Method and unit for driving piezoelectric transformer used...

Details Method and unit for driving piezoelectric transformer used... Method and unit for driving piezoelectric transformer used...

2001-04-13

2002-08-13

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S317000, C315S2090PZ

Reexamination Certificate

active

06433458

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and unit for driving a piezoelectric transformer used in various high-voltage transformer assemblies.
2. Related Background Art
FIG. 14
shows a configuration of a Rosen-type piezoelectric transformer that is a typical configuration of conventional piezoelectric transformers. This piezoelectric transformer has advantages of, for example, having a smaller size than that of an electromagnetic transformer, being incombustible, and generating no noise caused by electromagnetic induction.
In
FIG. 14
, a portion indicated with numeral
1
is a low impedance portion of the piezoelectric transformer and functions as an input part when the piezoelectric transformer is used for voltage step-up. The low impedance portion
1
is polarized in the thickness direction (PD), and electrodes
3
U and
3
D are disposed on its principal planes in the thickness direction. On the other hand, a portion indicated with a numeral
2
is a high impedance portion and functions as an output part when the piezoelectric transformer is used for voltage step-up. The high impedance portion
2
is polarized in the longitudinal direction (PL) and an electrode
4
is disposed on an end face in the longitudinal direction.
A piezoelectric transformer as shown in
FIG. 14
has characteristics that a very high step-up ratio can be obtained under an infinite load and the step-up ratio decreases with reduction in load. Due to those characteristics, recently such a piezoelectric transformer has been used as a power supply for a cold-cathode tube. An inverter with a piezoelectric transformer can generate a high voltage efficiently.
FIG. 15
is a block diagram showing a configuration of a conventional self-oscillation type drive for a piezoelectric transformer. In
FIG. 15
, numeral
13
indicates a variable oscillation circuit for producing a variable-frequency voltage signal. A voltage signal output from the variable oscillation circuit
13
generally has a pulse waveform. A high-frequency component in the voltage signal is removed by a wave shaping circuit
11
and thus the voltage signal is converted into an AC signal with a substantially sinusoidal waveform. An output signal from the wave shaping circuit
11
is converted to a voltage, the voltage is amplified to a sufficient level to drive a piezoelectric transformer
10
by a drive circuit
12
, and then the voltage thus amplified is input to one primary side electrode
3
U of the piezoelectric transformer
10
. The other primary side electrode
3
D of the piezoelectric transformer
10
is connected to a ground potential. A voltage stepped up by a piezoelectric effect of the piezoelectric transformer
10
is output from the secondary side electrode
4
.
A high voltage output from the secondary side electrode is applied to a series circuit including a cold-cathode tube
17
and a feedback resistance
18
and to an overvoltage protection circuit section
20
. The overvoltage protection circuit section
20
includes resistances
19
a
and
19
b
and a comparing circuit
15
. The comparing circuit
15
compares a voltage obtained through division by the resistances
19
a
and
19
b
with a reference voltage Vref
1
. The comparing circuit
15
outputs a signal to an oscillation control circuit
14
so that the high voltage output from the secondary side electrode
4
of the piezoelectric transformer is prevented from rising beyond a preset voltage determined depending on the reference voltage Vref
1
. This overvoltage protection circuit section
20
does not operate during emission by the cold-cathode tube
17
.
A voltage generated at both ends of the feedback resistance
18
by a current flowing in the series circuit including the cold-cathode tube
17
and the feedback resistance
18
is applied to one input terminal of a comparing circuit
16
as a feedback voltage. The comparing circuit
16
compares the feedback voltage with a reference voltage Vref
2
applied to the other input terminal and sends a signal to the oscillation control circuit
14
so that a substantially constant current flows in the cold-cathode tube
17
.
The oscillation control circuit
14
outputs a signal to the variable oscillation circuit
13
to allow the variable oscillation circuit
13
to oscillate at a frequency corresponding to the output signal from the comparing circuit
16
. This comparing circuit
16
does not operate before a start of emission by the cold-cathode tube
17
.
Thus, the cold-cathode tube
17
emits light stably. In the case where the piezoelectric transformer is driven by a self-oscillation system, even when the resonance frequency of the piezoelectric transformer varies depending on temperatures, a drive frequency automatically follows the resonance frequency.
As described above, an inverter with a configuration using a piezoelectric transformer allows driving of the piezoelectric transformer to be controlled so that a constant current flows in the cold-cathode tube
17
.
In order to prevent variations in luminance of the cold-cathode tube, for example, the following drive methods have been proposed. In one driving method, as shown in
FIG. 9
, two piezoelectric transformers
22
and
23
are driven in parallel with each other and a cold-cathode tube
21
is allowed to emit light with two AC voltage signals V
1
and V
2
whose phases are different from each other by 180°. In another driving method, using a piezoelectric transformer
61
with a configuration shown in
FIG. 10
, two output electrodes
4
L and
4
R of the piezoelectric transformer
61
are connected to two input terminals
641
and
642
of a cold-cathode tube
64
, respectively, as shown in FIG.
11
.
In such drives, in an operation carried out in the drive shown in
FIG. 15
, it is necessary to feedback a current flowing in the cold-cathode tube to control the frequency and voltage. Alternatively, feedback is carried out through detection of luminance of the cold-cathode tube.
In order to obtain a constant luminance of the cold-cathode tube, a current flowing in the cold-cathode tube is controlled through detection of an output current or voltage of the piezoelectric transformer (for example, by the output current detecting circuit
24
or the output voltage comparing circuit
25
shown in
FIG. 9
) or through detection of a current flowing in a reflector.
In the conventional drives for a piezoelectric transformer described above, the current flowing in the cold-cathode tube is controlled by the feedback of a voltage detected by the feedback resistance
18
(
FIG. 15
) connected to the cold-cathode tube.
However, due to stray capacitance Cx (
FIGS. 9 and 11
) between a cold-cathode tube and a reflector (a reflector
26
shown in
FIG. 9
, a reflector
65
shown in FIG.
11
), a current flows out to the reflector from the cold-cathode tube. As a result, there has been a problem of variations in, luminance of the cold-cathode tube.
In order to solve this problem, JP 11(1999)-8087 A proposes a means for inputting voltages whose phases are different by 180° from respective ends of a cold-cathode tube. As shown in
FIG. 12A
, however, when voltages are applied to one cold-cathode tube
51
as shown in
FIG. 12A
or to two cold-cathode tubes
51
and
52
connected with each other in series as shown in
FIG. 13A
, a current flows out from the cold-cathode tube to the reflector (a current Ixa on the (+) side shown in
FIG. 12B and a
current Ixb on the (+) side shown in
FIG. 12C
) on a higher-voltage side (the side to which a voltage V
1
is applied during a period ta, the side to which a voltage V
2
is applied during a period tb). On the other hand, a current flows into the cold-cathode tube from the reflector (a current Ixa on the (−) side shown in
FIG. 12B and a
current Ixb on the (−) side shown in
FIG. 12C
) on a lower-voltage side (the side to which a voltage V
1
is applied during a period tb, the side to which a voltage V
2
is applied during a period ta).
Therefore, an

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