JP2008041249A - Discharge lamp lighting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a discharge lamp lighting device capable of realizing stabilization of a tube current at a low cost without installing a ballast element on the secondary side of an inverter transformer. <P>SOLUTION: The discharge lamp lighting device 10 includes an inverter means 12 and the inverter transformer TR, and a discharge lamp La such as a cold-cathode tube is directly connected to the secondary winding 16 of the inverter transformer TR without interposing the ballast element. Moreover, a switching means 13 is included in the inverter means 12, and an inductor 18 is connected in series as a ballast impedance element between the switching means 13 and the primary winding 14 of the inverter transformer TR. Since the inductor 18 is connected not to the secondary side but to the primary side of the inverter transformer to which a high voltage is applied, there is no necessity to use an element of high withstand voltage, a parts cost is reduced, and safety of the device is increased. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a discharge lamp lighting device, and more particularly to a discharge lamp lighting device for lighting a cold cathode tube or the like used as a light source for a backlight of a liquid crystal display device.

  2. Description of the Related Art Conventionally, discharge lamps such as cold cathode tubes have been widely used as light sources for backlights of liquid crystal display devices. Generally, such discharge lamps are AC-lit by a discharge lamp lighting device having an inverter. . Such a discharge lamp lighting device usually has an inverter transformer for generating a high voltage on the secondary side, and inverter means for generating a high frequency voltage is connected to the primary side of the inverter transformer, and the secondary side is connected to the secondary side. Is connected to a so-called ballast element, for example, a ballast capacitor, for stabilizing the tube current of the discharge lamp and the discharge lamp having negative resistance characteristics. In such a circuit configuration, in order to obtain the tube voltage necessary for lighting the discharge lamp, it is necessary to generate an output voltage including the voltage drop of the ballast capacitor connected in series with the discharge lamp on the secondary side. As a result of the increase in the shape of the inverter transformer, miniaturization of the device was hindered. For this reason, it has been proposed to eliminate the ballast capacitor by providing a low-voltage constant current source on the primary side of the inverter transformer and supplying power from this low-voltage constant current source (see, for example, Patent Document 1).

FIG. 6 is a circuit configuration diagram of the cold-cathode tube lighting device described in Patent Document 1. The cold cathode tube lighting device includes a voltage resonance type Royer circuit including an inverter transformer 1, transistors 2, 3, resistors 4, 5, a capacitor 6 and a choke coil 7, and a low voltage constant current source 9. A cold cathode tube 8 is directly connected to the secondary side of the transformer 1. In this cold cathode tube lighting device, by supplying power from the constant current source 9 to the inverter transformer 1, the tube current is kept constant without connecting a ballast capacitor to the secondary side of the inverter transformer 1. .
Japanese Patent No. 32566992

  However, in general, since a constant voltage power supply common to a liquid crystal drive circuit or the like is used as a power source of a discharge lamp lighting device used as a backlight of a liquid crystal display, it is not possible to use a constant current source for the discharge lamp lighting device. This means that new components are added to the liquid crystal display device, and the cost of the entire device increases.

  In view of the above problems, an object of the present invention is to provide a discharge lamp lighting device capable of stabilizing tube current at a low cost without providing a ballast element on the secondary side of an inverter transformer.

  In order to achieve the above object, the present invention includes an inverter means for outputting a high-frequency voltage and an inverter transformer, and is connected to the secondary side of the inverter transformer by the inverter means connected to the primary side of the inverter transformer. In the discharge lamp lighting device for lighting the discharge lamp, the inverter means includes switching means, and a ballast impedance element is connected in series between the switching means and the primary winding of the inverter transformer. Features.

  Further, the ballast impedance element is formed of any one of a resistor, a capacitor, an inductor, or a combination thereof.

  The switching means is constituted by a full bridge circuit, and the full bridge circuit is driven by separately excited oscillation.

  According to the discharge lamp lighting device of the present invention, by connecting the ballast impedance element in series between the switching means and the primary winding of the inverter transformer, the tube current can be generated without connecting the ballast element to the secondary side. A stable discharge lamp lighting device can be realized without increasing the number of parts from the conventional configuration. In the present invention, the ballast impedance element is connected not to the secondary side of the inverter transformer to which a high voltage is applied, but to the primary side, so there is no need to use a high voltage resistance element and the component cost is reduced. At the same time, there is no risk of failure or ignition due to dielectric breakdown of the element, and the safety of the device is increased. Moreover, since it is not necessary to connect a ballast element in series with the discharge lamp on the secondary side of the inverter transformer, the output power of the inverter transformer can be kept low. Furthermore, even if a short circuit between windings (so-called rare short) occurs on the secondary side of the transformer, the overcurrent flowing through the winding is suppressed by the primary side ballast impedance element to prevent smoke and ignition of the inverter transformer. Can do.

  In particular, when an inductor is used as the ballast impedance element, the inductance can be made smaller than when the inductor is connected to the secondary side, so that the ballast impedance element can be reduced in size. In addition, since the higher-order harmonic components are suppressed by the primary-side inductor, noise can be removed from the input waveform applied to the inverter transformer, and the heat generation of the transformer due to the harmonic components is suppressed. As a result, the heat generation of the transformer is reduced.

  Further, by making the inverter means a separately excited type, it is possible to select an element having any appropriate impedance as the ballast impedance element according to the present invention without considering the influence on the resonance frequency on the primary side. it can. In particular, high-efficiency operation is possible by making the switching means a full bridge circuit.

  Hereinafter, an embodiment of a discharge lamp lighting device according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an embodiment of a discharge lamp lighting device according to the present invention. In FIG. 1, a discharge lamp lighting device 10 includes inverter means 12 and an inverter transformer TR, and a discharge lamp La such as a cold cathode tube is not passed through a ballast element in a secondary winding 16 of the inverter transformer TR. Connected directly. Further, the inverter means 12 includes a switching means 13, and an inductor 18 is connected in series as a ballast impedance element in this embodiment between the switching means 13 and the primary winding 14 of the inverter transformer TR. .

  In the present embodiment, the inverter unit 12 includes a full bridge circuit that is the switching unit 13 and a control circuit 21 that drives the full bridge circuit 13. As shown in FIG. 2, the full-bridge circuit 13 includes a set of switching elements Q1 and Q3 connected in series and a set of switching elements Q2 and Q4 connected in series in parallel. For example, the switching elements Q3 and Q4 are composed of NMOSFETs, and the switching elements Q1 and Q2 are composed of PMOSFETs. The inverter means 12 alternately turns on / off the switching element pairs (Q1, Q4) and (Q2, Q3) at a predetermined frequency (for example, about 60 kHz) according to the gate voltage output from the control circuit 21, and outputs it. A high frequency voltage is generated at terminals A and B.

  The discharge lamp lighting device 10 shown in FIG. 1 includes a dimming circuit 22, a current detection circuit 23, and a protection circuit 24 in addition to the above-described components. The discharge lamp lighting device according to the present invention is not limited to the presence or absence of these circuits 22 to 24, but the function of each circuit 22 to 24 will be briefly described as follows. First, the current detection circuit 23 generates an appropriate signal according to the current value detected by the current transformer 25 and outputs it to the control circuit 21, whereby the control circuit 21 is switched, for example, in the inverter means 12. The on-duty of the elements Q1 to Q4 is varied to adjust the power input to the inverter transformer TR. The protection circuit 24 generates an appropriate signal according to the voltage detected by the tertiary winding 26 of the inverter transformer TR and outputs it to the control circuit 21, so that the control circuit 21 can, for example, open the discharge lamp La or When an abnormality such as a short circuit is detected, the operation of the inverter means 21 is stopped to protect the device. The dimming circuit 22 outputs a signal for adjusting the luminance of La such as discharge by burst dimming, for example, to the control circuit 21, whereby the control circuit 21 has a frequency of about 150 to 300 Hz, for example. Thus, the average brightness of the discharge lamp La is adjusted by operating the inverter means 12 intermittently. In the illustrated example, the current detection circuit 23 detects the current by the current transformer 25, but may detect the tube current of the discharge lamp La.

The discharge lamp lighting device 10 in the present embodiment includes an inductor 18 on the primary side of the inverter transformer TR, and this inductor 18 functions as a ballast impedance element, thereby realizing stabilization of the tube current of the discharge lamp La. is there. That is, when the tube current (hereinafter also referred to as secondary current) increases for some reason, the current (hereinafter also referred to as primary current) flowing through the primary winding 14 of the inverter transformer TR increases, but the inverter means 12 Since the voltage applied by is constant, the impedance of the inductor 18 acts to decrease the primary current and lower the voltage drop, and as a result, an increase in secondary tube current is suppressed. Similarly, when the tube current decreases, the primary side current also decreases. At this time, the impedance of the inductor 18 increases the primary side current to increase the voltage drop, resulting in the secondary side tube. A decrease in current is suppressed. Here, if the winding ratio of the inverter transformer TR (number of turns of the secondary winding / number of turns of the primary winding) is n and the equivalent load resistance of the discharge lamp La is R, the primary side of the inverter transformer TR Since the viewed load impedance is R / n ² , the impedance required for the ballast impedance element only needs to be sufficiently larger than R / n ² .

The present invention is not limited to the type of impedance element to be used, and as the ballast impedance element according to the present invention, any of a resistor, a capacitor, an inductor, or a combination thereof can be used. Like the ballast impedance element in this embodiment, an inductor or a combination including an inductor is used. In the discharge lamp lighting device according to the present invention, it is not necessary to use a high breakdown voltage element because the ballast impedance element is connected to the primary side of the inverter transformer. The inductive inductor can be advantageously used as a ballast element while overcoming the conventional drawback of increased shape. In addition, as described above, since the load impedance viewed from the primary side of the inverter transformer is reduced to about 1 / n ² , the discharge lamp lighting device 10 according to the present embodiment includes an inductor having the same function as the ballast element. Compared with the case of connecting to the secondary side, the self-inductance can be reduced to about L / n ² , and the element can be further downsized. For example, in the discharge lamp lighting device 10, when the winding ratio n of the inverter transformer TR is 100 and the self-inductance L of the inductor 18 is about 30 μH, an inductor having a self-inductance L of about 300 mH is used as a ballast element on the secondary side. The same function as when connected is exhibited.

  As in the circuit configuration shown in FIG. 6, the conventional discharge lamp lighting device also includes a choke coil 7 on the primary side of the inverter transformer 1, and this choke coil 7 is related to the present invention. Unlike the ballast impedance element, it is connected between the switching means constituted by the transistors 2 and 3 and the DC power source 9 and is used as an impedance element only when an overcurrent flows instantaneously due to, for example, magnetic saturation of the inverter transformer 1. It functions to protect the transistors 2 and 3 from being damaged, and does not function as a ballast element that stabilizes the tube current by dividing the high-frequency voltage generated by the switching means. .

  Further, in the present embodiment, the inductor 18 functions as a low-pass filter. Therefore, the harmonic component of the output voltage of the inverter means 12 is cut, and the voltage waveform applied to the primary winding 14 of the inverter transformer TR is substantially sine. Can be wavy. As a result, noise is removed from the inverter transformer TR and heat generation of the inverter transformer TR due to harmonic components is suppressed.

Next, as one of the advantages of connecting the ballast impedance element on the primary side, the operation when a short-circuit between windings (so-called rare short) occurs on the secondary side of the inverter transformer TR will be described. When a short circuit occurs on the secondary side of the inverter transformer, the secondary circuit has the resistance Rs of the shorted portion of the secondary winding connected to the secondary side regardless of the impedance of the discharge lamp and the ballast element. Therefore, in the conventional discharge lamp lighting device, an excessive current flows through the inverter transformer, causing smoke and fire. At this time, if the voltage on the primary side of the inverter transformer is Vp and the resistance value when the load resistance due to the rare short is viewed from the primary side is Rp, the power loss in the shorted portion is
P = Vp ² / Rp
It is represented by However, since the discharge lamp lighting device 10 according to the present embodiment includes the ballast impedance element (in this case, the inductor) 18 on the primary side, the loss P in the short portion is
P = Rp · Vp ² / ((ωL) ² + Rp ² )
(Where L is the self-inductance of the inductor 18), it can be seen that power loss, that is, heat generation due to overcurrent is suppressed by the impedance of the inductor 18.

  In the discharge lamp lighting device 10 according to the present embodiment, the inverter means 12 is configured by a high-efficiency separately-excited circuit composed of a full bridge circuit 13 and a control circuit 21. Driven at a frequency of Therefore, for example, unlike the case of a Royer circuit in which the drive frequency of the inverter means is determined by the resonance frequency of the LC resonance circuit provided on the primary side of the inverter transformer 1 as shown in FIG. An element having any impedance suitable as a ballast can be connected to the primary side without considering the influence on the primary side.

  Hereinafter, another embodiment of the present invention will be described with reference to FIGS. 3 to 5. Throughout the following description, the same components as those in the first embodiment described above are denoted by the same reference numerals, The description will be omitted as appropriate, and only the differences will be described.

  FIG. 3 is a circuit configuration diagram showing a second embodiment of the discharge lamp lighting device according to the present invention. The discharge lamp lighting device 30 in the present embodiment is substantially the same as the discharge lamp lighting device 10 in the first embodiment described above, but is connected between the inverter means 12 and the primary winding 14 of the inverter transformer TR. The ballast impedance element is different in that a series circuit 32 including a capacitor 34 and an inductor 33 is connected.

  The discharge lamp lighting device 30 in the present embodiment has the following operation in addition to the same operations and effects as the discharge lamp lighting device 10 in the first embodiment described above. For example, as shown in FIG. 4, when the output waveform of the inverter means 12 has an asymmetry such that the voltage in one direction is V and the voltage in the other direction is V + ΔV, the output voltage is averaged. A DC voltage of ΔV ′ (where ΔV ′ is a time average of ΔV) is superimposed. For this reason, if the ballast impedance element is only the inductor 32, a large direct current is superimposed on the inverter transformer TR, causing magnetic saturation and a decrease in efficiency. At this time, in the discharge lamp lighting device 30, a DC component of an asymmetric voltage waveform is cut by adding a capacitor 34 connected in series to the inverter means 12 to the ballast impedance element, and the primary winding of the inverter transformer TR. The symmetry of the voltage applied to the line 14 is improved.

  FIG. 5 is a circuit configuration diagram showing a third embodiment of the discharge lamp lighting device according to the present invention. The discharge lamp lighting device 40 in the present embodiment is substantially the same as the discharge lamp lighting device 10 in the first embodiment described above, but a capacitor 42 is connected in parallel to the primary winding 14 of the inverter transformer TR. Is different.

  The discharge lamp lighting device 40 in the present embodiment has the following operations in addition to the same operations and effects as the discharge lamp lighting device 10 in the first embodiment described above. That is, when the tube current is not stable with respect to a predetermined drive frequency of the inverter means 12 by forming a resonance circuit on the secondary side of the inverter transformer TR due to the parasitic capacitance between the discharge lamp and the liquid crystal display device, As in the discharge lamp lighting device 40 in the present embodiment, a capacitor 42 having an appropriate capacity is connected in parallel to the primary winding 14 of the inverter transformer, thereby adjusting the resonance frequency of the secondary side resonance circuit and Can be stabilized. Further, the combination of the inductor 43 and the capacitor 44 cuts the harmonic component of the output voltage of the inverter means 12 more effectively, so that the voltage waveform applied to the primary winding 14 of the inverter transformer TR is substantially sinusoidal. can do.

It is a circuit block diagram which shows 1st Embodiment of the discharge lamp lighting device which concerns on this invention. It is a circuit block diagram which shows the inverter means of the lighting devices, such as a discharge, in the 1st Embodiment of this invention. It is a circuit block diagram which shows 2nd Embodiment of the discharge lamp lighting device which concerns on this invention. In the 2nd Embodiment of this invention, it is a graph which shows typically the asymmetrical voltage waveform by an inverter means. It is a circuit block diagram which shows 3rd Embodiment of the discharge lamp lighting device which concerns on this invention. It is a circuit block diagram which shows the conventional discharge lamp lighting device.

Explanation of symbols

10, 30, 40: discharge lamp lighting device, 12: inverter means, switching means (full bridge circuit): 13, ballast impedance elements: 18, 32, 43, inverter transformer: TR, discharge lamp: La

Claims (3)

In a discharge lamp lighting device comprising an inverter means for outputting a high-frequency voltage and an inverter transformer, and lighting the discharge lamp connected to the secondary side of the inverter transformer by the inverter means connected to the primary side of the inverter transformer. ,
The discharge lamp lighting device, wherein the inverter means includes switching means, and a ballast impedance element is connected in series between the switching means and a primary winding of the inverter transformer.   The discharge lamp lighting device according to claim 1, wherein the ballast impedance element is configured by any one of a resistor, a capacitor, and an inductor, or a combination thereof. The discharge lamp lighting device according to claim 1 or 2, wherein the switching means is configured by a full bridge circuit, and the full bridge circuit is driven by separately excited oscillation.

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