JP4832938B2 - Discharge lamp drive circuit - Google Patents

Description

  The present invention relates to a discharge lamp driving circuit for driving an illumination discharge lamp in a liquid crystal display panel, and in particular, a plurality of lamps housed in a lamp reflector having a substantially U-shaped cross section used as an edge light source for a liquid crystal display panel. The present invention relates to a discharge lamp driving circuit for driving the discharge lamp.

  Conventionally, several or more cold cathode discharge lamps (hereinafter referred to as discharge lamps) are arranged around a liquid crystal display panel for illumination of various liquid crystal display panels used in liquid crystal TVs, etc. A so-called edge light system that guides illumination light to the back of the liquid crystal display panel is known. According to such an edge light system, attention is paid to soft illumination by an indirect illumination effect as compared with a so-called direct system that directly illuminates the back surface of a liquid crystal display panel (see, for example, Patent Document 1 below).

  By the way, as a light source unit for such an edge light system, for example, as shown in FIG. 7, three discharge lamps 50A, B, and C are stored in a lamp reflector 60 having a U-shaped cross section. The illumination light emitted from the open end side of the reflector 60 is configured to enter the side end face (edge) of the light guide plate disposed on the back side of the liquid crystal display panel.

JP 2003-31014 A

However, such a reflector is grounded, and a parasitic capacitance corresponding to the distance between each discharge lamp and the reflector surface is generated. Therefore, for the discharge lamps 50A and C located on both sides as shown in FIG. 7, parasitic capacitance is generated not only between the reflector bottom wall 60B but also between the side walls 60A and C. compared to the discharge lamp 50B located at the center, if both ends of the discharge lamp 50A, the C, the bottom wall 60B and side walls 60A, when the distance d ₁ to C are equal to each other, for example, that 2 times the parasitic capacitance is generated Become.

Such a difference in parasitic capacitance causes a difference in leakage currents in the discharge lamps 50A, B, and C, thereby causing uneven brightness between the discharge lamps, and thus uneven brightness on the display surface of the liquid crystal display panel. It will occur.
Of course, if the current amount of each discharge lamp is individually controlled, the above-described problems can be solved. However, providing such a control circuit causes a significant increase in cost and complication of the circuit configuration.

  In addition, as described above, the difference in the parasitic capacitance between the discharge lamps is caused by the shape of the reflector and the position where the discharge lamp is disposed. Therefore, when these shapes and positions are changed, the difference in the parasitic capacitance is changed. Will also change. Therefore, there is a demand for a policy that can easily and quickly respond to changes in these shapes and positions, and that can be applied to mass production at low cost.

  The present invention has been made in view of such circumstances. When driving a plurality of discharge lamps housed in a lamp reflector having a substantially U-shaped cross section as an edge light source for a liquid crystal display panel, the lamp reflector is provided. It is an object of the present invention to provide a discharge lamp driving circuit that can easily prevent luminance unevenness between discharge lamps according to the position of the discharge lamp in the interior and can respond to mass production quickly and at low cost. .

The discharge lamp driving circuit of the present invention capable of achieving such an object is a light source for an edge light of the liquid crystal display panel arranged along the edge of the liquid crystal display panel, and is provided in a reflector having a substantially U-shaped cross section. A discharge lamp driving circuit for driving a plurality of discharge lamps stored in parallel,
In the transformer secondary side of the discharge lamp driving circuit, the capacitance for determining the resonance frequency against the plurality of discharge lamps each conductor pattern so as to face each other in the front and rear surfaces of the circuit board is formed, the discharge lamp With a pattern capacitor connected in parallel with
According to the size of the parasitic capacitance of each discharge lamp determined according to the distance between each of the plurality of discharge lamps and the wall surface of the reflector, a fixed capacity by the pattern capacitor on the transformer secondary side corresponding to each of the discharge lamps ; In order to reduce the difference in total capacitance from the parasitic capacitance, the overlapping area of the conductor pattern of the pattern capacitor is set to be larger than that of the other discharge lamps for discharge lamps whose parasitic capacitance is smaller than that of the other discharge lamps. It is characterized by having been adjusted to.

Further, the plurality of discharge lamps are arranged along two inner side walls facing the open end of the reflector and between the two side walls substantially orthogonal to the inner wall surface, and more than the discharge lamps closer to the side walls. It is preferable that the discharge lamp located in the middle portion has a configuration in which the overlapping area of the conductor pattern of the pattern capacitor is increased.
Here, “allocated along the inner wall surface facing the open end” means that they are arranged on a plane substantially parallel to the inner wall surface facing the open end.

  Moreover, it is preferable that the reflector is disposed along a pair of the edge portions facing each other of the liquid crystal display panel.

In the present invention , the difference in parasitic capacitance for each discharge lamp associated with the arrangement of the discharge lamp in the reflector is reduced by using a pattern capacitor on a circuit board arranged in parallel to the discharge lamp. . The value of the capacitance of the pattern capacitor can be easily adapted to the change by changing the front and back overlapping areas of the conductor patterns formed to face each other on the front and back surfaces of the circuit board . In addition, after changing the overlapping area of the conductor pattern, there is no need to install a separate capacitor element, etc., and respond to mass production quickly and at low cost according to the change in the parasitic capacitance. Can do.

  In particular, when three or more discharge lamps are configured to be arranged between both side walls on a plane parallel to the open end-facing wall surface of the substantially U-shaped reflector, they are arranged on both sides. This is useful in reducing the difference in parasitic capacitance between the discharge lamp and the discharge lamp disposed in the middle.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing a partial configuration of a discharge lamp driving circuit according to an embodiment of the present invention.
The discharge lamp drive circuit of the present embodiment shown in FIG. 1 is a drive circuit equipped with an inverter transformer for simultaneously discharging and lighting three discharge lamps (cold cathode discharge lamp: CCFL).

That is, this discharge lamp driving circuit drives three discharge lamps 5A, B, and C accommodated in parallel in a reflector 6 having a substantially U-shaped cross section as an edge light source for a liquid crystal display panel. The main part of the drive circuit is mounted on the circuit board 7, one end of the transformer secondary side is connected to one end of each discharge lamp 5A, B, C, and a predetermined terminal of the control IC 23 is connected to each discharge lamp. It is connected to the other end of 5A, B, C (see FIG. 4).
On this circuit board 7, pattern capacitors 9A, B, and C for determining the resonance frequencies of the discharge lamps 5A, B, and C are mounted. On the secondary side of the transformer, the discharge lamps 5A, B, C and the pattern capacitors 9A, B, C are arranged in parallel and are connected to each other via the terminal portions 8A, B, C of the circuit board 7.

  Edge light source parts 12A and 12B in which the discharge lamps 5A, B and C are housed in the reflector 6 are disposed along the opposite upper and lower edges of the liquid crystal display panel 11, as shown in FIG. The

  FIG. 3 is a schematic view of the liquid crystal display panel 11 as viewed from the side. On the back surface of the liquid crystal display panel 11, a light guide plate 13 and a reflection plate 14 are arranged in this order. Further, the edge light source parts 12A and 12B (open ends of the reflector 6) are arranged so as to face the upper and lower end faces (edges) of the light guide plate 13, respectively.

  Incidentally, the circuit configuration of the discharge lamp driving circuit is generally configured as shown in FIG. That is, this driving circuit drives a discharge lamp (one discharge lamp is shown for convenience in FIG. 4) 21 by using a step-up transformer 22, a transistor TR, and a driving IC 23.

In this discharge lamp driving circuit, a DC voltage is applied between the input terminals I ₁ and I ₂ , and based on the voltage applied across the primary winding 41 of the step-up transformer 22, the switching operation of the transistor TR is used. A high voltage is generated at both ends of the secondary winding 42 of the step-up transformer 22, and the discharge lamp 21 is driven and lit by this high voltage. In such a discharge lamp driving circuit, generally, a fixed capacitance by a pattern capacitor and a total capacitance of a parasitic capacitance C are connected between a non-grounded line in the secondary winding 42 of the step-up transformer 22 and GND. (The parasitic capacitance C is actually generated between the discharge lamp 21 and the reflector 6).

However, as shown in FIG. 5, the discharge lamps 5A, C located on both sides are not only between the bottom wall 6B of the reflector 6 (distance l ₁ ) but also between the side walls 6A, C (distance l ₂ ). Since parasitic capacitance is generated, the parasitic capacitance is larger by the amount generated between the side walls 6A and 6C than the discharge lamp 5B located at the center.

This will be described below using mathematical formulas.
Here, the distances between the discharge lamps 5A, B, and C in the reflector 6 and the bottom wall 6B and the side walls 6A and C of the reflector 6 are set to l ₁ and l ₂ , respectively, as described above. The distances between the discharge lamps 5A, B and C and the bottom wall 6B are substantially equal to each other, and the distances between the discharge lamps 5A and C and the side walls 6A and C are also approximately equal to each other. ing.

Here, the parasitic capacitance Cl ₁ between the discharge lamps 5A, B and C and the bottom wall 6B is originally represented by a distributed constant. However, when simply replaced by a primary constant, the following equation (1) It is expressed as follows. Further, when the parasitic capacitance Cl ₂ between the discharge lamps 5A and 5C and the side walls 6A and C is simply replaced with a primary constant, it is expressed by the following equation (2).

Here, the discharge lamp 5B is represented by the following equation (3) because its parasitic capacitance is represented only by Cl ₁ .

On the other hand, the discharge lamp 5A, C, since the parasitic capacitance is the sum of Cl ₁ and Cl _2, is expressed by the following equation (4).

  Therefore, the parasitic capacitance of the discharge lamp 5B: The parasitic capacitance of the discharge lamps 5A and C is expressed by the following equation (5).

  When a difference occurs in the parasitic capacitance in this way, a difference occurs in the leakage current in the discharge lamps 5A, B, and C, thereby causing uneven brightness between the discharge lamps and uneven brightness on the display surface of the liquid crystal display panel. The problem occurs.

  In reality, it is difficult to eliminate such a difference in parasitic capacitance by changing the value of the parasitic capacitance itself. Therefore, in the present embodiment, based on the idea that the total capacity on the secondary side of the transformer should be equal to prevent the above-described leakage current from being different between the discharge lamps 5A, B, and C. For each of the discharge lamps 5A, B, and C, predetermined pattern capacitors 9A, B, and C having different areas are connected in parallel with the discharge lamps 5A, B, and C (parasitic capacitance).

  That is, for the discharge lamps 5A and C having a large parasitic capacity, pattern capacitors 9A and C having a small area and a small capacity are connected via the terminals 8A and C, while the discharge lamp 5B has a small parasitic capacity. On the other hand, a pattern capacitor 9B having a large area and a large capacity is connected via a terminal 8B (refer to Japanese Patent Laid-Open No. 7-93671 etc. for the pattern capacitor itself).

  FIG. 6 shows this equivalently. In FIG. 6, coils 42A, B, and C schematically show the secondary winding of the transformer.

Hereinafter, description will be made using specific numerical values. When the parasitic capacitance Cl ₁ for the discharge lamp 5A is 5 pF and the parasitic capacitance Cl ₂ is 3 pF, if the capacitance of the pattern capacitor 9A is 4 pF, the total capacitance on the transformer secondary side for the discharge lamp 5A is 12 pF. It becomes. Since this is the same for the parasitic capacitances Cl ₁ and Cl ₂ for the discharge lamp 5C, the capacitance of the pattern capacitor 9C may be set to 4 pF. On the other hand, the parasitic capacitance for the discharge lamp 5B is only ₁ and is 5 pF. Therefore, when the total capacity on the transformer secondary side of the discharge lamp 5B is set to 12 pF similarly to the other discharge lamps 5A and C, the capacity of the pattern capacitor 9B needs to be 7 pF. That is, by setting 4 pF for the pattern capacitors 9A and 9C and 7 pF for the pattern capacitor 9B, the total capacity on the transformer secondary side for each of the discharge lamps 5A, B, and C can be made similar to each other. The currents in 5A, B, and C can be made substantially equal.

  As described above, in the present embodiment, in order to reduce the influence of the difference in parasitic capacitance between the discharge lamps 5A, B, and C, the pattern capacitor on the circuit board 7 that drives the discharge lamps 5A, B, and C is used. 9A, B, and C are used. The capacitance values of the pattern capacitors 9A, 9B, 9C are inversely proportional to the distance between the front and back surfaces of the substrate, that is, the thickness d of the substrate 7, and are proportional to the size (overlapping area) S of the opposing conductor patterns. Therefore, when a change occurs in the difference in parasitic capacitance between the discharge lamps 5A, B, and C, the change (the overlapping area) S of the conductor pattern can be easily adapted to the change. . In addition, after changing the size S of the conductor pattern, a process of mounting a capacitor element or the like is unnecessary, and it is possible to respond to mass production quickly and at low cost according to the change in the parasitic capacitance. Can do.

In addition, the resonance frequency (= 1 / (2π (LC)) ^1/2 ) may be affected (frequency deviation or the like) due to the difference in parasitic capacitance between the discharge lamps 5A, B, and C described above. In the embodiment, such an unexpected situation can be reliably and easily prevented by appropriately changing the size (overlapping area) S of the conductor pattern.

  The discharge lamp driving circuit of the present invention is not limited to the above embodiment, and various modifications can be made. For example, since the capacitance of the pattern capacitor is determined by the overlapping portions of the front and back conductor patterns, for example, one of the front and back (the ground side) of the conductor pattern of the pattern capacitors 9A, B, and C is connected to the other. Only the high-pressure side can be divided as shown in FIG. In this way, it is easier to change the pattern.

  Further, the number of discharge lamps housed in the reflector is not limited to the above three, but can be two or four or more, depending on the distance from the inner wall surface of the reflector. The present invention can be applied to various modes in which a difference occurs in the parasitic capacitance generated in each discharge lamp. In particular, three or more discharge lamps are arranged between both side walls (side walls 6A and C in FIG. 5) on a plane parallel to the open end facing wall surface (bottom wall 6B in FIG. 5) of the reflector having a substantially U-shaped cross section. In the case of being configured as described above, the present invention can be applied to alleviating the difference in parasitic capacitance between the discharge lamps arranged on both sides and the intermediate (one or more) discharge lamps.

Schematic which shows a partial structure of the discharge lamp drive circuit which concerns on embodiment of this invention. Schematic showing the positional relationship of the edge light source part with respect to the liquid crystal display panel Schematic diagram of the positional relationship of the edge light source unit with respect to the liquid crystal display panel shown in FIG. 2 when viewed from the side of the liquid crystal display panel 1 is a circuit diagram schematically showing an overall configuration of a discharge lamp driving circuit according to an embodiment of the present invention. The conceptual diagram which expands and shows the reflector and discharge lamp in FIG. The conceptual circuit diagram for demonstrating the point of the discharge lamp drive circuit which concerns on embodiment of this invention Conceptual diagram for explaining problems related to the prior art

Explanation of symbols

5A, 5B, 5C, 21, 50A, 50B, 50C Discharge lamp 6, 60 Reflector 6A, 6C, 60A, 60C Side wall 6B, 60B Bottom wall 7 Circuit board 8A, 8B, 8C Terminal portion 9A, 9B, 9C Pattern capacitor 11 Liquid crystal display panel 12A, 12B Edge light source 13 Light guide plate 14 Reflector 22 Step-up transformer 23 Driving IC
41 Primary winding 42 Secondary winding 42A, 42B, 42C Coil TR transistor

Claims (3)

A light source for driving a plurality of discharge lamps arranged in parallel in a reflector having an approximately U-shaped cross section, which is an edge light source of the liquid crystal display panel arranged along the edge of the liquid crystal display panel. An electric light driving circuit,
In the transformer secondary side of the discharge lamp driving circuit, the capacitance for determining the resonance frequency against the plurality of discharge lamps each conductor pattern so as to face each other in the front and rear surfaces of the circuit board is formed, the discharge lamp With a pattern capacitor connected in parallel with
According to the size of the parasitic capacitance of each discharge lamp determined according to the distance between each of the plurality of discharge lamps and the wall surface of the reflector, a fixed capacity by the pattern capacitor on the transformer secondary side corresponding to each of the discharge lamps ; In order to reduce the difference in total capacitance from the parasitic capacitance, the overlapping area of the conductor pattern of the pattern capacitor is set to be larger than that of the other discharge lamps for discharge lamps whose parasitic capacitance is smaller than that of the other discharge lamps. A discharge lamp driving circuit characterized by being adjusted to the above. The plurality of discharge lamps are arranged between two side walls that are along the inner wall facing the open end of the reflector and are substantially perpendicular to the inner wall, and are located at an intermediate portion than the discharge lamp near the side wall. The discharge lamp drive circuit according to claim 1, wherein the overlapping area of the conductor pattern of the pattern capacitor is made larger for the discharge lamp located at the position.   3. The discharge lamp driving circuit according to claim 1, wherein the reflector is disposed along a pair of the edge portions facing each other of the liquid crystal display panel.

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