Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch cut-out
Reexamination Certificate
1999-11-16
2002-11-26
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch cut-out
C315S127000, C315S276000
Reexamination Certificate
active
06486612
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 fluorescent lamp.
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 fluorescent lamp as a so-called back light in order to illuminate a liquid crystal display panel from the back. Turning on this cold-cathode fluorescent lamp requires an 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) such that the boosting ratio is high for a small output load (large load resistance) and is low for a large output load (small 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 fluorescent 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. An example of a control circuit for this piezoelectric transformer will be described below with reference to FIG.
1
.
FIG. 1
is a block diagram of a piezoelectric transformer control circuit as the prior art.
In
FIG. 1
, reference numeral
101
denotes a piezoelectric transformer;
102
, a load such as a cold-cathode fluorescent lamp connected to the output terminal of the piezoelectric transformer
101
;
103
, a detecting resistor Rdet for detecting a current flowing in the load;
104
, a rectifying circuit for converting an AC voltage generated in the detecting resistor
103
into a DC voltage;
105
, an error amplifier for comparing a voltage Vri rectified by the rectifying circuit
104
with a reference voltage Vref and amplifying the difference as a comparison result;
106
, a voltage-controlled oscillation circuit for outputting an oscillation signal in accordance with the output voltage from the error amplifier
105
; and
107
, a driving circuit for driving the piezoelectric transformer
101
in accordance with the oscillation signal from the voltage-controlled oscillation circuit
106
. The operation of the control circuit with the above configuration will be described below with reference to
FIGS. 2A and 2B
.
FIGS. 2A and 2B
are graphs for explaining an example of frequency characteristics for the output voltage and load current of a piezoelectric transformer.
As shown in
FIG. 2A
, the piezoelectric transformer
101
has a hilly resonance frequency characteristic whose peak is the resonance frequency of the piezoelectric transformer
101
. It is generally known that a current flowing in the load
102
due to the output voltage from the piezoelectric transformer
101
also has a similar hilly characteristic. In
FIG. 2B
, this load current is represented by a load current detection voltage Vri. Control using a right-side (falling) portion in this characteristic will be described below. When the power supply of this control circuit is turned on, the voltage-controlled oscillation circuit
106
starts oscillating at an initial frequency fa. Since no current flows in the load
102
at that time, the voltage generated in the detecting resistor (Rdet)
103
is zero. Accordingly, the error amplifier
105
outputs a negative voltage, as a result of comparison of the load current detection voltage Vri with the reference voltage Vref, to the voltage-controlled oscillation circuit
106
. In accordance with this voltage, the voltage-controlled oscillation circuit
106
shifts the oscillation frequency of an oscillation signal to a lower frequency. Therefore, as the frequency is shifted to a lower frequency, the output voltage from the piezoelectric transformer
101
rises, and the load current (load current detection voltage Vri) also increases. When the load current (load current detection voltage Vri) become equal to the reference voltage Vref, the frequency stabilizes (fb). Even if the resonance frequency changes due to a temperature change or a change with time, the frequency shifts accordingly to always hold the load current substantially constant.
In the control circuit shown in
FIG. 1
, therefore, frequency control is so performed that the load current detection voltage Vri becomes equal to the reference voltage Vref, and the load current is held at a predetermined value by this frequency control. When a cold-cathode fluorescent lamp is used as a load in this piezoelectric transformer control circuit and the control circuit is used as a lighting device for the cold-cathode fluorescent lamp, a function of holding the luminance of the cold-cathode fluorescent lamp at a predetermined luminance can be achieved since the luminance of the cold-cathode fluorescent lamp is proportional to a lamp current flowing in the cold-cathode fluorescent lamp.
In this conventional piezoelectric transformer control circuit, however, when the load connected to the output terminal of the piezoelectric transformer is disconnected due to some reason, and the output terminal changes to a so-called open state, a high voltage is generated at the output terminal of the piezoelectric transformer in accordance with the open state.
At this time, if the frequency of the oscillation signal for driving the piezoelectric transformer does not vary, such a high voltage as to induce physical damage to the piezoelectric transformer is not generated because the operating point of the piezoelectric transformer shifts from the resonance frequency. However, when the output terminal of the piezoelectric transformer
101
actually changes to the open state, for example, the above control circuit shown in
FIG. 1
detects that no current flows through the load current detecting resistor (Rdet)
103
, and the oscillation frequency (operating point) of the voltage-controlled oscillation circuit
106
is swept to a lower frequency by the function of keeping the load current substantially constant. As a result, the operating point of the piezoelectric transformer
101
becomes equal to the resonance frequency to generate such a high voltage (10 kV or higher) as to physically damage the piezoelectric transformer
101
at the output terminal.
In the control circuit of
FIG. 1
, assume that the output of the piezoelectric transformer
101
is short-circuited to, e.g., ground (GND) (so-called grounding) due to some reason (note that the output may be short-circuited to a negative potential). Even in this state, if the frequency of the oscillation signal for driving the piezoelectric transformer
101
does not vary, no physical damage to the piezoelectric transformer
101
is induced. In practice, however, similar to the open state, when the control circuit detects that no current flows through the load current detecting resistor (Rdet)
103
, the operating point of the piezoelectric transformer
101
is swept to a lower frequency by the voltage-controlled oscillation circuit
106
with the function of keeping the load current substantially constant, and the operating point of the piezoelectric transformer
101
may pass a resonance point (near the peaks in FIGS.
2
A and
2
B). Since the piezoelectric transformer
101
is in a series resonance state at a resonance frequency unique to the piezoelectric transformer, and the output of the piezoelectric transformer
101
is sh
Fujimura Takeshi
Ishikawa Katsuyuki
Toyama Masaaki
Burns Doane , Swecker, Mathis LLP
Taiheiyo Cement Corporation
Vu David
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