Vref
2
and outputs an error voltage. The error voltage output from the comparison circuit
1196
is applied to the oscillation control circuit
1194
. The variable oscillation circuit
1193
is controlled by the oscillation control circuit
1194
so that a nearly constant current flows through the cold cathode tube
1197
.
As described above, the oscillation control circuit
1194
operates on the basis of the output from the comparison circuit
1195
before the lighting start of the-cold cathode tube
1197
, and the oscillation control circuit
1194
operates on the basis of the output from the comparison circuit
1196
while the cold cathode tube
1197
is lit.
In this way, the cold cathode tube
1197
is lit stably. Even if the resonance frequency is changed depending on the change in the load of the piezoelectric transformer, ambient temperature and the like, the drive frequency can follow the resonance frequency automatically by driving the cold cathode tube
1197
using the above-mentioned drive apparatus.
Next, the operation of this drive apparatus will be described referring to FIG. B
19
. FIG. B
19
is a graph showing the operation characteristic of the piezoelectric transformer. As clearly shown in FIG. B
19
, the step-up ratio has the maximum value at the resonance frequency according to the operation characteristic of the piezoelectric transformer. Usually, drive control is carry out by using a frequency higher than the resonance frequency of the piezoelectric transformer.
When driving the piezoelectric transformer, its drive frequency is set at a frequency (fa) higher than the resonance frequency at the time of start. When the voltage divided by the voltage division resistors
1199
a
and
1199
b
is smaller than the set voltage Vref
1
, the drive frequency is lowered close to the resonance frequency by the oscillation control circuit
1194
and the variable oscillation circuit
1193
. When the drive frequency is close to the resonance frequency, the step-up ratio of the piezoelectric transformer increases, and its output voltage rises When the output voltage reaches the lighting start voltage (Vb) of the cold cathode tube
1197
, the cold cathode tube
1197
is lit. As a result, the load of the piezoelectric transformer lowers from an infinite value to about several hundred k&OHgr;. Therefore, the operation characteristic of the piezoelectric transformer shifts from the curve P
1
to curve P
2
.
Accordingly, the operation of the oscillation control circuit
1194
is shifted from the operation depending on the output of the comparison circuit
1195
to the operation depending on the output of
Asahi Toshiyuki
Kawasaki Osamu
Moritoki Katsunori
Nakatsuka Hiroshi
Okuyama Kojiro
Budd Mark O.
Matsushita Electric - Industrial Co., Ltd.
Smith , Gambrell & Russell, LLP
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