Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
2000-03-31
2001-08-28
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S276000, C315S282000, C310S314000, C310S316010, C310S318000
Reexamination Certificate
active
06281637
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric transformer for driving a discharge tube such as a cold-cathode tube or the like for use as the back light of a liquid crystal display or the like, and a discharge lamp device including the same.
2. Description of the Related Art
Cold-cathode tubes for use as a light source for the back light of a liquid crystal display have a high impedance of several M&OHgr; when they are lit, and require a high voltage of at least 1 kV. The impedance while they are lit is decreased to about dozen k&OHgr; to several hundred k&OHgr;, and they are driven at a several hundred volts. Conventionally, as a high voltage source for driving such a discharge tube, a piezoelectric transformer which can be easily reduced in size and for which the required power can be simply reduced has been used.
FIG. 11
shows an example of the piezoelectric transformer, which is a so-called Rosen type piezoelectric transformer, and makes use of a primary longitudinal vibration mode in the longitudinal direction. As regards the constitution of the piezoelectric transformer, an input voltage is applied across input electrodes
2
a
and
2
b
through terminals IN-GND on the input side. An output voltage boosted and generated at an output electrode, caused by the piezoelectric effect and the inverse piezoelectric effect, is output through terminals OUT-GND on the output side.
However, when a cold-cathode tube is connected directly to such a piezoelectric transformer, the tube current of the cold-cathode tube is oscillated in some cases as shown in
FIG. 8
, depending of the input voltage to the piezoelectric transformer and its drive frequency. In
FIG. 8
, time is plotted as abscissa, and the tube current as ordinate. The high frequency component of this periodic waveform is the drive frequency of the piezoelectric transformer. The current flows as if the tube current is amplitude-modulated at a lower frequency than the drive frequency.
Japanese Unexamined Patent Publication Nos. 10-125970, 10-14448, and 10-150230 describes piezoelectric transformers and cold-cathode tube lighting devices of which the purpose lies in that the above-described abnormal oscillation of a tube current is prevented.
In the piezoelectric transformers and the cold-cathode tube lighting devices using the piezoelectric transformers described in the above-mentioned official gazettes, basically, a capacitor element is inserted between a terminal on the output side of a piezoelectric transformer and a cold-cathode tube, so that the output impedance of the piezoelectric transformer is effectively enhanced, and the tube current is stabilized. Such a method of stabilizing a tube current by insertion of a capacitor element is very effective. However, it is necessary to provide an additional capacitor element. Thus, there arises the problem that the number of components is increased, and the manufacturing process is complicated.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a piezoelectric transformer and a discharge lamp device, in which the tube current of a discharge tube such as a cold-cathode tube or the like is stabilized, and the piezoelectric transformer itself causes a stable tube current to flow through the discharge tube without need of inserting an additional capacitor element between a terminal on the output side of the piezoelectric transformer and the cold-cathode tube.
According to a first aspect of the present invention, a piezoelectric transformer for outputting a drive voltage to a discharge tube as a load is characterized in that the value obtained by dividing the output current of the piezoelectric transformer at a load resistance of about zero into the output voltage of the piezoelectric transformer at a load resistance of about infinity is set to be larger than about one-third of the maximum impedance of the discharge tube during discharging.
The reason of this requirement will be described in the embodiments. The output impedance of the piezoelectric transformer is defined as a value obtained by dividing the output current of the piezoelectric transformer at a load resistance of about zero into the output voltage at a load resistance of about infinity. The output characteristic of the piezoelectric transformer becomes approximately that of a constant current source by setting the above-defined output impedance of the piezoelectric transformer to be larger than about one-third of the maximum impedance of the discharge tube during discharging (during lighting in the case where the discharge tube is a cold-cathode tube), so that the tube current of the discharge tube is stabilized. When the discharge tube is a discharge lamp comprising a cold-cathode tube, the lighting condition is stabilized.
Preferably, the condition that the load resistance is about infinite is obtained under the condition that a capacitive load is connected in parallel to the piezoelectric transformer. For example, if the cold-cathode tube is incorporated into a panel unit for which measures against noise have been taken, it means that a capacitive load is equivalently connected to the piezoelectric transformer. The tube current in the practical use condition can be stabilized by determining the above-described output impedance including the capacitive load.
According to a second aspect of the present invention, the absolute value of the gradient of a change in output current of the piezoelectric transformer based on a change in output voltage thereof at an impedance substantially equivalent to the impedance of the discharge tube during discharging is set to be smaller than the absolute value of the gradient of a change in tube current of the discharge tube during discharging based on a change in tube voltage of the discharge tube during discharging.
The reason of this requirement will be also described in the embodiments. With the above-described configuration, a fluctuation in input source voltage to the piezoelectric transformer, if occurs, is prevented from significantly changing the tube current of the discharge tube. Thus, the tube current of the discharge tube is stabilized.
Preferably, the impedance of the discharge tube during discharging is the impedance of the discharge tube measured under the condition that a capacitive load is connected in parallel to the piezoelectric transformer. Thus, the tube current in the practical use condition can be stabilized.
REFERENCES:
patent: 5705879 (1998-01-01), Abe et al.
patent: 5923542 (1999-07-01), Saskai et al.
patent: 5923546 (1999-07-01), Shimada et al.
patent: 6028398 (2000-02-01), Kawasaki et al.
patent: 10-144484 (1998-05-01), None
patent: 10-125970 (1998-05-01), None
patent: 10-150230 (1998-06-01), None
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Philogene Haissa
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