JP2000164384A - Fluorescent tube lighting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fluorescent tube lighting device capable of driving a load with a low current even when the impedance of the load is higher than the output impedance of a piezoelectric transformer. SOLUTION: The AC voltage applied to the primary side electrode of a piezoelectric transformer 1 is voltage-converted, to supply a high AC to the secondary side electrode and a fluorescent tube 3 in this cold-cathode tube lighting device is lit. ballast capacitor is connected between the secondary side electrode of the piezoelectric transformer 1 and the fluorescent tube 3, and a load is driven by a low current even when the impedance of the load is higher than the output impedance of the piezoelectric transformer 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorescent tube lighting apparatus, and more particularly, to a fluorescent tube load, particularly a thin tube cold cathode tube having a high tube voltage and a small load current, which converts a direct current into an alternating current using a piezoelectric transformer. The present invention relates to a fluorescent tube lighting device for lighting.

[0002]

2. Description of the Related Art Fluorescent tube lighting devices (hereinafter, abbreviated as inverters) using a wound electromagnetic transformer or a piezoelectric transformer are:
It is widely used for lighting cold cathode fluorescent lamps for liquid crystal backlights of portable devices and notebook personal computers. In recent years, as the demand for lower power consumption of these information devices has increased, the power consumption of cold cathode tubes has been rapidly reduced.

Specifically, cold-cathode tubes, such as a thin-tube cold-cathode tube and a double tube, capable of keeping the temperature of the plasma inside the tube higher than before and increasing the brightness even with the same tube current have been widely used. I have. These cold-cathode tubes are characterized in that the tube voltage is high and the tube current is small as compared with conventional ones. That is, when viewed from the inverter side, it is necessary to drive a load having a higher voltage and a higher equivalent impedance.
On the other hand, there is also a strong demand for downsizing of information devices, and there is also a demand for downsizing and thinning of inverters.

Conventionally, winding inverters have been the mainstream as inverters. In general, a method of generating a constant high voltage by a self-excited oscillating circuit and absorbing a load characteristic of a cold cathode tube by a ballast capacitor inserted between the wound inverter and the cold cathode tube is used (ballast).

On the other hand, in a piezoelectric inverter which has been developed recently and has been widely used, an external ballast capacitor is not required since the piezoelectric transformer itself has a ballast effect.

[0006]

However, these conventional techniques have the following problems.

[0007] As the size of the winding inverter is reduced, the height and the size of the winding transformer must be reduced. In the winding transformer, it is necessary to secure a certain distance between the windings to secure insulation between the windings, and it is difficult to reduce the size.
Further, when the height is reduced, there is a problem that the output voltage cannot be increased, and it is difficult to apply the present invention to a recent high tube voltage type cold cathode tube.

The piezoelectric inverter does not require the problem of insulation between windings, and can maintain the maximum output voltage relatively high even when the height is reduced. However, there is a problem that it is difficult to perform impedance matching with a cold cathode tube load.
This is described below.

The cold cathode tube has a very high equivalent impedance when turned off, and changes to an impedance of about several tens to several hundreds kΩ when turned on. As described above, since the impedance change of the load is large, the output impedance of the inverter needs to be equal to or higher than the equivalent impedance of the cold-cathode tube in order to stably turn on the cold-cathode tube at a constant current. . In a wound inverter, the output impedance can be easily increased by reducing the capacitance of the ballast capacitor. However, it is relatively difficult to increase the output impedance of the piezoelectric inverter for the above reason.

When the thickness of the piezoelectric transformer is reduced to increase the output impedance, the volume of the piezoelectric transformer is reduced,
The output power that can be handled is insufficient. If the piezoelectric transformer is lengthened to increase the output impedance, the operating frequency decreases. It is said that driving a cold-cathode tube at a frequency of about 40 kHz or less extremely shortens the life of the tube, and the operating frequency cannot be reduced very much. Enlarging the dimensions goes against the demand for miniaturization.

If the dielectric constant of the piezoelectric transformer is reduced in order to increase the output impedance, the material composition that can be used for the piezoelectric transformer is limited, and the dielectric constant cannot be easily reduced.

SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide a fluorescent tube lighting device capable of driving a load with a low current even when the load impedance is higher than the output impedance of a piezoelectric transformer.

[0013]

The invention according to claim 1 is
In a cold-cathode tube lighting device that turns on a fluorescent tube load by using a piezoelectric transformer that converts an AC voltage applied between the primary side electrodes and supplies a high-voltage AC voltage to the secondary side electrode, the secondary of the piezoelectric transformer A ballast capacitor is connected between the side electrode and the load.

[0014]

FIG. 1 is a block diagram of one embodiment of the present invention. In FIG. 1, a piezoelectric transformer 1 uses a piezoelectric effect to boost an AC voltage applied to a primary electrode, outputs the boosted voltage from a secondary electrode, and supplies an AC current to a fluorescent tube load 3 via a ballast capacitor 2. I do. The tube current control means 4 detects / rectifies the current flowing through the fluorescent tube load 3, and varies the driving frequency so that the current flowing through this load becomes constant. The drive means 5 applies a drive voltage to the primary electrode of the piezoelectric transformer 1 at the oscillation frequency of the tube current control means.

The circuit shown in FIG. 1 uses a tube current stabilizing circuit of a frequency control type widely used in a piezoelectric transformer inverter. Since the piezoelectric transformer 1 is a transformer using mechanical resonance, the boosting ratio can be largely changed by slightly changing the driving frequency from the resonance frequency.

In the piezoelectric transformer 1, when the frequency is increased, the step-up ratio decreases. This region is generally used because the portion where the step-up ratio is not inclined has high efficiency. In the example shown in FIG. 1, when the tube current decreases due to some disturbance, the output frequency of the tube current control means 4 decreases. As a result, the step-up ratio of the piezoelectric transformer 1 increases, and the tube current increases. Conversely, when the tube current increases, the frequency decreases, and control is applied in the direction in which the step-up ratio of the piezoelectric transformer 1 decreases and the tube current decreases.
By this operation, the cold-cathode tube current is controlled to be constant.

However, even if such control is applied,
In an application where the load impedance of the cold cathode tube is sufficiently larger than the output impedance of the piezoelectric transformer 1, there is a problem that the current of the cold cathode tube fluctuates and becomes unstable. Therefore, in the present invention, the ballast capacitor 2 is inserted between the output of the piezoelectric transformer 1 and the fluorescent tube 3. Because of the ballast capacitor 2, the impedance seen from the fluorescent tube 3 increases, and the fluorescent tube 3 is driven with a substantially low current.

[0018]

As described above, according to the present invention, by inserting a ballast capacitor between the load and the piezoelectric transformer, the load can be reduced even when the load impedance is higher than the output impedance of the piezoelectric transformer. It can be driven by current. Further, the characteristics of the piezoelectric transformer, such as the ability to output a low profile and a high voltage, can be utilized as it is. For this reason, a low-profile and compact inverter can be realized,
In addition, it is possible to stably light a thin tube cold cathode tube having a high equivalent impedance.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric transformer 2 Ballast capacitor 3 Fluorescent tube 4 Tube current control means 5 Driving means

Claims (1)

[Claims] 1. A fluorescent tube lighting device for lighting a fluorescent tube load using a piezoelectric transformer that converts an AC voltage applied to a primary-side electrode tube to supply a high-voltage AC voltage to a secondary-side electrode, A lighting device for a fluorescent tube, wherein a ballast capacitor is connected between a secondary electrode of a piezoelectric transformer and the load.