Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-08-16
2004-04-13
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S318000, C310S359000
Reexamination Certificate
active
06720705
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving a piezoelectric transformer used in a high-voltage generator, and to a power source apparatus employing the piezoelectric transformer.
2. Description of the Related Art
FIG. 16
illustrates a structure of a Rosen piezoelectric transformer which is one of typical piezoelectric transformers. Favorably, such the piezoelectric transformers can be sized smaller in the dimensions than electromagnetic transformers as is nonflammable and free from noises due to electromagnetic induction.
As shown in
FIG. 16
, the piezoelectric transformer comprises a low-impedance section
301
and a high-impedance section
302
. The low-impedance section
301
acts as an input section when used for stepping up. The section
301
is polarized along the thickness direction, and has two electrodes
303
u
and
303
d
provided on both principle faces of the thickness direction. The high-impedance section
302
acts as an output section when used for stepping up. The section
302
is polarized along the long direction, and has an electrode
304
provided on a face of the long direction.
FIG. 17
shows a frequency response of the above described piezoelectric transformer. Recently the piezoelectric transformer has been used as a power source for a cold cathode ray tube because it has characteristics in that, the greater the load, the higher the step-up ratio increases (curve P
1
) and the smaller the load, the lower the step-up ratio decreases (curve P
2
).
The piezoelectric transformer may be driven by a separately-excited oscillator circuit which is provided with an external oscillator.
FIG. 18
is a block diagram of a conventional drive circuit with a separately-excited oscillation method employing the Rosen-type piezoelectric transformer.
As shown in
FIG. 18
, a variable oscillator circuit
221
generates an alternating-current drive signal of which frequency is close to the resonant frequency of a piezoelectric transformer
110
. The output signal of the variable oscillator circuit
221
contains other alternating-current signal components than the driving frequency signal. Those signal components generate heat or a loss in the piezoelectric transformer
110
. To reduce the loss in the piezoelectric transformer
110
, the output signal is shaped to substantially a sine wave by a waveform shaping circuit
224
. The waveform shaping circuit
224
may simply be a band-pass filter for reducing harmonic components. The output of the waveform shaping circuit
224
is then amplified by a drive circuit
225
to a current level or a voltage level enough to actuate the piezoelectric transformer
110
. The drive circuit
225
may comprise only a normal amplifier circuit composed of transistors, or a combination of an amplifier circuit and a step-up transformer. The output of the drive circuit
225
is stepped up by the piezoelectric transformer
110
, applied to a load such as a cold cathode fluorescent lamp
108
to light on.
The piezoelectric transformer
110
may be varied in the resonant frequency depending on the ambient conditions including the temperature and the load. Therefore when the piezoelectric transformer
110
is driven at a constant frequency by the circuit shown in
FIG. 18
, relative relation between the resonant frequency and the driving frequency will change. More specifically, in case that the driving frequency is largely differentiated from the resonant frequency of the piezoelectric transformer
110
, the voltage step-up ratio of the piezoelectric transformer
110
will decline significantly thus lowering the output voltage. As a result, the cold cathode fluorescent lamp
108
as a load may be supplied with insufficient current and fail to provide a desired level of luminance.
FIG. 19
is a block diagram of another conventional circuit for driving the piezoelectric transformer different from the circuit shown in
FIG. 16
which can prevail over a change in the resonant frequency of the piezoelectric transformer
110
. The cold cathode fluorescent lamp
108
serving as a load in the circuit is connected in series with a feedback resistor
109
having a small resistance. The feedback resistor
109
detects a current across the cold cathode fluorescent lamp
108
. A resultant voltage across the feedback resistor
109
which is proportional to the current flowing through the cold cathode fluorescent lamp
108
is fed into a current detector circuit
232
. An output of the current detector circuit
232
is applied to an oscillation control circuit
214
. The oscillation control circuit
214
in turn controls the frequency of the output of the variable oscillator circuit
221
so that the voltage across the feedback resistor
109
or the current across the cold cathode fluorescent lamp
108
can be constant. This control permits the cold cathode fluorescent lamp
108
to light up at substantially a uniform level of luminance.
FIG. 20
is a block diagram of another modification of the conventional circuit for driving the piezoelectric transformer shown in
FIG. 16
which can prevail over a change in the resonant frequency of the piezoelectric transformer. In this modification, the current across the cold cathode fluorescent lamp
108
is detected by the feedback resistor
109
. When the resonant frequency of the piezoelectric transformer
110
is varied with a change in the load or the ambient conditions, the current across the cold cathode fluorescent lamp
108
may change. The voltage across the feedback resistor
109
which is proportional to the current across the cold cathode fluorescent lamp
108
is fed into the current detector circuit
232
. The output of the current detector circuit
232
is then fed into a pulse width control circuit
223
. The pulse width control circuit
223
in turns generates and delivers a control signal to a pulse width modifying circuit
222
so that the voltage across the feedback resistor
109
or the current across the cold cathode fluorescent lamp
108
can be constant. Upon receiving the control signal, the pulse width modifying circuit
222
adjusts the pulse width of the output signal to determine the amplitude of the voltage applied to the cold cathode fluorescent lamp
108
. This control permits the cold cathode fluorescent lamp
108
to light up at substantially a uniform level of luminance.
As described above, the conventional drive circuit using the piezoelectric transformer controls the driving frequency of the piezoelectric transformer to keep a level of output current flowing through the load connected to the piezoelectric transformer constant. That is, to increase the output current, the driving frequency is kept away from the resonant frequency. In the conventional circuit, however, once the source voltage is declined, it becomes impossible to flow a sufficient level of current through the piezoelectric transformer even though the driving frequency is forced to match the resonant frequency. Thus, a desired level of the output current can not be provided. In reverse, when the source voltage is increased, the driving frequency is differentiated from the resonant frequency of the piezoelectric transformer hence lowering the driving efficiency. Also, in case that the load to the piezoelectric transformer is largely changed, simultaneously, the output current of the piezoelectric transformer can hardly be controlled to a specified value, and the driving frequency may be differentiated from the resonant frequency of the piezoelectric transformer, hence lowering largely the driving efficiency.
There are some techniques to modify the output voltage of the drive circuit without changing the driving frequency when the source voltage is varied or the load to the piezoelectric transformer is varied. One of the most known techniques is to modify the pulse width of the output voltage of the drive circuit. In this case, the narrower the pulse width or the smaller the duty, the greater harmonic components other than the basic driving frequencies will
Moritoki Katsunori
Nakatsuka Hiroshi
Okuyama Kojiro
Takeda Katsu
Yamaguchi Takeshi
Aguirrechea J.
Greenblum & Bernstein P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Mullins Burton S.
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