U.S. patent application number 09/834820 was filed with the patent office on 2001-11-01 for method and unit for driving piezoelectric transformer used for controlling luminance of cold-cathode tube.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Asahi, Toshiyuki, Kawasaki, Osamu, Moritoki, Katsunori, Nakatsuka, Hiroshi, Okuyama, Kojiro.
Application Number | 20010035698 09/834820 |
Document ID | / |
Family ID | 18637036 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035698 |
Kind Code |
A1 |
Nakatsuka, Hiroshi ; et
al. |
November 1, 2001 |
Method and unit for driving piezoelectric transformer used for
controlling luminance of cold-cathode tube
Abstract
A method and unit for driving a small high-efficiency
piezoelectric transformer allowing a cold-cathode tube to have
stable luminance by detecting only an active current flowing in the
cold-cathode tube based on a phase difference between an output
current and voltage of the piezoelectric transformer, removing a
reactive current caused by stray capacitance formed between the
cold-cathode tube and a reflector, and accurately controlling
driving of the piezoelectric transformer so that a constant active
current is detected.
Inventors: |
Nakatsuka, Hiroshi; (Osaka,
JP) ; Moritoki, Katsunori; (Osaka, JP) ;
Asahi, Toshiyuki; (Hyogo, JP) ; Okuyama, Kojiro;
(Nara, JP) ; Kawasaki, Osamu; (Kyoto, JP) |
Correspondence
Address: |
Merchant & Gould P.C.
P.O. Box 2903
Minneapolis
MN
55402-0903
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
18637036 |
Appl. No.: |
09/834820 |
Filed: |
April 13, 2001 |
Current U.S.
Class: |
310/318 |
Current CPC
Class: |
H05B 41/2822 20130101;
H01L 41/107 20130101; H01L 41/044 20130101; H05B 41/3925
20130101 |
Class at
Publication: |
310/318 |
International
Class: |
H01L 041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 27, 2000 |
JP |
2000-127444 |
Claims
What is claimed is:
1. A method of driving a piezoelectric transformer, comprising:
stepping up a voltage input from a primary terminal of a
piezoelectric transformer by using a piezoelectric effect and
outputting a voltage stepped up by using the piezoelectric effect
to two terminals of a cold-cathode tube from two secondary
terminals of the piezoelectric transformer, detecting a phase
difference between the voltage applied to the cold-cathode tube and
a current flowing in the cold-cathode tube; detecting an active
current flowing in the cold-cathode tube based on the phase
difference; comparing the active current with a predetermined set
value; and controlling driving of the piezoelectric transformer so
that the active current flowing in the cold-cathode tube has a
value equal to the predetermined set value.
2. The method of driving a piezoelectric transformer according to
claim 1, wherein the cold-cathode tube comprises one or more
cold-cathode tubes connected in series.
3. A method of driving piezoelectric transformers, comprising:
inputting a voltage from a primary terminal of a first
piezoelectric transformer by using a piezoelectric effect and
outputting a voltage stepped up by using the piezoelectric effect
to one terminal of a cold-cathode tube from a secondary terminal of
the first piezoelectric transformer; inputting a voltage from a
primary terminal of a second piezoelectric transformer by using the
piezoelectric effect and outputting a voltage stepped up by using
the piezoelectric effect to the other terminal of the cold-cathode
tube from a secondary terminal of the second piezoelectric
transformer; detecting a phase difference between the voltage
applied to the cold-cathode tube and a current flowing in the
cold-cathode tube; detecting an active current flowing in the
cold-cathode tube based on the phase difference; comparing the
active current with a predetermined set value; and controlling
driving of the first and second piezoelectric transformers so that
the active current flowing in the cold-cathode tube has a value
equal to the predetermined set value.
4. The method of driving piezoelectric transformers according to
claim 3, wherein the cold-cathode tube comprises one or more
cold-cathode tubes connected in series.
5. A drive for a piezoelectric transformer, comprising: a
piezoelectric transformer with a primary terminal and two secondary
terminals, for inputting a voltage from the primary terminal by
using a piezoelectric effect and outputting a voltage stepped up by
using the piezoelectric effect from the two secondary terminals; a
drive circuit for driving the piezoelectric transformer; a variable
oscillation circuit for outputting a variable-frequency voltage to
the drive circuit; a cold-cathode tube with two terminals to which
the voltage output from the two secondary terminals of the
piezoelectric transformer is applied; a current detecting circuit
for detecting a current flowing in the cold-cathode tube; a voltage
detecting circuit for detecting the voltage applied to the
cold-cathode tube; a phase difference detecting circuit for
detecting a phase difference between a current signal output from
the current detecting circuit and a voltage signal output from the
voltage detecting circuit; an active current detecting circuit for
detecting an active current flowing in the cold-cathode tube based
on the current signal output from the current detecting circuit and
the phase difference detected in the phase difference detecting
circuit; and an oscillation control circuit for comparing the
active current detected in the active current detecting circuit
with a predetermined set value and controlling an oscillation
frequency of the variable oscillation circuit so that the active
current has a value equal to the predetermined set value.
6. A drive for piezoelectric transformers, comprising: a first
piezoelectric transformer for inputting a voltage from its primary
terminal by using a piezoelectric effect and outputting a voltage
stepped up by using the piezoelectric effect from its secondary
terminal; a second piezoelectric transformer for inputting a
voltage from its primary terminal by using the piezoelectric effect
and outputting a voltage stepped up by using the piezoelectric
effect from its secondary terminal; drive circuits for driving the
first and second piezoelectric transformers with signals whose
phases are different from each other by 180.degree., respectively;
variable oscillation circuits for outputting variable-frequency
voltages to the drive circuits, respectively; a cold-cathode tube
with one terminal to which the voltage output from the secondary
terminal of the first piezoelectric transformer is applied and the
other terminal to which the voltage output from the secondary
terminal of the second piezoelectric transformer is applied; a
current detecting circuit for detecting a current flowing in the
cold-cathode tube; a voltage detecting circuit for detecting the
voltage applied to the cold-cathode tube; a phase difference
detecting circuit for detecting a phase difference between a
current signal output from the current detecting circuit and a
voltage signal output from the voltage detecting circuit; an active
current detecting circuit for detecting an active current flowing
in the cold-cathode tube based on the current signal output from
the current detecting circuit and the phase difference detected in
the phase difference detecting circuit; and an oscillation control
circuit for comparing the active current detected in the active
current detecting circuit with a predetermined set value and
controlling oscillation frequencies of the variable oscillation
circuits so that the active current has a value equal to the
predetermined set value.
7. A method of driving piezoelectric transformers, comprising:
inputting a voltage from a primary terminal of a first
piezoelectric transformer by using a piezoelectric effect and
outputting a voltage stepped up by using the piezoelectric effect
to one terminal of a cold-cathode tube from a secondary terminal of
the first piezoelectric transformer; inputting, by using the
piezoelectric effect, a voltage from a primary terminal of a second
piezoelectric transformer having a phase identical with that of the
voltage input to the first piezoelectric transformer, and
outputting a voltage stepped up by using the piezoelectric effect
having a different phase from that of the voltage output from the
first piezoelectric transformer by 180.degree. to the other
terminal of the cold-cathode tube from a secondary terminal of the
second piezoelectric transformer; and allowing the cold-cathode
tube to emit light.
8. The method of driving piezoelectric transformers according to
claim 7, further comprising: detecting a phase difference between
the voltage applied to the cold-cathode tube and a current flowing
in the cold-cathode tube; detecting an active current flowing in
the cold-cathode tube based on the phase difference; comparing the
active current with a predetermined set value; and controlling
driving of the first and second piezoelectric transformers so that
the active current flowing in the cold-cathode tube has a value
equal to the predetermined set value.
9. A drive for piezoelectric transformers, comprising: a first
piezoelectric transformer for inputting a voltage from its primary
terminal by using a piezoelectric effect and outputting a voltage
stepped up by using the piezoelectric effect from its secondary
terminal; a second piezoelectric transformer for inputting, by
using a piezoelectric effect, a voltage from its primary terminal
having a phase identical with that of the voltage input to the
first piezoelectric transformer and outputting, from its secondary
terminal, a voltage stepped up by using the piezoelectric effect
having a different phase from that of the voltage output from the
first piezoelectric transformer by 180.degree.; drive circuits for
driving the first and second piezoelectric transformers with
signals whose phases are identical with each other, respectively; a
variable oscillation circuit for outputting a variable-frequency
voltage to the drive circuits; a cold-cathode tube with one
terminal to which the voltage output from the secondary terminal of
the first piezoelectric transformer is applied and the other
terminal to which the voltage output from the secondary terminal of
the second piezoelectric transformer is applied; a current
detecting circuit for detecting a current flowing in the
cold-cathode tube; a voltage detecting circuit for detecting the
voltage applied to the cold-cathode tube; a phase difference
detecting circuit for detecting a phase difference between a
current signal output from the current detecting circuit and a
voltage signal output from the voltage detecting circuit; an active
current detecting circuit for detecting an active current flowing
in the cold-cathode tube based on the current signal output from
the current detecting circuit and the phase difference detected in
the phase difference detecting circuit; and an oscillation control
circuit for comparing the active current detected in the active
current detecting circuit with a predetermined set value and
controlling an oscillation frequency of the variable oscillation
circuit so that the active current has a value equal to the
predetermined set value.
10. A drive for piezoelectric transformers, comprising: a pair of
current amplifying circuits for amplifying currents converted from
AC voltages input thereto having different phases from each other
by 180.degree., respectively; a pair of step-up transformers for
amplifying voltages converted from signals output from the pair of
current amplifying circuits and outputting voltage signals whose
phases are different from each other by 180.degree., respectively;
and a pair of piezoelectric transformers each of which includes a
piezoelectric body in which a primary side electrode and a
secondary side electrode are formed, steps up the voltage signal
input to the primary side electrode from one of the pair of step-up
transformers, and outputs a voltage signal stepped up from the
secondary side electrode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a method and unit
for driving a piezoelectric transformer used in various
high-voltage transformer assemblies.
[0003] 2. Related Background Art
[0004] 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.
[0005] 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 3U and 3D 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.
[0006] 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.
[0007] 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 3U of the
piezoelectric transformer 10. The other primary side electrode 3D
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.
[0008] 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 19a and 19b and a comparing circuit 15. The comparing
circuit 15 compares a voltage obtained through division by the
resistances 19a and 19b with a reference voltage Vref1. 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 Vref1. This overvoltage protection circuit
section 20 does not operate during emission by the cold-cathode
tube 17.
[0009] 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 Vref2 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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 V1 and V2 whose phases are different from
each other by 180.degree.. In another driving method, using a
piezoelectric transformer 61 with a configuration shown in FIG. 10,
two output electrodes 4L and 4R 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] In order to solve this problem, JP 11(1999)-8087 A proposes
a means for inputting voltages whose phases are different by
180.degree. 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 V1 is applied during a period ta, the side to which a
voltage V2 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 V1 is applied during a period tb, the side to
which a voltage V2 is applied during a period ta).
[0019] Therefore, an output current from the piezoelectric
transformer contains both a current Ia flowing only in the
cold-cathode tube and leakage currents Ixa and Ixb (Ix) flowing in
the stray capacitance Cx. During emission by the cold-cathode tube,
the cold-cathode tube is handled as a resistive load. Therefore, a
current participating in the luminance of the cold-cathode tube is
only an active current Ia=Icos.theta. (.theta. indicates a phase
difference between an output voltage and an output current from the
piezoelectric transformer as shown in FIG. 16) of a current (I)
output from the piezoelectric transformer. In other words, the
leakage current Ix flowing in the reflector through the stray
capacitance Cx becomes a reactive current and thus does not
participate in the luminance of the cold-cathode tube.
[0020] Hence, in the drives for a piezoelectric transformer with
the configurations as shown in FIGS. 9 and 11, the output current
detecting circuit 24, 62 detects the current Ia flowing in the
cold-cathode tube together with the leakage current Ix caused by
the stray capacitance Cx formed by, for example, the cold-cathode
tube and the reflector. If the stray capacitance Cx formed by the
reflector were constant, the current flowing in the cold-cathode
tube would be controlled to be constant with consideration given to
the constant capacitance Cx. However, the stray capacitance Cx
varies and it therefore is difficult to control the current Ia
flowing in the cold-cathode tube so that the current Ia is
constant. This causes variations in luminance among inverters, or
the like. In addition, similarly in the case of a drive with two
piezoelectric transformers, it is difficult to control a tube
current.
[0021] In JP 11(1999)-27955 A, a leakage current and a tube current
are detected by a stray capacitance current detecting circuit and a
tube current detecting circuit, respectively, and thus a tube
current is controlled. In the method disclosed in JP 11(1999)-27955
A, however, in a piezoelectric transformer allowing an output
voltage to be constant by control of a drive frequency, the
impedance depending on stray capacitance varies when the frequency
of a leakage current caused by the stray capacitance varies.
Accordingly, the magnitude of the leakage current varies. As a
result, when a circuit is intended to be configured with
consideration also given to an influence of the frequency, a
complicated control circuit is required.
SUMMARY OF THE INVENTION
[0022] Therefore, with the foregoing in mind, it is an object of
the present invention to provide a method and unit for driving a
small high-efficiency piezoelectric transformer allowing a
cold-cathode tube to have a stable luminance through removal of an
influence of a reactive current as a leakage current caused by
stray capacitance between the cold-cathode tube and a reflector and
through accurate control to obtain a constant tube current.
[0023] In order to achieve the above-mentioned object, a first
method of driving a piezoelectric transformer according to the
present invention includes: stepping up a voltage input from a
primary terminal of a piezoelectric transformer by using a
piezoelectric effect and outputting a voltage stepped up by using
the piezoelectric effect to two terminals of a cold-cathode tube
from two secondary terminals of the piezoelectric transformer;
detecting a phase difference between the voltage applied to the
cold-cathode tube and a current flowing in the cold-cathode tube;
detecting an active current flowing in the cold-cathode tube based
on the phase difference; comparing the active current with a
predetermined set value; and controlling the driving of the
piezoelectric transformer so that the active current flowing in the
cold-cathode tube has a value equal to the predetermined set
value.
[0024] In order to achieve the above-mentioned object, a second
method of driving piezoelectric transformers according to the
present invention includes: inputting a voltage from a primary
terminal of a first piezoelectric transformer by using a
piezoelectric effect and outputting a voltage stepped up by using
the piezoelectric effect to one terminal of a cold-cathode tube
from a secondary terminal of the first piezoelectric transformer;
inputting a voltage from a primary terminal of a second
piezoelectric transformer by using the piezoelectric effect and
outputting a voltage stepped up by using the piezoelectric effect
to the other terminal of the cold-cathode tube from a secondary
terminal of the second piezoelectric transformer; detecting a phase
difference between the voltage applied to the cold-cathode tube and
a current flowing in the cold-cathode tube; detecting an active
current flowing in the cold-cathode tube based on the phase
difference; comparing the active current with a predetermined set
value; and controlling driving of the first and second
piezoelectric transformers so that the active current flowing in
the cold-cathode tube has a value equal to the predetermined set
value.
[0025] In the first and second driving methods, preferably, the
cold-cathode tube includes one or more cold-cathode tubes connected
in series.
[0026] In order to achieve the above-mentioned object, a first
drive for a piezoelectric transformer according to the present
invention includes: a piezoelectric transformer for inputting a
voltage from its primary terminal by using a piezoelectric effect
and outputting a voltage stepped up by using the piezoelectric
effect from its two secondary terminals; a drive circuit for
driving the piezoelectric transformer; a variable oscillation
circuit for outputting a variable-frequency voltage to the drive
circuit; a cold-cathode tube with two terminals to which the
voltage output from the two secondary terminals of the
piezoelectric transformer is applied; a current detecting circuit
for detecting a current flowing in the cold-cathode tube; a voltage
detecting circuit for detecting the voltage applied to the
cold-cathode tube; a phase difference detecting circuit for
detecting a phase difference between a current signal output from
the current detecting circuit and a voltage signal output from the
voltage detecting circuit; an active current detecting circuit for
detecting an active current flowing in the cold-cathode tube based
on the current signal output from the current detecting circuit and
the phase difference detected in the phase difference detecting
circuit; and an oscillation control circuit for comparing the
active current detected in the active current detecting circuit
with a predetermined set value and controlling an oscillation
frequency of the variable oscillation circuit so that the active
current has a value equal to the predetermined set value.
[0027] In order to achieve the above-mentioned object, a second
drive for piezoelectric transformers according to the present
invention includes: a first piezoelectric transformer for inputting
a voltage from its primary terminal by using a piezoelectric effect
and outputting a voltage stepped up by using the piezoelectric
effect from its secondary terminal; a second piezoelectric
transformer for inputting a voltage from its primary terminal by
using the piezoelectric effect and outputting a voltage stepped up
by using the piezoelectric effect from its secondary terminal;
drive circuits for driving the first and second piezoelectric
transformers with signals whose phases are different from each
other by 180.degree., respectively; variable oscillation circuits
for outputting variable-frequency voltages to the drive circuits,
respectively; a cold-cathode tube with one terminal to which the
voltage output from the secondary terminal of the first
piezoelectric transformer is applied and the other terminal to
which the voltage output from the secondary terminal of the second
piezoelectric transformer is applied; a current detecting circuit
for detecting a current flowing in the cold-cathode tube; a voltage
detecting circuit for detecting the voltage applied to the
cold-cathode tube; a phase difference detecting circuit for
detecting a phase difference between a current signal output from
the current detecting circuit and a voltage signal output from the
voltage detecting circuit; an active current detecting circuit for
detecting an active current flowing in the cold-cathode tube based
on the current signal output from the current detecting circuit and
the phase difference detected in the phase difference detecting
circuit; and an oscillation control circuit for comparing the
active current detected in the active current detecting circuit
with a predetermined set value and controlling oscillation
frequencies of the variable oscillation circuits so that the active
current has a value equal to the predetermined set value.
[0028] In order to achieve the above-mentioned object, a third
method of driving piezoelectric transformers according to the
present invention includes: inputting a voltage from a primary
terminal of a first piezoelectric transformer by using a
piezoelectric effect and outputting a voltage stepped up by using
the piezoelectric effect to one terminal of a cold-cathode tube
from a secondary terminal of the first piezoelectric transformer;
inputting, by using the piezoelectric effect, a voltage from a
primary terminal of a second piezoelectric transformer having a
phase identical with that of the voltage input to the first
piezoelectric transformer, and outputting a voltage stepped up by
using the piezoelectric effect having a different phase from that
of the voltage output from the first piezoelectric transformer by
180.degree. to the other terminal of the cold-cathode tube from a
secondary terminal of the second piezoelectric transformer; and
allowing the cold-cathode tube to emit light.
[0029] Preferably, the third driving method further includes:
detecting a phase difference between the voltage applied to the
cold-cathode tube and a current flowing in the cold-cathode tube;
detecting an active current flowing in the cold-cathode tube based
on the phase difference; comparing the active current with a
predetermined set value; and controlling driving of the first and
second piezoelectric transformers so that the active current
flowing in the cold-cathode tube has a value equal to the
predetermined set value.
[0030] In order to achieve the above-mentioned object, a third
drive for piezoelectric transformers according to the present
invention includes: a first piezoelectric transformer for inputting
a voltage from its primary terminal by using a piezoelectric effect
and outputting a voltage stepped up by using the piezoelectric
effect from its secondary terminal; a second piezoelectric
transformer for inputting, by using the piezoelectric effect, a
voltage from its primary terminal having a phase identical with
that of the voltage input to the first piezoelectric transformer,
and outputting, from its secondary terminal, a voltage stepped up
by using the piezoelectric effect having a different phase from
that of the voltage output from the first piezoelectric transformer
by 180.degree.; drive circuits for driving the first and second
piezoelectric transformers with signals whose phases are identical
with each other, respectively; a variable oscillation circuit for
outputting a variable-frequency voltage to the drive circuits; a
cold-cathode tube with one terminal to which the voltage output
from the secondary terminal of the first piezoelectric transformer
is applied and the other terminal to which the voltage output from
the secondary terminal of the second piezoelectric transformer is
applied; a current detecting circuit for detecting a current
flowing in the cold-cathode tube; a voltage detecting circuit for
detecting the voltage applied to the cold-cathode tube; a phase
difference detecting circuit for detecting a phase difference
between a current signal output from the current detecting circuit
and a voltage signal output from the voltage detecting circuit; an
active current detecting circuit for detecting an active current
flowing in the cold-cathode tube based on the current signal output
from the current detecting circuit and the phase difference
detected in the phase difference detecting circuit; and an
oscillation control circuit for comparing the active current
detected in the active current detecting circuit with a
predetermined set value and controlling an oscillation frequency of
the variable oscillation circuit so that the active current has a
value equal to the predetermined set value.
[0031] In order to achieve the above-mentioned object, a fourth
drive for piezoelectric transformers according to the present
invention includes: a pair of current amplifying circuits for
amplifying currents converted from AC voltages input thereto having
different phases from each other by 180.degree., respectively; a
pair of step-up transformers for amplifying voltages converted from
signals output from the pair of current amplifying circuits and
outputting voltage signals whose phases are different from each
other by 180.degree., respectively; and a pair of piezoelectric
transformers each of which includes a piezoelectric body in which a
primary side electrode and a secondary side electrode are formed,
steps up the voltage signal input to the primary side electrode
from one of the pair of step-up transformers, and outputs a voltage
signal stepped up from the secondary side electrode.
[0032] According to the configurations described above, only an
active current flowing in the cold-cathode tube is detected based
on the phase difference between an output current and voltage of a
piezoelectric transformer, a reactive current caused by the stray
capacitance formed between the cold-cathode tube and a reflector
can be removed, and the driving of the piezoelectric transformer is
controlled accurately so that a constant tube current is obtained.
Thus, there can be provided a method and unit for driving a small
high-efficiency piezoelectric transformer allowing the cold-cathode
tube to have a stable luminance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a block diagram showing a configuration example of
a drive for a piezoelectric transformer according to a first
embodiment of the present invention.
[0034] FIG. 2 is a schematic diagram showing reactive currents Ixa
and Ixb during periods ta and tb caused by stray capacitance Cx
when voltage is applied to a cold-cathode tube from a piezoelectric
transformer.
[0035] FIG. 3 is a circuit diagram showing a specific configuration
example of the periphery of a drive circuit 112 shown in FIG.
1.
[0036] FIG. 4 is a block diagram showing a configuration example of
a drive for piezoelectric transformers according to a second
embodiment of the present invention.
[0037] FIG. 5 is a perspective view of piezoelectric transformers
used in another drive according to the second embodiment of the
present invention.
[0038] FIG. 6 is a block diagram showing a configuration example of
another drive with the piezoelectric transformers shown in FIG.
5.
[0039] FIG. 7 is a configuration diagram showing a drive circuit
600 and its periphery in a drive for piezoelectric transformers
according to a third embodiment of the present invention.
[0040] FIG. 8 is a block diagram showing a configuration example of
a drive for piezoelectric transformers with the drive circuit
configuration shown in FIG. 7.
[0041] FIG. 9 is a block diagram showing a configuration of a
conventional drive for piezoelectric transformers.
[0042] FIG. 10 is a perspective view showing a configuration of a
conventional piezoelectric transformer.
[0043] FIG. 11 is a block diagram showing a conventional drive
circuit for a piezoelectric transformer with the piezoelectric
transformer shown in FIG. 10.
[0044] FIG. 12A is a schematic diagram showing leakage currents Ixa
and Ixb caused by stray capacitance Cx when voltages whose phases
are different from each other by 180.degree. are applied to ends of
one cold-cathode tube, respectively.
[0045] FIG. 12B is a diagram showing a magnitude and direction of
the leakage current Ixa with respect to a length direction in the
cold-cathode tube during a period ta shown in FIG. 12A.
[0046] FIG. 12C is a diagram showing a magnitude and direction of
the leakage current Ixb with respect to the length direction in the
cold-cathode tube during a period tb shown in FIG. 12A.
[0047] FIG. 13A is a schematic diagram showing leakage currents Ixa
and Ixb caused by stray capacitance Cx when voltages whose phases
are different from each other by 180.degree. are applied to ends of
two cold-cathode tubes connected to each other in series,
respectively.
[0048] FIG. 13B is a diagram showing a magnitude and direction of
the leakage current Ixa with respect to a length direction in the
cold-cathode tubes during a period ta shown in FIG. 13A.
[0049] FIG. 13C is a diagram showing a magnitude and direction of
the leakage current Ixb with respect to the length direction in the
cold-cathode tubes during a period tb shown in FIG. 13A.
[0050] FIG. 14 is a perspective view showing a configuration of a
conventional piezoelectric transformer.
[0051] FIG. 15 is a block diagram showing a configuration of a
conventional drive for a piezoelectric transformer.
[0052] FIG. 16 is a graph showing waveforms of an output voltage
and current of a piezoelectric transformer.
DETAILED DESCRIPTION OF THE INVENTION
[0053] Suitable embodiments of the present invention are described
with reference to the drawings as follows.
[0054] First Embodiment
[0055] FIG. 1 is a block diagram showing a configuration example of
a drive for a piezoelectric transformer according to a first
embodiment of the present invention.
[0056] FIG. 2 is a schematic diagram showing a state of connection
between a piezoelectric transformer 110 shown in FIG. 1 and a
cold-cathode tube 118 as a load, and reactive currents Ixa and Ixb
caused by stray capacitance Cx.
[0057] As shown in FIG. 2, the piezoelectric transformer 110
includes a rectangular plate formed of a piezoelectric material
such as lead zirconate titanate (PZT) processed in a rectangular
form, input electrodes 1100U and 1100D formed in the vicinity of
the center of the rectangular plate, and output electrodes 1100L
and 1100R provided on end faces of the rectangular plate,
respectively. A driving part of the piezoelectric transformer 110
is polarized with the input electrodes 1100U and 1100D in the
thickness direction. A power generation part of the piezoelectric
transformer 110 is polarized with the input electrodes 1100U and
1100D and the respective output electrodes 1100L and 1100R. In the
piezoelectric transformer 110, when an AC voltage with a
half-wavelength oscillation mode is applied between the input
electrodes 1100U and 1100D, the AC voltage is output from the two
output electrodes 1100L and 1100R formed on the end faces as
voltages whose phases are different from each other by
180.degree..
[0058] As shown in FIG. 2, output voltages from the piezoelectric
transformer having different phases from each other by 180.degree.
obtained through multiplication of an input voltage by a step-up
ratio are applied to respective input terminals of the cold-cathode
tube 118. Generally, the cold-cathode tube 118 has stray
capacitance Cx caused by a reflector 120 or the like. In such a
case, when plus and minus voltages are applied to ends of the
cold-cathode tube 118, respectively, currents Ixa2 and Ixb1 flow
out from the cold-cathode tube 118 to the reflector 120 on a
higher-voltage side (the side to which a voltage V2 is applied
during a period ta, the side to which a voltage V1 is applied
during a period tb) of the cold-cathode tube 118, and currents Ixa1
and Ixb2 flow into the cold-cathode tube 118 from the reflector 120
on a lower-voltage side (the side to which a voltage V1 is applied
during a period ta, the side to which a voltage V2 is applied
during a period tb). Consequently, an output current from the
piezoelectric transformer 110 includes a current Ia (an active
current) contributing to emission by the cold-cathode tube 118 and
a current (a reactive current Ix) flowing in the stray capacitance
Cx formed between the cold-cathode tube 118 and the reflector
120.
[0059] As a result, when the luminance of the cold-cathode tube 118
is intended to be kept constant, it is necessary to detect only the
active current Ia contributing to the emission by the cold-cathode
tube 118 and to feedback it.
[0060] In FIG. 1, numeral 113 is a variable oscillation circuit for
producing a variable-frequency voltage signal. An output signal
from the variable oscillation circuit 113 generally is a voltage
signal with a pulse waveform. A drive circuit 112 removes high
frequency components from the output signal to convert it to an AC
signal with a substantially sinusoidal waveform. The output signal
from the variable oscillation circuit 113 is input to the drive
circuit 112. An output signal from the drive circuit 112 is
converted to a voltage and the voltage is amplified to a sufficient
level to drive the piezoelectric transformer 110 and then is input
to the primary side electrode 1100U of the piezoelectric
transformer 110. In this case, the piezoelectric transformer with
the configuration shown in FIG. 2 is used as the piezoelectric
transformer 110.
[0061] The output voltages stepped up by the piezoelectric effect
of the piezoelectric transformer 110 are output from the secondary
side electrodes 1100L and 1100R. Two high voltages whose phases are
different from each other by 180.degree. output from the secondary
side electrodes 1100L and 1100R are applied to two input terminals
of the cold-cathode tube 118. Thus, the cold-cathode tube 118 emits
light.
[0062] During emission by the cold-cathode tube 118, plus and minus
voltages whose phases are different from each other by 180.degree.
are applied alternately from the two input terminals. An output
signal from a current detecting circuit 116 for detecting a current
flowing in the cold-cathode tube 118 and an output signal from a
voltage detecting circuit 117 for detecting the voltage applied to
the respective ends of the cold-cathode tube 118 are supplied to a
phase difference detecting circuit 119 for detecting the phase
difference between the voltage and current in the cold-cathode tube
118. An output signal from the phase difference detecting circuit
119 and the output signal from the current detecting circuit 116
are supplied to an active current detecting circuit 115 and thus an
active current flowing in the cold-cathode tube 118 is
detected.
[0063] An output signal from the active current detecting circuit
115 is supplied to one input terminal of an oscillation control
circuit 114 and is compared with a reference voltage Vref supplied
to the other input terminal of the oscillation control circuit 114.
According to the comparison result, the oscillating frequency of
the variable oscillation circuit 113 is controlled so that a
constant active current flows in the cold-cathode tube 118.
[0064] The oscillation control circuit 114 controls the variable
oscillation circuit 113 so that the oscillating frequency varies in
a direction apart from the resonance frequency of the piezoelectric
transformer 110 when the active current flowing in the cold-cathode
tube 118 exceeds a set value determined depending on the reference
voltage Vref. On the other hand, the oscillation control circuit
114 controls the variable oscillation circuit 113 so that the
oscillating frequency approaches the resonance frequency of the
piezoelectric transformer 110 when the active current becomes lower
than the set value. As described above, the driving of the
piezoelectric transformer 110 is controlled by a self-exciting
system allowing an active current flowing in the cold-cathode tube
118 to be constant, so that the cold-cathode tube 118 can emit
light stably even when the load on the cold-cathode tube 118 varies
or the characteristics of the piezoelectric transformer 110 vary
depending on temperatures.
[0065] FIG. 3 is a circuit diagram showing a specific configuration
example of the periphery of the drive circuit 112 shown in FIG. 1.
In FIG. 3, the current detecting circuit 116 includes a current
transformer CT and a resistance R1. The current transformer CT
includes a primary winding with one end connected to the
piezoelectric transformer 110 and the other end connected to the
cold-cathode tube 118. The resistance R1 is connected between ends
of a secondary winding of the current transformer CT as a load for
current detection.
[0066] A current signal detected by the secondary winding of the
current transformer CT is supplied to one input terminal of an AND
gate included in the phase difference detecting circuit 119. A
signal of a voltage obtained through division by resistances R2 and
R3 included in the voltage detecting circuit 117 is supplied to the
other input terminal of the AND gate. In this case, the output
voltage from the piezoelectric transformer 110 is used for the
detection of the phase difference between an output voltage and an
output current. Therefore, the absolute value of the output voltage
is not required. The resistances R2 and R3 divide the output
voltage to an input threshold level of the AND gate.
[0067] The active current detecting circuit 115 includes a peak
hold circuit, a switching element Q1, and a resistance R4. The peak
hold circuit includes a diode D1, a capacitor C1, and a resistance
R5. A current signal detected by the secondary winding of the
current transformer CT is supplied to the peak hold circuit to be
used for the detection of an absolute value of the current.
[0068] An output signal from the AND gate is input to the switching
element Q1 to turn on and off the switching element Q1 according to
the input levels of the voltage signal and the current signal, i.e.
according to the phase difference therebetween. Thus, the peak hold
circuit detects only an active current component of the current
signal.
[0069] In the present embodiment, the piezoelectric transformer was
formed with piezoelectric ceramic such as PZT. However, output
voltages whose phases are different from each other by 180.degree.
also can be obtained using a single crystal material such as
LiNbO.sub.3 or the like as long as the material has
piezoelectricity.
[0070] The piezoelectric transformer is not limited to those with a
half-wavelength oscillation mode as shown in FIG. 2. The same
effects as described above can be obtained with another
piezoelectric transformer as long as the piezoelectric transformer
outputs voltages whose phases are different from each other by
180.degree. and inputs voltages to respective ends of a
cold-cathode tube.
[0071] In addition, even when two cold-cathode tubes are connected
as a load on a piezoelectric transformer, the same effects as
described above can be obtained by the following process: a voltage
applied to the two cold-cathode tubes and a current flowing in the
cold-cathode tubes are detected; using the phase difference between
the voltage and the current, only an active current component
contained in the output current from the piezoelectric transformer
is detected; and the detection result is used for luminance
control.
[0072] Second Embodiment
[0073] FIG. 4 is a block diagram showing a configuration example of
a drive for piezoelectric transformers according to a second
embodiment of the present invention.
[0074] In FIG. 4, piezoelectric transformers 315 and 316 are made
of a piezoelectric material having piezoelectricity such as PZT or
the like. In each piezoelectric transformer, a voltage applied to
its primary side electrode is multiplied by a step-up ratio and
then the voltage thus obtained is output from its secondary side
electrode.
[0075] To the two piezoelectric transformers 315 and 316 shown in
FIG. 4, input voltages whose phases are different from each other
by 180.degree. are applied by a phase inverting circuit 317,
respectively. As a result, output voltages whose phases are
different from each other by 180.degree. are output from the
piezoelectric transformers 315 and 316 and are input to input
terminals of a cold-cathode tube 118, respectively. The present
embodiment is different from the first embodiment in that the
cold-cathode tube 118 is driven with two piezoelectric
transformers, and the other controls in the present embodiment are
carried out in the same manner as in the first embodiment.
[0076] Similarly in the case of driving according to the present
embodiment, the cold-cathode tube 118 generally has stray
capacitance Cx caused by a reflector 120 or the like. Hence, when
plus and minus voltages are applied alternately to ends of the
cold-cathode tube 118, a current flows out to the reflector 120
from the cold-cathode tube 118 on a higher-voltage side, and a
current flows into the cold-cathode tube 118 from the reflector 120
on a lower-voltage side.
[0077] Thus, an output current from the piezoelectric transformers
315 and 316 includes a current Ia (an active current) contributing
to emission by the cold-cathode tube 118 and a current (a reactive
current Ix) flowing in the stray capacitance Cx formed between the
cold-cathode tube 118 and the reflector 120.
[0078] As a result, when the luminance of the cold-cathode tube 118
is intended to be kept constant, it is necessary to detect only the
active current Ia contributing to the emission by the cold-cathode
tube 118 and to feedback it.
[0079] In FIG. 4, numerals 311 and 312 are variable oscillation
circuits for producing variable-frequency voltage signals. Output
signals from the variable oscillation circuits 311 and 312
generally are voltage signals with pulse waveforms. Drive circuits
313 and 314 remove high frequency components from the output
signals to convert them to AC signals with a substantially
sinusoidal waveform. The output signals from the drive circuits 314
and 313 are converted to voltages and the voltages are amplified to
a sufficient level to drive the piezoelectric transformers 315 and
316 and then are input to primary side electrodes of the
piezoelectric transformers 315 and 316, respectively. In this case,
the respective voltages input to the two piezoelectric transformers
315 and 316 have the same amplitude and are different in phase from
each other by 180.degree..
[0080] Output voltages stepped up by the piezoelectric effect of
the piezoelectric transformers 315 and 316 are output from their
secondary side electrodes. With input voltages whose phases are
different from each other by 180.degree., voltages whose phases are
different from each other by 180.degree. are output from
piezoelectric transformers with the same configuration. The two
high voltages output from the secondary side electrodes are applied
to two input terminals of the cold-cathode tube 118. Thus, the
cold-cathode tube 118 emits light.
[0081] During emission by the cold-cathode tube 118, plus and minus
voltages whose phases are different from each other by 180.degree.
are applied alternately from the two input terminals. An output
signal from a current detecting circuit 116 for detecting a current
flowing in the cold-cathode tube 118 and an output signal from a
voltage detecting circuit 117 for detecting the voltage applied to
the respective ends of the cold-cathode tube 118 are supplied to a
phase difference detecting circuit 119 for detecting the phase
difference between the voltage and current in the cold-cathode tube
118. An output signal from the phase difference detecting circuit
119 and the output signal from the current detecting circuit 116
are supplied to an active current detecting circuit 115 and thus an
active current flowing in the cold-cathode tube 118 is
detected.
[0082] An output signal from the active current detecting circuit
115 is supplied to one input terminal of an oscillation control
circuit 114. The output signal is compared with a reference voltage
Vref supplied to the other input terminal of the oscillation
control circuit 114. According to the comparison result, the
oscillating frequencies of the variable oscillation circuits 311
and 312 are controlled so that a constant active current flows in
the cold-cathode tube 118. An output signal from the oscillation
control circuit 114 is input to the phase inverting circuit 317 and
then output signals from the phase inverting circuit 317, which
have different phases from each other by 180.degree., are input to
the variable oscillation circuits 311 and 312, respectively.
[0083] The oscillation control circuit 114 controls the variable
oscillation circuits 312 and 311 so that the oscillating
frequencies vary in a direction apart from the resonance
frequencies of the piezoelectric transformers 315 and 316 when the
active current flowing in the cold-cathode tube 118 exceeds a set
value determined depending on the reference voltage Vref. On the
other hand, the oscillation control circuit 114 controls the
variable oscillation circuits 312 and 311 so that the oscillating
frequencies approach the resonance frequencies of the piezoelectric
transformers 315 and 316 when the active current becomes lower than
the set value. As described above, the driving of the piezoelectric
transformers 315 and 316 is controlled by the self-exciting system
allowing the active current flowing in the cold-cathode tube 118 to
be constant, so that the cold-cathode tube 118 can emit light
stably even when the load on the cold-cathode tube 118 varies or
the characteristics of the piezoelectric transformers 315 and 316
vary depending on temperatures.
[0084] In the description above, since two piezoelectric
transformers with the same configuration were used, voltages whose
phases are different from each other by 180.degree. were applied to
the input terminals of the piezoelectric transformers,
respectively. However, as a modified example of the present
embodiment, piezoelectric transformers with different polarization
structures can be used. In this case, as shown in FIG. 5, one
piezoelectric transformer 416 has input side electrodes 4161U and
4161D and an output side electrode 4162, and the other
piezoelectric transformer 415 has input side electrodes 4151U and
4151D and an output side electrode 4152. The piezoelectric
transformers 416 and 415 are polarized in the same direction PD in
the thickness direction and in directions PL1 and PL2 opposite to
the direction PL1 in the longitudinal direction, respectively.
Thus, output voltages whose phases are different from each other by
180.degree. can be obtained from input voltages with the same
phase.
[0085] Consequently, as in a drive for piezoelectric transformers
shown in FIG. 6, the phase inverting circuit 317 (FIG. 4) can be
omitted and one variable oscillation circuit 113 to be shared
between two piezoelectric transformers 415 and 416 can be used
instead of the two variable oscillation circuits 311 and 312 (FIG.
4).
[0086] In the present embodiment, the piezoelectric transformers
were formed with piezoelectric ceramic such as PZT. However, output
voltages whose phases are different from each other by 180.degree.
also can be obtained using a single crystal material such as
LiNbO.sub.3 or the like as long as the material has
piezoelectricity.
[0087] The same effects as described above can be obtained with
other piezoelectric transformers as long as they input a voltage to
each of the ends of a cold-cathode tube.
[0088] In addition, even when two cold-cathode tubes are connected
as a load on the piezoelectric transformers, the same effects as
described above can be obtained by the following process: a voltage
applied to the two cold-cathode tubes and a current flowing in the
cold-cathode tubes are detected; using the phase difference between
the voltage and current, only an active current component contained
in the output current from the piezoelectric transformers is
detected; and according to the detection result, the luminance is
controlled to be constant.
[0089] Third Embodiment
[0090] FIG. 7 is a configuration diagram of a drive circuit and its
periphery in a drive for piezoelectric transformers according to a
third embodiment of the present invention.
[0091] In FIG. 7, numerals 605 and 606 indicate piezoelectric
transformers. The piezoelectric transformers 605 and 606 have a
resonance characteristic as other piezoelectric elements do. In
each piezoelectric transformer, when an AC current with a frequency
close to the resonance frequency is input to its primary electrode,
an output voltage multiplied by a step-up ratio due to the
piezoelectric effect is output from its secondary side electrode.
However, frequency components other than the resonance frequency
are lost in the piezoelectric transformers 605 and 606, are
converted to heat, or cause unwanted stresses, resulting in
deterioration in reliability. Thus, it is desirable to drive the
piezoelectric transformers 605 and 606 with a sine wave with a
frequency close to the resonance frequency as far as possible.
[0092] As described in the second embodiment, however, in a unit
using two piezoelectric transformers, each piezoelectric
transformer requires one drive circuit.
[0093] In such a case, as shown in FIG. 7, a drive circuit 600 is
configured with a pair of FETs 603 and 604 and a pair of step-up
transformers 601 and 602.
[0094] In the drive circuit 600 shown in FIG. 7, a center tap is
provided in the middle of each secondary side winding of the
step-up transformers 601 and 602 to be grounded, and voltages whose
phases are different from each other by 180.degree. are generated
from the two terminals of each secondary side winding and are
applied to the respective piezoelectric transformers 605 and 606.
In this case, the inductances of the secondary side windings of the
step-up transformers 601 and 602 are set to allow the voltages to
resonate at a desired frequency with consideration given to the
primary side capacitance of the piezoelectric transformers 605 and
606.
[0095] Rectangular wave signals CLK and /CLK with phases opposite
to each other are input to gate terminals of the pair of FETs 603
and 604, respectively. When the FET 603 is in an on state, the FET
604 is in an off state. When the FET 603 or 604 is in the on state,
a current flows to the primary side winding of the corresponding
step-up transformer 601 or 602 from a power source Vd and thus
energy is stored. When the FET in the on state is changed to the
off state, the energy stored in the inductor is converted to a
voltage and the voltage is output to the corresponding
piezoelectric transformer 605 or 606 from the secondary side
winding.
[0096] Thus, the piezoelectric transformers 605 and 606 are driven
by the pair of FETs 603 and 604 and the pair of step-up
transformers 601 and 602 using a sine-wave voltage. Voltages whose
phases are different from each other by 180.degree. are output from
the output terminals of the piezoelectric transformers 605 and 606
and a cold-cathode tube 607 is driven with signals of the
voltages.
[0097] As in a drive for piezoelectric transformers shown in FIG.
8, when using the drive circuit 600 with the above-mentioned
configuration, a drive circuit 600 can be shared between
piezoelectric transformers 605 and 606 and hence, voltages with the
same driving waveform can be applied to the two piezoelectric
transformers 605 and 606 in driving the piezoelectric transformers.
Accordingly, output voltages of the two piezoelectric transformers
605 and 606 can be made substantially equal and thus voltages to be
applied to the cold-cathode tube 607 can be made substantially
equal. In addition, there are effects of reducing the size of the
drive for the piezoelectric transformers and reducing the number of
parts, or the like.
[0098] The invention may be embodied in other forms without
departing from the spirit or essential characteristics thereof. The
embodiments disclosed in this application are to be considered in
all respects as illustrative and not limiting. The scope of the
invention is indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of equivalency of the claims are intended to be
embraced therein.
* * * * *