Cold-cathode tube lighting device for use in a plurality of cold-cathode tubes lit by two low-impedance power sources Number:7,436,130 from the United States Patent and Trademark Office (PTO) owispatent A cold-cathode tube lighting device uniformly lights a plurality of cold-cathode tubes using a common power source, and maintains the luminance of each cold-cathode tube uniformly in the longitudinal direction thereof at high precision. A first block converts a direct-current voltage to one pair of alternating-voltages. Since leakage impedances of step-up transformers are low, the first block functions as one pair of low-impedance power sources. Each second block is connected to each cold-cathode tube. A ballast inductor stabilizes tube current by resonating with a matching capacitor during lighting of the cold-cathode tube. A combined impedance of the matching capacitor and a peripheral stray capacitance is matched with an impedance of the ballast inductor, for each cold-cathode tube. Since a delay circuit shifts phases of two pulse waves with respect to each other, a phase difference between the alternating-voltages is shifted from 180.degree..<H impedance, plurality, Cold, cathode, tu, 7436130.html, Patent, pto, 7436130

Title: Cold-cathode tube lighting device for use in a plurality of cold-cathode tubes lit by two low-impedance power sources

Abstract: A cold-cathode tube lighting device uniformly lights a plurality of cold-cathode tubes using a common power source, and maintains the luminance of each cold-cathode tube uniformly in the longitudinal direction thereof at high precision. A first block converts a direct-current voltage to one pair of alternating-voltages. Since leakage impedances of step-up transformers are low, the first block functions as one pair of low-impedance power sources. Each second block is connected to each cold-cathode tube. A ballast inductor stabilizes tube current by resonating with a matching capacitor during lighting of the cold-cathode tube. A combined impedance of the matching capacitor and a peripheral stray capacitance is matched with an impedance of the ballast inductor, for each cold-cathode tube. Since a delay circuit shifts phases of two pulse waves with respect to each other, a phase difference between the alternating-voltages is shifted from 180.degree..

Patent Number: 7,436,130 Issued on 10/14/2008 to Komatsu, et al.

Inventors: Komatsu; Akeyuki (Osaka, JP), Miyake; Eiji (Osaka, JP), Kawataka; Kenji (Osaka, JP)
Assignee: Matsushita Electric Industrial Co., Ltd. (Osaka, JP)

Appl. No.: 10/578,200

Filed: April 21, 2005

PCT Filed: April 21, 2005

PCT No.: PCT/JP2005/007652

371(c)(1),(2),(4) Date: July 11, 2006

PCT Pub. No.: WO2005/109967

PCT Pub. Date: November 17, 2005

Foreign Application Priority Data


May 10, 2004 [JP]			2004-139406

Current U.S. Class: 315/307 ; 315/246

Current International Class: H05B 37/02 (20060101)

Field of Search: 315/209R,224-225,246-247,276,291,307-308,312 345/102,87,204

References Cited [Referenced By]

U.S. Patent Documents


4508996	April 1985	Clegg et al.
5959412	September 1999	Ushijima
6222327	April 2001	Shoji et al.
6559606	May 2003	Chou et al.
6661181	December 2003	Shin
6714111	March 2004	Suzuki
6864644	March 2005	Kernahan
6979957	December 2005	Lee et al.
7034800	April 2006	Nakatsuka et al.
7038397	May 2006	Komatsu et al.
2002/0047615	April 2002	Yokozeki et al.
2004/0095081	May 2004	Kernahan
2004/0207339	October 2004	Lin et al.
2004/0232853	November 2004	Hur et al.
2005/0134192	June 2005	Ellams
2005/0156534	July 2005	Oh
2005/0285478	December 2005	Takeda et al.
2007/0052664	March 2007	Hirakata et al.

Foreign Patent Documents


5-90897	Dec., 1993	JP
6-19299	Mar., 1994	JP
6-301034	Oct., 1994	JP
8-122776	May., 1996	JP
8-273862	Oct., 1996	JP
8-288086	Nov., 1996	JP
2002-353044	Dec., 2002	JP
2004-119206	Apr., 2004	JP
2004-241136	Aug., 2004	JP
2005-63941	Mar., 2005	JP
02/078403	Oct., 2002	WO
03/056885	Jul., 2003	WO

Primary Examiner: Tran; Thuy Vinh
Assistant Examiner: Le; Tung X
Attorney, Agent or Firm: Wenderoth, Lind & Ponack, L.L.P.

Claims

The invention claimed is:

1. A cold-cathode tube lighting device comprising: a plurality of ballasts, at least one of said ballasts being connected to an electrode at one end of each of a plurality of cold-cathode tubes; a first low-impedance power source having an output impedance lower than a combined impedance of said cold-cathode tubes, said first low-impedance power source being connected to the electrode at one end of each of said cold-cathode tubes via said ballasts; a second low-impedance power source having an output impedance lower than the combined impedance of said cold-cathode tubes, said second low-impedance power source being connected to an electrode at the other end of each of said cold-cathode tubes; and a phase correction circuit for adjusting a phase difference between an output from said first low-impedance power source and an output from said second low-impedance power source, so that electrode potentials at both ends of each of said cold-cathode tubes change in opposite phase with respect to each other, wherein said phase correction circuit comprises a delay circuit for delaying one of a first pulse signal for instructing an output timing with respect to said first low-impedance power source and a second pulse signal for instructing an output timing with respect to said second low-impedance power source, from the other signal by a constant quantity.

2. The cold-cathode tube lighting device as claimed in claim 1, wherein said first low-impedance power source, said second low-impedance power source, and said phase correction circuit are mounted on a first substrate, and wherein said ballasts are mounted on a second substrate.

3. The cold-cathode tube lighting device as claimed in claim 2, wherein one end of each of said cold-cathode tubes is connected to said second substrate.

4. The cold-cathode tube lighting device as claimed in claim 1, further comprising a detector for detecting current flowing through said cold-cathode tubes, or an electrode potential at one end of each of said cold-cathode tubes, wherein said phase correction circuit changes the phase difference based on a detected value detected by said detector.

5. The cold-cathode tube lighting device as claimed in claim 1, wherein each of said first low-impedance power source and said second low-impedance power source comprises a transformer connected to said ballasts, and said transformer has an output impedance lower than the combined impedance of said plurality of cold-cathode tubes.

6. The cold-cathode tube lighting device as claimed in claim 5, wherein said transformer comprises a core, a primary winding being wound around said core, and a secondary winding being wound around at least one of an inside and outside of said primary winding.

7. The cold-cathode tube lighting device as claimed in claim 6, wherein said secondary winding has one configuration of a sectional winding and a honeycomb winding.

8. The cold-cathode tube lighting device as claimed in claim 1, wherein each of said first low-impedance power source and said second low-impedance power source comprises a power transistor connected to said ballasts.

9. The cold-cathode tube lighting device as claimed in claim 1, wherein each of said ballasts comprises an inductor.

10. The cold-cathode tube lighting device as claimed in claim 9, wherein said inductor has one configuration of a sectional winding and a honeycomb winding.

11. The cold-cathode tube lighting device as claimed in claim 10, wherein said inductor comprises a saturable reactor.

12. The cold-cathode tube lighting device as claimed in claim 1, wherein each of said ballasts comprises a capacitor.

13. The cold-cathode tube lighting device as claimed in claim 12, wherein said capacitor has an inter-layer capacity of a substrate.

14. The cold-cathode tube lighting device as claimed in claim 1, further comprising: matching capacitors, at least one of said matching capacitors being connected across a ground potential and the electrode at one end of each cold-cathode tube connected to said ballast.

15. The cold-cathode tube lighting device as claimed in claim 14, wherein each of said matching capacitors has an inter-layer capacity of a substrate.

16. The cold-cathode tube lighting device as claimed in claim 14, wherein an impedance of said ballast and an impedance of said matching capacitor are matched with each other.

17. The cold-cathode tube lighting device as claimed in claim 14, wherein an impedance of said ballast, a combined impedance of said matching capacitor and a stray capacitance in the periphery of said cold-cathode tube, and an impedance of said cold-cathode tube during lighting are matched with each other.

18. A liquid crystal display comprising: a plurality of cold-cathode tubes; a liquid crystal panel installed on the front side of said cold-cathode tubes, said liquid crystal panel shielding light emitted from said cold-cathode tubes using a predetermined pattern; and a cold-cathode tube lighting device, wherein said cold-cathode tube lighting device comprises: a plurality of ballasts, at least one of said ballasts being connected to an electrode at one end of each of said plurality of cold-cathode tubes; a first low-impedance power source having an output impedance lower than a combined impedance of said cold-cathode tubes, said first low-impedance power source being connected to the electrode at one end of each of said cold-cathode tubes via said ballasts; a second low-impedance power source having an output impedance lower than the combined impedance of said cold-cathode tubes, said second low-impedance power source being connected to an electrode at the other end of each of said cold-cathode tubes; and a phase correction circuit for adjusting a phase difference between an output from said first low-impedance power source and an output from said second low-impedance power source, so that electrode potentials at both ends of each of said cold-cathode tubes change in opposite phase with respect to each other, wherein said phase correction circuit comprises a delay circuit for delaying one of a first pulse signal for instructing an output timing with respect to said first low-impedance power source and a second pulse signal for instructing an output timing with respect to said second low-impedance power source, from the other signal by a constant quantity.

Description

TECHNICAL FIELD

The present invention relates to a cold-cathode tube lighting device. In particular, the present invention relates to a device for lighting a plurality of cold-cathode tubes.

BACKGROUND ART

Fluorescent tubes are classified roughly into hot-cathode tubes and cold-cathode tubes depending on the configuration of the electrodes thereof. The electrodes of a cold-cathode tube (also referred to as a CCFL) are formed of substances that emit numerous electrons through the application of high voltage. Namely, the electrodes do not include any filaments for emitting thermal electrons, unlike the case of the hot-cathode tubes. For this reason, the cold-cathode tubes are particularly advantageous over the hot-cathode tubes in terms of very small tube diameter, long life and low power consumption. Because of the advantages, the cold-cathode tubes are mainly used frequently for products strongly requested to be made thinner (or smaller in size) and lower in power consumption, such as the backlights of liquid crystal displays, the light sources of facsimiles and scanners.

The cold-cathode tubes have electrical characteristics of higher firing potential, smaller discharge current (referred to as tube current hereinafter) and higher impedance than the hot-cathode tubes. In particular, the cold-cathode tubes have such negative resistance characteristics that the resistance value thereof drops abruptly as the tube current thereof increases. The configuration of a cold-cathode tube lighting device is devised so as to conform to these electrical characteristics of the cold-cathode tubes. In particular, since thinning (downsizing) and electric power saving are emphasized for devices to which the cold-cathode tubes are applied, the cold-cathode tube lighting device is also strongly requested to be made smaller in size (particularly thinner) and lower in power consumption.

For example, as a cold-cathode tube lighting device according to a prior art, the device described below has been known (for example, see Patent documents 1 and 2). FIG. 14 is a circuit diagram showing a configuration of the cold-cathode tube lighting device according to the prior art. The cold-cathode tube lighting device according to the prior art includes a high-frequency oscillation circuit 100, a step-up transformer "T" and an impedance matching part 200.

The high-frequency oscillation circuit 100 converts a direct-current voltage supplied from a direct-current power source DC into an alternating-voltage having a high frequency, and applies the alternating-voltage to a primary winding L1 of the step-up transformer "T". The step-up transformer "T" generates a voltage, which is extremely higher than a primary voltage, across both ends of a secondary winding L2 thereof. The high secondary voltage "V" is applied across both ends of a cold-cathode tube FL via the impedance matching part 200. For example, the impedance matching part 200 includes a series circuit of a choke coil "L" and a capacitor "C". In this case, the capacitor "C" includes stray capacitances in the periphery of the cold-cathode tube FL. Impedance matching is performed between the step-up transformer "T" and the cold-cathode tube FL by adjusting the inductance of the choke coil "L" and the capacitance of the capacitor "C".

During the time when the cold-cathode tube FL is off, when a voltage is applied to the primary winding L1 of the transformer "T", a voltage VR across both ends of the cold-cathode tube FL is raised abruptly by a resonance of the choke coil "L" and the capacitor "C" of the impedance matching part 200, and the voltage VR exceeds a firing potential. As a result, the cold-cathode tube FL starts discharging and begins to emit light. Then, a resistance value of the cold-cathode tube FL drops abruptly as the tube current IR increases (negative resistance characteristics). Along with this drop in the resistance value of the cold-cathode tube FL, the voltage VR across both ends of the cold-cathode tube FL drops. At that time, the tube current IR is maintained stably by the action of the impedance matching part 200, regardless of the change in the voltage VR across both ends of the cold-cathode tube FL. Namely, the luminance of the cold-cathode tube FL is maintained stably.

In FIG. 14, the secondary winding L2 of the step-up transformer "T" and the choke coil "L" are shown as circuit elements different from each other. However, in an actual cold-cathode tube lighting device, a secondary winding of one leakage flux transformer was used for three purposes of step-up, choking and impedance matching. Accordingly, both the number of components and the size were reduced. Namely, in the cold-cathode tube lighting device according to the prior art, the leakage flux transformer was regarded as particularly advantageous in downsizing and thus used frequently.

Generally speaking, in the cold-cathode tube FL, the stray capacitance between the tube wall and the external grounding conductor (such as a case or reflecting plate of a liquid crystal display) is caused. For example, in such a configuration that one of the electrodes of the cold-cathode tube FL is grounded as in the cold-cathode tube lighting device disclosed in the patent document 1, only the electric potential of the other electrode fluctuates greatly with respect to the ground potential. Accordingly, when the stray capacitance between the tube wall and the external part is excessive, the leakage current flowing between the tube wall and the external increases excessively particularly near above-mentioned the other electrode. Particularly when the code cathode tube FL is long, the excessive increase of the leakage current may impair the uniformity of the tube current in the longitudinal direction. As a result, an imbalance in luminance may occur in the longitudinal direction of the cold-cathode tube FL.

In order to further raise the uniformity of the luminance in the longitudinal direction the cold-cathode tube, an intermediate point of the electrode potentials at both ends of the cold-cathode tube FL is preferably maintained at the ground potential. For example, with regards to the cold-cathode tube lighting device according to the prior art shown in FIG. 14, the secondary winding L2 of the step-up transformer "T" is grounded at a neutral point M2 thereof, and equivalent ballasts are connected to both ends of the cold-cathode tube FL, respectively (See patent document 2). By this configuration, the intermediate point of the electrode potentials at both ends is maintained at the ground potential. Namely, the electrode potentials at both ends are maintained asymmetrically with respect to the ground potential and the electrode potentials are fluctuated equally. Accordingly, in the cold-cathode tube FL, the distribution of the leakage current flowing between each part of the tube wall and the external is symmetrical with respect to the central part of the cold-cathode tube FL. Accordingly, in each cold-cathode tube, the imbalance in luminance in the longitudinal direction thereof is reduced, and this leads to the improved uniformly.

Further, when the intermediate point of the electrode potentials at both ends of the cold-cathode tube FL is maintained at the ground potential, the amplitude of the electrode potential with respect to the ground potential is halved while the amplitude of the voltage across both ends of the cold-cathode tube FL is maintained, unlike the case where the electrode at one end of the cold-cathode tube FL is grounded. Accordingly, since the leakage current is reduced, the imbalance of the distribution of the leakage current is reduced. Accordingly, the imbalance in luminance in the longitudinal direction of the cold-cathode tube FL is further reduced, and this leads to the further improved uniformly.

Patent document 1: Japanese patent laid-open publication No. 8-273862.

Patent document 2: Japanese patent laid-open publication No. 8-122776.

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

High luminance is particularly requested for the backlight of a liquid crystal display. Accordingly, when cold-cathode tubes are used as the backlight, it is desired that a plurality of cold-cathode tubes are installed. At that time, the luminance values of the plurality of cold-cathode tubes must be made uniform. In addition, the cold-cathode tube lighting device thereof must be small in size. For the purpose of meeting these needs, it is desired that the plurality of cold-cathode tubes are driven in parallel using a common power source.

However, the parallel driving of the plurality of cold-cathode tubes using the common power source was difficult because of the following reasons.

The cold-cathode tubes have the negative resistance characteristics as described above. Accordingly, when the plurality of cold-cathode tubes are simply connected in parallel, current concentration occurs in only one of the cold-cathode tubes, and the only one cold-cathode tube can be lit eventually. Further, when the plurality of cold-cathode tubes are connected to the common power source, wires connected among them are different from each other, more particularly, their lengths are different from each other. Accordingly, stray capacitances of the cold-cathode tubes are different from each other. Accordingly, when the plurality of cold-cathode tubes are driven in parallel, it is necessary to control the tube current for each cold-cathode tube so as to suppress the variation in the tube current.

It was difficult to perform the following of (a) using one leakage flux transformer as a common choke coil for a plurality of cold-cathode tubes, (b) attaining highly accurate impedance matching between the leakage flux transformer and each of the cold-cathode tubes, and (c) highly accurately controlling the tube currents of the individual tubes. In this case, the difficulty remained similarly even when a piezoelectric transformer is used instead of the leakage flux transformer. Accordingly, in the cold-cathode tube lighting device according to the prior art, each of the cold-cathode tubes is provided with a power source (a leakage flux transformer, in particular), while tube current of each of the cold-cathodes tube is controlled to be uniform using the power source. Namely, in the cold-cathode tube lighting device according to the prior art, the power sources as many as the cold-cathode tubes were required. As a result, it was difficult to reduce the number of components so as to further downsize the whole device.

Further, for example, when the neutral point of the secondary winding of the leakage flux transformer is grounded and the ballasts are connected to each of both ends of the cold-cathode tube so as to maintain the intermediate point of the electrode potentials at both ends of the cold-cathode tube at ground potential, the neutral point of the secondary winding and the impedance of the ballast must be determined at high precision. In particular, the impedances should be coincided at high precision between the secondary windings divided into two. In a manner similar to above, the impedances should be coincided between two ballasts at high precision. Such high precision settings further make it difficult to lighten a plurality of cold-cathode tubes with using a common leakage flux transformer.

In addition, two ballasts are required for each cold-cathode tube, and thus reduction in the number of components is difficult, and thus further downsizing of the whole device is difficult.

An object of the present invention is to provide a cold-cathode tube lighting device that uniformly lights a plurality of cold-cathode tubes with using a common power source, that maintains uniformly the luminance, particularly, in the longitudinal direction of each of the plurality of cold-cathode tubes, and that can realize further downsizing and quality improvement.

Means for Solving the Problems

A cold-cathode tube lighting device according to the present invention includes a plurality of ballasts, a first low-impedance power source having an output impedance lower than a combined impedance of the cold-cathode tubes, a second low-impedance power source having an output impedance lower than the combined impedance of the cold-cathode tubes, and a phase correction circuit for adjusting a phase difference between an output from the first low-impedance power source and an output from the second low-impedance power source, so that electrode potentials at both ends of each of the cold-cathode tubes change in opposite phase with respect to each other. At least one of the ballasts is connected to an electrode at one end of each of a plurality of cold-cathode tubes. The first low-impedance power source is connected to the electrode at one end of each of the cold-cathode tubes via the ballasts, and the second low-impedance power source is connected to an electrode at the other end of each of the cold-cathode tube.

The cold-cathode tube lighting device is preferably installed into a liquid crystal display as described below. The liquid crystal display includes the plurality of cold-cathode tubes and a liquid crystal panel installed on the front side of the cold-cathode tubes, for shielding light emitted from the cold-cathode tubes using a predetermined pattern. The cold-cathode tube lighting device according to the present invention drives the above-mentioned plurality of cold-cathode tubes serving as the backlight of the liquid crystal display.

Generally speaking, the properties of the plurality of cold-cathode tubes vary among the plurality of cold-cathode tubes, and variation in the peripheral stray capacitance occurs by the difference in wirings. Further, the change in the environment condition, such as temperature, causes variation in the operating states of the cold-cathode tubes.

In the cold-cathode tube lighting device according to the present invention, the output impedance of the power source is suppressed, contrary to the presumption in the device according to the prior art. Instead, the ballast is connected to each of the cold-cathode tubes. In this case, since the output impedance of the power source is low, each of the ballasts operates substantially independently. Thus, the above-mentioned variations are canceled out precisely for each cold-cathode tube. Namely, no variation in tube current occurs among the plurality of cold-cathode tubes. Accordingly, luminance is maintained uniformly among the plurality of cold-cathode tubes.

Thus, the above-mentioned cold-cathode tube lighting device according to the present invention uniformly lights the plurality of cold-cathode tubes using the common low-impedance power sources.

In the cold-cathode tube lighting device according to the present invention, since the output impedance of the power source is low, each of the ballasts operates substantially independently. Thus, even if wires between the low-impedance power source and each ballast are long, and further, greatly differs for each ballast, no variation in tube current occurs among the plurality of cold-cathode tubes.

Preferably, the first low-impedance power source, the second low-impedance power source, and the phase correction circuit are mounted on a first substrate, and the ballast is mounted on a second substrate. More preferably, one end of each of the cold-cathode tubes is connected to the second substrate.

Generally speaking, the other circuit element such as a ballast has a size smaller than that of the low-impedance power source. Accordingly, when the first substrate mounted with the low-impedance power source is separated away from the other substrate, a part constructed by the second substrate and the cold-cathode tubes can be made thinner easily. For example, when the cold-cathode tubes are used as the back light of the liquid crystal display, the thinning of the display is realized easily.

Thus, in the cold-cathode tube lighting device according to the present invention, the layout of the wiring thereof is high in flexibility. In particular, separation of the above-mentioned substrates is easily achieved, while maintaining the luminance of the plurality of cold-cathode tubes uniform.

In the cold-cathode tube lighting device according to the present invention, the first low-impedance power source changes the electrode potential at one of each of the plurality of cold-cathode tubes, and the second low-impedance power source changes the electrode potential at the other end of each of the plurality of cold-cathode tubes. Frequencies of the outputs from the two low-impedance power source are set to be equal to each other. On the other hand, amplitudes of the outputs from the two low-impedance power source are set independently, so that the electrode potentials at both ends of each cold-cathode tube change at the same amplitude.

Further, the phase correction circuit adjusts the phase difference between the outputs from the two low-impedance power sources, so as to change the electrode potentials at both ends at each cold-cathode tube in opposite phase with respect to each other.

Thus, in each cold-cathode tube, an intermediate point of the electrode potentials at both ends thereof is maintained at the ground potential at high precision. Namely, the electrode potentials at both ends are maintained asymmetrically with respect to the ground potential.

In particular, in the cold-cathode tube lighting device according to the present invention, a circuit configuration between the first low-impedance power source and the electrode at one end of the cold-cathode tube may differ greatly from a circuit configuration between the second low-impedance power source and the electrode at the other end of the cold-cathode tube. Preferably, each ballast is connected only between the first low-impedance power source and the electrode at one of each cold-cathode tube.

In this case, the output of the first low-impedance power source is set to the amplitude different from that of the output of the second low-impedance power source. For example, when the ballast is an inductor, the output of the first impedance power source has the amplitude set to be smaller than that of the output of the second low-impedance power source. In addition, when the ballast is a capacitor, the output of the first low-impedance power source has the amplitude set to be greater than the amplitude of the output of the second low-impedance power source. Thus, since a difference in the amplitudes of the outputs from the two low-impedance power sources cancels out the change in amplitude by the ballast, the electrode potentials at both ends of the cold-cathode tube is changed at the same amplitude at high precision.

Further, the phase correction circuit shifts the phase difference between the outputs from the two low-impedance power sources by a predetermined amount from, for example, 180.degree.. Thus, since the phase difference between the outputs from the low-impedance power sources cancel out the phase shift by the ballast, the phase difference between the electrode potentials at both ends of the cold-cathode tube is maintained equal to 180.degree. at high precision.

In this case, since the low-impedance power source has a low output impedance, the settings of the amplitudes and the phase difference may be common for all the pairs of the cold-cathode tube and the ballast.

Thus, the electrode potentials at both ends of each of the cold-cathode tubes vary uniformly, while they are maintained asymmetrically with respect to the ground potential. Accordingly, in each of the cold-cathode tubes, the distribution of the leakage current flowing between each part of the tube wall and the external is symmetrical with respect to the central part of the cold-cathode tube. Accordingly, in each of the cold-cathode tubes, the imbalance in luminance in the longitudinal direction of each of the cold-cathode tubes is reduced, and this leads to the improved uniformity.

Further, when the intermediate point of the electrode potentials at both ends of each cold-cathode tube is maintained at the ground potential, the amplitude of the electrode potential with respect to the ground potential is halved while the amplitude of the voltage across both ends of each cold-cathode tube is maintained, unlike the case where the electrodes at one end of the cold-cathode tubes are grounded. Accordingly, since the leakage current is reduced, the imbalance of the distribution of the leakage current is reduced. Accordingly, the imbalance in luminance in the longitudinal direction of each of the cold-cathode tubes is further reduced, and this leads to the further improved uniformity.

In addition, by using the two step-up transformers, withstand voltages of the circuit elements included in the step-up transformers can be reduced by half compared to those when one step-up transformer is used. On the other hand, since the ballast needs to be connected only to one of the electrodes of each cold-cathode tube, the number of ballasts may be the same as the number of cold-cathode tubes. Accordingly, the downsizing of the cold-cathode tube lighting device according to the present invention is easily realized.

Preferably, in the cold-cathode tube lighting device according to the present invention, the phase correction circuit includes a delay circuit for delaying one of a first pulse signal for instructing an output timing with respect to the first low-impedance power source and a second pulse signal for instructing an output timing with respect to the second low-impedance power source, from the other signal by a constant quantity.

In the cold-cathode tube lighting device according to the present invention, the fluctuation of the operating state of each of the cold-cathode tubes is absorbed by the ballasts each connected to each of the cold-cathode-tubes. Accordingly, the phase difference between the outputs from the two low-impedance power sources is hardly affected by the variations in the operating state among the plurality of cold-cathode tubes. Accordingly, the phase difference simply needs to be maintained substantially at a constant quantity for all the cold-cathode tubes. The phase correction circuit easily maintains the phase difference between the outputs from the two low-impedance power sources to be equal to the constant quantity using the delay circuit.

Further preferably, the cold-cathode tube lighting device according to the present invention further includes a detector for detecting current flowing through the cold-cathode tubes, or an electrode potential at one end of each of the cold-cathode tubes, and the phase correction circuit changes the phase difference based on a detected value detected by the detector.

During the time when the cold-cathode tube is off, since the tube current thereof is small, the space between the electrodes at both ends thereof is opened. At that time, the amplitude of each electrode potential is large. Further, the phase shift by the ballast does not occur.

The phase correction circuit may stop the adjustment of the phase difference between the outputs from the power sources and fix the phase difference to 180.degree., during the period the detector does not detect the tube current of greater than or equal to a constant threshold, or during the period the detector does not detect the amplitude of the electrode potential at one end of the cold-cathode tube within a predetermined range. Here, since the phase shift of the output by the ballast does not occur, regardless of the presence of the action of the phase correction circuit, the electrode potentials at both ends of the cold-cathode tube change in opposite phase with respect to each other. In particular, when the phase correction circuit maintains the phase difference between the outputs from the two low-impedance power sources equal to the constant quantity using the delay circuit, the malfunction thereof can be avoided by stopping the delay circuit during the above period.

Preferably, in the cold-cathode tube lighting device according to the present invention, each of the first low-impedance power source and the second low-impedance power source comprises a transformer connected to the ballast capacitors, and the transformer has an output impedance lower than the combined impedance of the plurality of cold-cathode tubes. Thus, the output impedance of the transformer is suppressed, contrary to the presumption in the device according to the prior art. Accordingly, the power source having low output impedance is realized.

As a means effective for reducing the output impedance of the transformer, the transformer may includes a core, a primary winding being wound around the core, and a secondary winding being wound around at least one of the inside and outside of the primary winding. Thus, the leakage flux is reduced, and the output impedance is suppressed. Further, the adverse effect (for example, occurrence of noise) on the peripheral devices by the leakage flux is suppressed.

In this case, the secondary winding of the transformer may have one configuration of a sectional winding and a honeycomb winding. Thus, the line capacity is reduced. Accordingly, the self-resonance frequency of the secondary winding can be set to be sufficiently high. Therefor, according to the cold-cathode tube lighting device according to the present invention, the drive frequency of each of the cold-cathode tubes can be sufficiently high while maintaining stably the light emission of the plurality of cold-cathode tubes. The downsizing of the transformer and the downsizing of the whole device are therefore are easily realized.

In the cold-cathode tube lighting device according to the present invention, the low-impedance power source may include a power transistor connected to the ballasts, instead of the transformer. The use of the power transistor easily and effectively reduces the output impedance. Accordingly, the cold-cathode tube lighting device according to the present invention can uniformly light a greater number of cold-cathode tubes.

Preferably, in the cold-cathode tube lighting device according to the present invention, each of the ballasts includes an inductor. Thus, the inductor functions as a choke coil. Namely, the inductor and the stray capacitance in the periphery of the cold-cathode tube cause resonance, and this leads to application of a voltage of greater or equal to a firing potential to the cold-cathode tube. In this case, the actual firing potential varies among the plurality of cold-cathode tubes. However, in the cold-cathode tube lighting device according to the present invention, at least one ballast is connected to each cold-cathode tube. Accordingly, regardless of the variation of the actual firing potential, the voltage application from the common low-impedance power source reliably lights all the plurality of cold-cathode tubes.

In the above ballast, the inductor may have one configuration of a sectional winding and a honeycomb winding. Thus, the line capacity is reduced. Accordingly, the self-resonance frequency of the inductor can be set to be sufficiently high. Therefor, according to the cold-cathode tube lighting device according to the present invention, the drive frequency of each of the cold-cathode tubes can be sufficiently high while maintaining stably the light emission of the plurality of cold-cathode tubes. The downsizing of the ballast and the downsizing of the whole device are therefore are easily realized.

Further, in the above ballast, the inductor may include a saturable reactor. When the electric discharge in the cold-cathode tube is suddenly interrupted, and a voltage across both ends of the cold-cathode tube is drastically increased, the inductance of the ballast inductor LB is saturated so that further increase in the voltage is suppressed. Thus, the over voltage is prevented. The cold-cathode tube lighting device is high safety.

In the cold-cathode tube lighting device according to the present invention, the above ballast may include a capacitor (referred to as ballast capacitor hereinafter). The ballast capacitor preferably has an inter-layer capacity of a substrate. In this case, the substrate is, for example, a multi-layer circuit board or a flexible printed circuit board, and the cold-cathode tube lighting device according to the present invention, in particular, the connection parts thereof to the cold-cathode tubes are mounted on the substrate. Thus, the ballast capacitor is easily downsized, and the downsizing of the whole cold-cathode tube lighting device according to the present invention is easily realized.

The cold-cathode tube lighting device according to the present invention preferably includes matching capacitors, at least one of which is connected across a ground potential and the electrode at one end of each cold-cathode tube connected to the ballast. The each of the matching capacitors may have, for example, an inter-layer capacity of a substrate. In particular, an impedance of the ballast and an impedance of the matching capacitor are preferably matched with each other. Further, an impedance of the ballast, a combined impedance of the matching capacitor and a stray capacitance in the periphery of the cold-cathode tube, and an impedance of the cold-cathode tube during lighting are matched with each other. Thus, impedance matching between the ballast and the cold-cathode tube (and stray capacitance in the periphery thereof is realized for each combination of the ballast and the cold-cathode tube. Accordingly, the tube current is uniformly maintained among the plurality of cold-cathode tubes, and the luminance is uniformly maintained, regardless of the variations in the properties, the peripheral stray capacitance, and a voltage across both ends of the cold-cathode tube, among the plurality of cold-cathode tubes.

Effects of the Invention

Unlike the device according to the prior art, the above-mentioned cold-cathode tube lighting device according to the present invention includes a plurality of ballast capacitors, at least one of which is connected to each of a plurality of cold-cathode tubes, and a common low-impedance power source so as to uniformly lighten the plurality of cold-cathode tubes using the common power source. Further, the wires between the power source and the ballast capacitors may be long, and may be significantly different for each ballast capacitor, and therefore the layout of the wiring is high in flexibility. Accordingly, the downsizing of the whole device is realized more easily than that of the device according to the prior art.

Further, in the cold-cathode tube lighting device according to the present invention, the two low-impedance power sources separately change the electrode potentials at both ends of each of the plurality of cold-cathode tubes. In this case, the frequencies of the outputs from the two low-impedance power sources are set to be equal to each other. On the other hand, amplitudes of the outputs from the two low-impedance power source are set independently. In addition, the phase correction circuit adjusts the phase difference between the outputs from the two low-impedance power sources. Thus, even if a circuit configuration of a part between one low-impedance power source and the cold-cathode tubes, and a circuit configuration of a part between the other low-impedance power source and the cold-cathode tubes are greatly different from each other, the electrode potentials at both ends of each cold-cathode tube are maintained asymmetrically with respect to the ground potential. Accordingly, the uniformity of the luminance in the longitudinal direction of each cold-cathode tube is further raised.

Preferably, in the cold-cathode tube lighting device according to the present invention, the ballast is connected only to between the first low-impedance power source and the electrode at one end of each cold-cathode tube. Thus, only by using the ballasts of the same number as the cold-cathode tubes, the uniformity of the luminance in the longitudinal direction of each cold-cathode tube can be raised.

Further, when two low-impedance power sources are used as described above, the withstand voltages of the circuit elements included in the power sources are reduced by half compared to those when one low-impedance power source is used.

The downsizing of the cold-cathode tube lighting device according to the present invention is further easily realized.

For example, when the cold-cathode tube lighting device according to the present invention is used as the back light of the liquid crystal display, the display can be easily made thin.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view showing an internal part of a liquid crystal display provided with a cold-cathode tube lighting device according to a first preferred embodiment of the present invention.

FIG. 2 is a sectional view of the liquid crystal display, taken along line II-II shown in FIG. 1.

FIG. 3 is a circuit diagram showing a configuration of the cold-cathode tube lighting device according to the first preferred embodiment of the present invention.

FIG. 4 is a wave form chart showing an original pulse signal P0, a first pulse signal P1, a delay pulse signal Pd, output Pe of a comparator 8A, and a second pulse signal P2 for the cold-cathode tube lighting device according to the first preferred embodiment of the present invention.

FIG. 5 is an exploded view schematically showing a configuration of a step-up transformer 5 adopted in the cold-cathode tube lighting device according to the first preferred embodiment of the present invention.

FIG. 6 is a sectional view of the step-up transformer 5, taken along line VI-VI shown in FIG. 5.

FIG. 7 is a schematic view showing an equivalent circuit on the secondary side of each of step-up transformers 5A and 5B for the cold-cathode tube lighting device according to the first preferred embodiment of the present invention.

FIG. 8 is a graph showing a voltage-current property of a cold-cathode tube 20 for the cold-cathode tube lighting device according to the first preferred embodiment of the present invention.

FIG. 9 is a wave form chart showing each of changes in a secondary voltage VA of the step-up transformers 5A, a secondary voltage VB of the step-up transformers 5B, an electric potential V1 at a first electrode 21 of the cold-cathode tube 20, and a voltage VF across both ends of the cold-cathode tube 20 for the cold-cathode tube lighting device according to the first preferred embodiment of the present invention.

FIG. 10 is a circuit diagram showing a configuration of a cold-cathode tube lighting device according to a second preferred embodiment of the present invention.

FIG. 11 is a circuit diagram showing a configuration of a cold-cathode tube lighting device according to a third preferred embodiment of the present invention.

FIG. 12 is a circuit diagram showing a configuration of a cold-cathode tube lighting device according to a fourth preferred embodiment of the present invention.

FIG. 13 is a circuit diagram showing a configuration of a cold-cathode tube lighting device according to a fifth preferred embodiment of the present invention.

FIG. 14 is a circuit diagram showing a configuration of a cold-cathode tube lighting device according to a prior art.

DESCRIPTION OF NUMERICAL REFERENCES

DC direct-current power source 1 first block (low-impedance power source) 4A first high-frequency oscillator circuit 4B second high-frequency oscillator circuit Q1 first transistor Q2 second transistor In inverter Lr inductor Cr resonance capacitor 5A first step-up transformer 5B second step-up transformer 51A first primary winding 51B second primary winding M1 neutral point of primary winding 52A first secondary winding 52B second secondary winding 6 phase collection circuit Os oscillator 7 delay circuit Rd resistor Cd capacitor Vr reference voltage source 8A comparator 8B first flip-flop 8C second flip-flop 2 second block LB ballast inductor CP overcurrent protection capacitor CM matching capacitor 3 connection terminal 20 cold-cathode tube

BEST MODE FOR CARRYING OUT THE INVENTION

Best preferred embodiments of the present invention will be described below referring to the drawings.

First Preferred Embodiment

FIG. 1 is a front view showing an internal part of a liquid crystal display provided with a cold-cathode tube lighting device according to a first preferred embodiment of the present invention. FIG. 2 is a sectional view of the liquid crystal display, taken along line II-II shown in FIG. 1 (the arrows shown in FIG. 1 show a visual line direction).

The liquid crystal display includes a case 10, a plurality of cold-cathode tubes 20, a reflecting plate 30, a first substrate 40, a second substrate 50, a third substrate 60, and a liquid crystal panel 70. The cold-cathode tube lighting device according to the first preferred embodiment of the present invention is mainly divided into two blocks 1 and 2, and the blocks 1 and 2 are mounted on the first substrate 40 and the second substrate 50, respectively.

The case 10 is, for example, a box made of metal, and the box 10 is grounded. The front side of the case 10 is open, and the reflecting plate 30, the cold-cathode tube 20, and the liquid crystal panel 70 (not shown in FIG. 1) are accommodated inside in this order from the rear. The cold-cathode tubes 20 include a plurality of cold-cathode tubes (for example, 16 cold cathode tubes). Each of the cold-cathode tubes 20 is held horizontally, and the cold-cathode tubes 20 are arranged at equal intervals in the vertical direction. The second substrate 50 and third substrate 60 are disposed on both sides of the case 10. Both ends of each of the cold-cathode tubes 20 are fixed to the second substrate 50 and the third substrate 60, respectively. An electrode 21 at one end of each of the cold-cathode tubes 20 is connected to the second block 2 of the cold-cathode tube lightening device. An electrode 22 at the other end of each of the cold-cathode tubes 20 is connected to a connection terminal 3 on the third substrate 60. The second block 2 and the connection terminal 3 are connected to the first block 1 on the first substrate 40 (the wirings therefor are not shown). The first substrate 40 is mounted at a location different from the case 10, for example, at a power source unit (not shown) of the liquid crystal display. The first block 1 is connected to the direct-current power source (not shown). The cold-cathode tube lighting device distributes the electric power supplied from the direct-current power source to each of the cold-cathode tubes 20 via the two blocks 1 and 2, and the connection terminal 3 so that each of the cold-cathode tubes 20 emits light. The light emitted from the cold-cathode tube 20 enters the liquid crystal panel 70 directly or after being reflected by the reflecting plate 30 (See arrows shown in FIG. 2). The liquid crystal panel 70 shields the incident light emitted from the cold-cathode tubes 20 using a predetermined pattern so as to display the pattern on the front face of the liquid crystal panel 70.

FIG. 3 is a circuit diagram showing a configuration of the cold-cathode tube lighting device according to the first preferred embodiment of the present invention. The cold-cathode tube lighting device mainly includes the two blocks 1 and 2 described above.

The first block 1 includes a pair of high-frequency oscillator circuits 4A and 4B, a pair of step-up transformers 5A and 5B, and a phase correction circuit 6.

The high-frequency oscillator circuits 4A and 4B have configurations similar to each other, and each of the high-frequency oscillator circuits 4A and 4B includes an inductor Lr, a resonance capacitor Cr, a first transistor Q1, a second transistor Q2, and an inverter In.

The step-up transformers 5A and 5B has configurations similar to each other. A primary wingding of each of the step-up transformers 5A and 5B is divided into two primary windings 51A and 51B at the neutral point M1 thereof.

A positive electrode of the direct-current power source DC is connected to one terminal of the inductor Lr, and the negative electrode thereof is grounded. The other terminal of the inductor Lr is connected to the neutral point M1 of the primary windings 51A and 51B of the step-up transformer 5A (or 5B). The resonance capacitor Cr is connected across the other terminal 53A of the first primary winding 51A and the other terminal 53B of the second primary winding 51B. The terminal 53A of the first primary winding 51A is further connected to one terminal of the first transistor Q1. The terminal 53B of the second primary winding 51B is further connected to one terminal of the second transistor Q2. The other terminals of each of the first transistor Q1 and the second transistor Q2 are both grounded. In this case, the first transistor Q1 and the second transistor Q2 are preferably MOS FETs. In addition, they may also be IGBTs or bipolar transistors.

The phase correction circuit 6 includes an oscillator Os, a delay circuit 7, a comparator 8A, two flip-flops 8B and 8C, and a reference voltage source Vr.

The oscillator Os is connected to the first flip-flop 8B and the delay circuit 7 so as to send an original pulse signal P0 to them.

The first flip-flop 8B generates a first pulse signal P1 based on the original pulse signal P0. The first pulse signal P1 is sent to the first high-frequency oscillator circuit 4A. Then, the first pulse signal P1 is directly transmitted to a control terminal of the first transistor Q1, and the first pulse signal P1 is transmitted to a control terminal of the second transistor Q2 via the inverter In.

The delay circuit 7 generates a delay pulse signal Pd based on the original pulse signal P0. For example, the delay circuit 7 is a so-called RC filter, and the delay circuit 7 includes a series connection of a resistor Rd and a capacitor Cd. A terminal on the resistor Rd side of the series connection is connected to the oscillator Os, and a terminal on the capacitor Cd side thereof is grounded. The delay pulse signal Pd indicates a change in the electric potential at a connection point "J" between the resistor Rd and the capacitor Cd.

One of input terminals of the comparator 8A is connected to the connection point "J" of the resistor Rd and the capacitor Cd, and the other input terminal of the comparator 8A is connected to a positive electrode of the reference voltage source Vr. A negative electrode of the reference voltage source Vr is grounded. An output terminal of the comparator 8A is connected to the second flip-flop 8C. The comparator 8A compares a level of the delay pulse signal Pd to a voltage of the reference voltage source Vr and, indicates a comparison result with a level of an output Pe.

The second flip-flop 8C generates a second pulse signal P2 based on the output Pe from the comparator 8A. The second pulse signal P2 is sent to the second high-frequency oscillator circuit 4B. Then, the second pulse signal P2 is directly transmitted to the control terminal of the first transistor Q1, and the second pulse signal P2 is transmitted to the control terminal of the second transistor Q2 via the inverter In.

FIG. 4 is a wave form chart showing the original pulse signal P0, the first pulse signal P1, the delay pulse signal Pd, the output Pe of the comparator 8A, and the second pulse signal P2.

The original pulse signal P0 is a square pulse signal, and has a constant frequency (for example, 90 [kHz]), a constant pulse width, and a constant pulse height.

The first flip-flop 8B synchronizes risings and fallings of the first pulse signal P1 to risings of the first pulse signal P0 so as to generate the first pulse signal P1 of a square pulse signal similar to the original pulse signal P0. In particular, a frequency of the first pulse signal P1 is 1/2 times (for example, 45 [kHz]) a frequency of the original pulse signal P0, and a duty of the first pulse signal P1 is 50%.

The delay pulse signal Pd is in phase with the original pulse signal P0. However, a rise time and a decay time of the delay pulse signal Pd is longer than a rise time and a decay time of the original pulse signal P0, respectively.

When the level of the delay pulse signal Pd is lower than the reference voltage Vr, the output Pe from the comparator 8A is maintained at a constant low level. On the other hand, when the level of the delay pulse signal Pd is higher than the reference voltage Vr, the output Pe from the comparator 8A is maintained at a constant high level.

The second flip-flop 8C synchronizes risings and fallings of the second pulse signal P2 to risings of the output Pe from the comparator 8A so as to generate the second pulse signal P2 of a square pulse signal similar to the first pulse signal P1. Namely, a frequency of the second pulse signal P2 is equal to the frequency of the first pulse signal P1 (for example, 45 [kHz]). Further, a duty of the second pulse signal is 50%. However, the risings of the second pulse signal P2 are delayed by a constant delay time Td from the risings of the first pulse signal P1. The delay time Td is determined by a time constant (a product of a resistance value "R" of the resistor Rd and a capacity "C" of the capacitor Cd) of the delay circuit 7, a pulse height Vp of the delay pulse signal Pd, and the reference voltage Vr by the following equation: Td=-RC.times.In(1-Vr/Vp).

The direct-current power source DC maintains its output voltage Vi at a constant value (for example, 16[V]).

In the first high-frequency oscillator circuit 4A, the first pulse signal P1 is applied to the control terminal of the first transistor Q1 in the original polarity, and the first pulse signal P1 is applied to the control terminal of the second transistor Q2 with the polarity inverted by the inverter In.

In the second high-frequency oscillator circuit 4B, the second pulse signal P2 is applied to the control terminal of the first transistor Q1 in the original polarity, and is applied to the control terminal of the second transistor Q2 with the polarity inverted by the inverter In.

In this case, in each of the high-frequency oscillator circuits 4A and 4B, the two transistors Q1, Q2 are turned on and off alternately at 1/2 times the frequency of the oscillator OS (for example, 45 [kHz]). Further, on-duties of the two transistors Q1 and Q2 are both equal to 50%. As a result, in each of the step-up transformers 5A and 5B, the input voltage Vi is alternately applied to the two primary windings 51A and 51B. The inductor Lr and the resonance capacitor Cr cause resonance at each application of the voltage, a