Electric lamp and discharge devices: systems – Current and/or voltage regulation – Automatic regulation
Reexamination Certificate
1999-02-26
2001-05-29
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
Automatic regulation
C315S224000, C315SDIG004, C310S316020, C310S318000
Reexamination Certificate
active
06239558
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a control circuit and method for a piezoelectric transformer and, more particularly, to a control circuit and method for a piezoelectric transformer suited for use in a driving apparatus for a cold-cathode fluoresent lamp(CCFL).
BACKGROUND ART
Recently, liquid crystal displays are extensively used as display devices of, e.g., portable notebook personal computers. These liquid crystal display devices incorporate a cold-cathode fluoresent lamp as a so-called back light in order to illuminate a liquid crystal display panel from the back. Turning on this cold-cathode fluoresent lamp requires a inverter capable of converting a low DC voltage of a battery or the like into a high AC voltage of 1,000 Vrms or more in an initial lighting state and about 500 Vrms in a steady lighting state. Conventionally, a winding transformer is used as a boosting transformer of this inverter. In recent years, however, a piezoelectric transformer which performs electric conversion via mechanical energy and thereby performs boosting is beginning to be used. This piezoelectric transformer has a generally unpreferable characteristic, i.e., largely changes its boosting ratio in accordance with the magnitude of an output load (load resistance). On the other hand, this dependence upon a load resistance is suited to the characteristics of an inverter power supply for a cold-cathode fluoresent lamp. Accordingly, a piezoelectric transformer has attracted attention as a small-sized, high-voltage power supply meeting the demands for a low profile and a high efficiency of a liquid crystal display device. A basic configuration of a control circuit for this piezoelectric transformer will be described below with reference to
FIGS. 1
to
4
.
FIG. 1
is a block diagram of a piezoelectric transformer control circuit as the first prior art.
In
FIG. 1
, reference numeral
101
denotes a piezoelectric transformer;
102
, a load such as a cold-cathode fluoresent lamp connected to the output terminal of the piezoelectric transformer
101
;
103
, an oscillation circuit for oscillating an AC signal such as a rectangular wave; and
104
, a driving circuit for driving the piezoelectric transformer
101
in accordance with the oscillation signal from the oscillation circuit
103
.
It is generally known that a piezoelectric transformer largely changes the output voltage in the form of a hill in accordance with the frequency of an input AC voltage, the output voltage takes a maximum value when the piezoelectric transformer is driven by its resonance frequency, and the resonance frequency changes in accordance with the temperature or the magnitude (load resistance) of an output load. Accordingly, the general approach is to cause the oscillation circuit
103
to output an oscillation signal equal in frequency to the resonance frequency and drive the piezoelectric transformer
101
by the driving circuit
104
on the basis of this oscillation signal, thereby generating a high voltage at the output terminal of the piezoelectric transformer
101
.
In the block diagram of
FIG. 1
, the driving circuit
104
can have an arrangement as shown in FIG.
2
.
FIG. 2
is a block diagram of a piezoelectric transformer control circuit as the second prior art. In
FIG. 2
, a driving circuit
104
includes a p-type transistor (FET: Field-Effect Transistor)
104
a
and an n-type transistor (FET)
104
b
which are so connected as to form a half-bridge circuit. These two transistors (
104
a
and
104
b
) alternately perform switching in accordance with the state of an output oscillation signal from an oscillation circuit
103
. By this switching operation of the driving circuit
104
, a driving voltage (AC voltage) whose amplitude is an input voltage Vi is applied to a piezoelectric transformer
101
.
In the control circuits having the above configurations, it is necessary to change a lamp current (load current) flowing in a cold-cathode fluoresent lamp connected as the load
102
in order to control the brightness of the cold-cathode fluoresent lamp. To this end, the applied voltage (the output voltage from the piezoelectric transformer
101
) to the cold-cathode fluoresent lamp must be adjusted. To adjust the applied voltage, it is necessary to regulate the oscillation signal from the oscillation circuit
103
as the basis of the applied voltage. The area in an ON period of this oscillation signal can be regarded as an energy amount supplied to the piezoelectric transformer
101
. Accordingly, the output voltage from the piezoelectric transformer
101
can be changed by changing this energy amount. In conventional piezoelectric transformer control circuits, therefore, the brightness of a cold-cathode fluoresent lamp is controlled by methods as shown in
FIGS. 3 and 4
.
FIGS. 3 and 4
are timing charts for explaining conventional methods of controlling the brightness of a cold-cathode fluoresent lamp.
In the method shown in
FIG. 3
, the energy amount supplied from a driving circuit to a piezoelectric transformer is regulated by changing the amplitude of an oscillation signal, and thereby (the amplitude of) an output voltage is adjusted. In the method shown in
FIG. 4
, as disclosed in, e.g., Japanese Patent Laid-Open No. 5-64437 or 7-220888, a PWM (Pulse Width Modulation) circuit (not shown) is arranged in the control circuit shown in
FIG. 1
or
2
. In accordance with a signal from this PWM circuit, the duty ratio (Ton/(Ton+Toff)) of an oscillation signal from an oscillation circuit is changed to regulate the energy amount supplied from a driving circuit to a piezoelectric transformer, thereby adjusting (the amplitude of) an output voltage.
Unfortunately, when the brightness of a cold-cathode fluoresent lamp as a load is lowered by dropping the output voltage from a piezoelectric transformer by the above conventional methods, the lighting state becomes unstable if the output voltage from the piezoelectric transformer becomes lower than a voltage necessary to keep the cold-cathode fluoresent lamp discharging. The resultant flickering is of a problem to a human visual sense. Accordingly, adjusting the brightness by the above conventional methods has the problem that a dimming range within which a stable brightness is obtained is narrow.
A piezoelectric transformer control circuit as still another prior art capable of holding the brightness of a cold-cathode fluoresent lamp at a predetermined brightness will be described below with reference to
FIGS. 5
to
10
.
FIG. 5
is a block diagram of a piezoelectric transformer control circuit as the third prior art.
In
FIG. 5
, reference numeral
201
denotes a piezoelectric transformer;
202
, a load such as a cold-cathode fluoresent lamp connected to the output terminal of the piezoelectric transformer
201
;
203
, a detecting resistor Rdet for detecting a current flowing in the load;
204
, a rectifying circuit for converting an AC voltage generated in the detecting resistor
203
into a DC voltage;
205
, an error amplifier for comparing a voltage Vri rectified by the rectifying circuit
204
with a reference voltage Vref and amplifying the difference as a comparison result;
206
, a voltage-controlled oscillation circuit for outputting an oscillation signal in accordance with the output voltage from the error amplifier
205
; and
207
, a driving circuit for driving the piezoelectric transformer
201
in accordance with the oscillation signal from the voltage-controlled oscillation circuit
206
. The operation of the control circuit with the above configuration will be described below with reference to
FIGS. 6A and 6B
.
FIG. 6A
is a graph for explaining the relationship between the frequency and the output voltage of a piezoelectric transformer.
FIG. 6B
is a graph for explaining the relationship between the frequency of a piezoelectric transformer and the load current of a load connected to the piezoelectric transformer.
As shown in
FIG. 6A
, the piezoelectric transformer
201
has a hilly resonance frequency characteristic whose peak is
Fujimura Takeshi
Ishikawa Katsuyuki
Toyama Masaaki
Burns Doane , Swecker, Mathis LLP
Taiheiyo Cement Corporation
Vu David
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