1. Field of the Invention
The present invention relates to a lamp lighting apparatus to drive a plurality of discharge lamps (multiple discharge lamp lighting apparatus), and particularly to a multiple discharge lamp lighting apparatus to drive cold cathode lamps or the like used as a light source of a backlight system for a liquid crystal display device.
2. Description of the Related Art
A discharge lamp, for example, a cold cathode lamp, is extensively used as a light source of a backlight system for a liquid crystal display (LCD) device, and such a discharge lamp is usually AC driven by a discharge lamp lighting apparatus provided with an inverter. Recently, as an LCD device becomes larger in size for a higher brightness, a multiple lamp backlight device adapted to drive a plurality of discharge lamps is more and more used as a lighting source for an LCD device.
Generally, a high voltage is required for driving a discharge lamp, and therefore a discharge lamp lighting apparatus usually includes an inverter transformer to generate a high voltage at the secondary side. An inverter means to generate a high frequency voltage is provided at the primary side of the inverter transformer, while a discharge lamp having a negative resistance characteristic, and a so-called ballast element, such as a ballast capacitor, to stabilize the lamp current of the discharge lamp are provided at the secondary side of the inverter transformer. In a conventional multiple discharge lamp lighting apparatus to drive a plurality of discharge lamps, a ballast capacitor is connected to each of the discharge lamps (refer to, for example, Patent Document 1).
A multiple discharge lamp lighting apparatus is required to provide a uniform lamp current for all discharge lamps in order to achieve a uniform brightness among all the discharge lamps. However, if an individual ballast capacitor is connected to each of the plurality of discharge lamps, the characteristics variation among the individual ballast capacitors may possibly cause lamp current variation among the discharge lamps. To cope with this variation problem, a multiple discharge lamp lighting apparatus is disclosed which includes a circuitry in which a balance coil is provided at the secondary side of an inverter thereby uniformizing the lamp currents of all the discharge lamps (refer to, for example, Patent Document 2). Also, another multiple discharge lamp lighting apparatus is disclosed which includes a circuitry in which electric power is supplied from a low voltage constant current source provided at the primary side of an inverter thereby eliminating requirement of a ballast capacitor (refer to, for example, Patent Document 3), and this circuitry is expected to have a certain effect on achieving a uniform lamp current for the plurality of discharge lamps.
However, the multiple discharge lamp lighting apparatuses described above are accompanied with the following problems.
The multiple discharge lamp lighting apparatus disclosed in Patent Document 1 encounters, in addition to the aforementioned lamp current variation, a problem that an output voltage including the voltage drop of the ballast capacitor connected in series to the discharge lamp must be generated at the secondary side, which causes an increase in the dimension of the inverter transformer thus hindering downsizing of the apparatus.
Also, the multiple discharge lamp lighting apparatus disclosed in Patent Document 2 faces a problem that the balance coil provided at the secondary side is required to have a large inductance and so must be constituted by a large-size element thus inviting an increase in cost and a difficulty in downsizing.
And, the multiple discharge lamp lighting apparatus disclosed in Patent Document 3 may be free from the problems described above but has the following problem with its circuitry. Since a discharge lamp lighting apparatus, when used as a backlight for an LCD device, usually shares a power supply, specifically a constant voltage power supply, with a liquid crystal drive circuit, and the like, provision of a constant current source for the discharge lamp lighting apparatus results in adding an extra component to the entire assembly device thus increasing the total cost.
The present invention has been made in light of the problems described above, and it is an object of the present invention to provide a multiple discharge lamp lighting apparatus in which the lamp currents of a plurality of discharge lamps are stabilized and uniformed inexpensively without providing a ballast capacitor at the secondary side of an inverter transformer.
In order to achieve the object described above, according to an aspect of the present invention, a multiple discharge lamp lighting apparatus to drive a plurality of discharge lamps is provided, which includes an inverter means to output a high frequency voltage, and a plurality of inverter transformers each having a discharge lamp connected at the secondary side thereof. The multiple discharge lamp lighting apparatus described above further includes a plurality of balance inductance elements each of which includes a tight coupling section and a loose coupling section, and each of which is disposed between primary side wirings of adjacent two of the plurality of inverter transformers.
In the aspect of the present invention, the tight coupling section and the loose coupling section of the balance inductance element may be constituted respectively by a tight coupling section and a loose coupling section of a pair of windings disposed around a magnetic core forming an open magnetic path, and the pair of windings may be connected in series to respective primary side wirings of the two adjacent inverter transformers.
In the multiple discharge lamp lighting apparatus according to the present invention, a balance inductance element including a tight coupling section and a loose coupling section is disposed between the primary wirings of two adjacent inverter transformers, thereby stabilizing and equalizing the lamp currents of discharge lamps without a ballast element connected at the secondary side and without increasing the number of constituent members.
In the multiple discharge lamp lighting apparatus according to the present invention, since the loose coupling portion of the balance inductance element functions as a ballast impedance element and is connected at the primary winding of the inverter transformer, the inductance value can be reduced compared with a case when a inductance element as a ballast impedance element is connected at the secondary side, thus enabling downsizing of a ballast impedance element. Also, since high order harmonic component can be reduced by inductance at the primary side, the waveform of input applied to the inverter transformer can be denoised thus reducing heat generation due to the harmonic component, and consequently heat generation at the transformer can be reduced as a whole.
Further, in the multiple discharge lamp lighting apparatus according to the present invention, since the tight coupling section of the balance inductance element functions as a balance coil, currents flowing in the primary windings of the inverter transformers can be equalized without regard to the variation of the ballast impedance elements connected at the primary windings. Also, since each discharge lamp is connected at the secondary winding of the inverter transformer without a ballast element provided therebetween, the output power of the inverter transformer can be reduced, and the lamp current of each discharge lamp can be freed from the influence due to the characteristics variation of a ballast element, thus achieving a uniform lamp current among the discharge lamps. And, the inductance of a balance oil provided at the primary side of the inverter transformer can be reduced compared to when provided at the secondary side for equalizing the lamp currents, thus enabling downsizing of the element.
Accordingly, in the multiple discharge lamp lighting apparatus according to the present invention, since the ballast impedance element and the balance coil provided at the primary side of the inverter transformer can be integrally structured as one balance inductance element including the tight coupling section and the loose coupling section, the number of constituent members can be reduced compared to when the ballast impedance element and the balance coil are provided as separate members.
And, in the multiple discharge lamp lighting apparatus according to the present invention, since the balance inductance element is provided at the primary side of the inverter transformer, rather than at the secondary side to which a high voltage is applied, an element with a high withstand voltage is not necessary, the cost of constituent members can be reduced, and at the same time the malfunction and the fire hazard due to the insulation breakdown of the element is reduced enhancing the safety of the apparatus. Also, even when a short circuit (layer short) occurs in a winding of the secondary side of the inverter transformer, an excessive current flowing in the winding can be suppressed by the ballast impedance element provided at the primary side, thus preventing the inverter transformer from fuming and firing.
a) is a schematic plan view of a balance inductance element of the multiple discharge lamp lighting apparatus of
Exemplary embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.
A first embodiment of the present invention will be described with reference to
The inverter means 12 includes a full bridge circuit constituting the aforementioned switch means 13 (switch means 13 may hereinafter be referred to as full bridge circuit 13 as appropriate), and a control circuit 21 to drive the full bridge circuit 13. Referring to
The balance inductance element BIi includes a pair of windings, specifically a primary winding Wpi and a secondary winding Wsi, and a magnetic core around which the primary and secondary windings Wpi and Wsi are disposed. The structure and operation of the balance inductance element BIi will be detailed later.
The inverter transformers TRi to TRn, which are connected in parallel to the switch means 13, have the following connection mode. For example, as to the connection of the inverter transformer TR2, one terminal of a primary winding Np2 of the inverter transformer TR2 is connected in series to one terminal of a secondary winding Ws1 of a balance inductance element BI1, with the other terminal of the secondary winding Ws1 connected to the output terminal A of the inverter means 12, while the other terminal of the primary winding Np2 of the inverter transformer TR2 is connected to one terminal of a primary winding Wp2 of a balance inductance element BI2, with the other terminal of the primary winding Wp2 connected to the output terminal B of the inverter means 12. The inverter transformers TR3 to TRn−1 are connected in the same way as the inverter transformer TR2, though not entirely illustrated. As to the inverter transformers TR1 and TRn, since the inverter transformer TR1 has its primary side wiring connected to the primary side wiring of the inverter transformer TR2 alone, one terminal of a primary winding Np1 of the inverter transformer TR1 is connected directly to the output terminal A of the inverter means 12, and since the inverter transformer TRn has its primary side wiring connected to the primary side wiring of the inverter transformer TRn−1 alone, one terminal of a primary winding Npn of the inverter transformer TRn is connected directly to the output terminal B of the inverter means 12.
Referring back to
The current detecting circuit 23 generates an adequate signal according to the value of a current detected by a current transformer 25 and outputs the signal to the control circuit 21, which then, according to the signal, varies the on-duty of the switching elements Q1 to Q4 of the inverter means 12, thereby regulating the electric power applied to the inverter transformers TR1 to TRn. The protection circuit 24 generates an adequate signal according to the value of a voltage detected by tertiary windings Nt1 to Ntn of the inverter transformers TR1 and TRn and outputs the signal to the control circuit 21, which then deactivates the inverter means 12 according to the signal when a malfunction, for example, an open circuit or a short circuit at the discharge lamps La1 to Lan, is detected, thereby protecting the device associated. The dimmer circuit 22 outputs a signal to modulate the brightness of the discharge lamp La by, for example, burst dimming, to the control circuit 21, which then, according to the signal, activates intermittently the inverter means 12 at a frequency, for example, 150 to 300 Hz, thereby averaging the brightness of the discharge lamps La1 to Lan. The current detecting circuit 23 detects a current at the current transformer 25 in the embodiment shown, but may alternatively be adapted to detect a lamp current at the discharge lamp La.
The structure and operation of the balance inductance elements BI1 to BIn−1 in the present embodiment will now be described with reference to
a) is a schematic top plan view of the balance inductance element BI1, and
The magnetic core constituting the balance inductance element BI1 of the present invention is not limited in configuration to the squared-C shown in
Currents I1 and I2 flow respectively in the primary and secondary windings Wp1 and Ws1 in the directions opposite to each other as shown in
On the other hand, the inductors LB1 and LB2 of the balance inductance element BI1 function as a ballast impedance element to stabilize lamp currents of the discharge lamps La1 and La2. For example, when the lamp current of the discharge lamp La1 (hereinafter referred to as “secondary side current” as appropriate) is increased for some reason, the current flowing in the primary winding Np1 (hereinafter referred to as “primary side current” as appropriate) is caused to increase also, wherein since the voltage applied by the inverter means 12 is constant, and since the impedance of the balance coil BC1 is regarded as zero as described above, the impedance due to the inductance of the inductor LB1 acts to decrease the primary side current, which results in suppressing the increase of the secondary side current. And, when the secondary side current is decreased, the primary side current is caused to decrease also, and the impedance due to the inductance of the inductor LB1 acts to increase the primary side current resulting in suppressing the decrease of the secondary side current.
The equivalent load resistance seen from the primary side of the inverter transformer TR1 is defined as R/N2 where: N is the winding ratio (secondary winding number/primary winding number) of the inverter transformer TR1; and R is the equivalent resistance of the discharge lamp La1, and so a ballast impedance element must have an impedance value appropriate for R/N2.
Provision of a ballast impedance element at the primary side of an inverter transformer eliminates the necessity of using a high withstand voltage element and accordingly allows an inductor, which is lower in power loss than a resistor, to be used favorably as a ballast element without paying attention to the consideration that an inductor for high voltage use is inevitably subject to an increase in dimension, which is a drawback of an inductor. In addition, since the load resistance seen from the primary side of an inverter transformer is reduced to about 1/N2 as described above, the inductance can be reduced to about L/N2 compared with the case where an inductor functioning equivalently to a ballast element is provided at the secondary side, thus enabling further downsizing of the element. The multiple discharge lamp lighting apparatus 10, if arranged, for example, such that the winding ratio N of the inverter transformer TR1 is set to 100, and that the inductance L of the inductor LB1 is set to about 30 μH, produces a functional capability equivalent to that achieved when an inductor with an inductance L of about 300 mH is provided at the secondary side as a ballast element.
Also, provision of a balance coil at the primary side, rather than at the secondary side, of an inverter transformer eliminates the necessity of using a high withstand voltage element, and an inductance for achieving a practical current equilibration can be reduced, thus enabling downsizing of the element.
In the multiple discharge lamp lighting apparatus 10, a ballast impedance element and a balance coil are integrated into each of the balance inductance elements BI1 to BIn−1, whereby the effect and advantage described above can be achieved with a reduced number of components.
For the purpose of showing one of the advantages achieved by providing a ballast impedance element at the primary side, description will now be made on how the multiple discharge lamp lighting apparatus 10 operates when a short circuit in a winding (what is called “layer short”) is caused at the secondary side of the inverter transformers TR1 to TRn.
In a conventional multiple discharge lamp lighting apparatus, when a layer short is caused at the secondary winding of any one of inverter transformers, a resistor rs at the area of the secondary winding having a short circuit becomes connected to the secondary side thus causing an excessive current to flow in the inverter transformers and possibly prompting fuming and firing hazard. At this time, the power loss at the short circuit is represented as:
P=Vp2/rp
where Vp is the voltage at the primary side of the inverter transformer, and rp is the load resistance due to a layer short seen from the primary side. On the other hand, in the multiple discharge lamp lighting apparatus 10 according to the present embodiment, if a layer short occurs, for example, in the secondary winding Ns1 of the inverter transformer TR1, the power loss at the short circuit area is represented as:
P=rp·Vp2/((ωL)2+rp2)
where L is the inductance of the inductor LB1, which shows that the power loss, that is to say heat generation due to an excessive current, is reduced by the impedance of the inductor LB1.
Also, the inductors LB1 and LB2 function as s low pass filter and are adapted to reject the harmonic component of the output voltage of the inverter means 12 thereby making the waveform of the voltage applied to the primary windings Np1 and Np2 into a substantially sinusoidal waveform. Accordingly, the inverter transformers TR1 and TR2 are denoised and also suppressed from suffering heat generation caused due to the harmonic component.
Further, the inverter means 12 is a high efficiency separately excited circuit including the full bridge circuit 13 and the control circuit 21, wherein the full bridge circuit 13 is driven by the control circuit 21 at a predetermined frequency. Accordingly, unlike, for example, a Royer circuit in which a driving frequency for an inverter means is determined by the resonance frequency of an LC resonance circuit provided at the primary side of an inverter transformer, an element having an impedance and suitable as a ballast can be provided at the primary side without giving consideration to the impact on a resonance frequency.
A second embodiment of the present invention will be described with reference to
The present invention is not limited in structure to the multiple discharge lamp lighting apparatuses 10 and 40 described above, and some constituent elements may be provided additionally.
For example, a capacitor may be connected in series between the inverter means 12 and each of the primary windings Np1 to Npn of the inverter transformers TR1 to TRn. When the inverter means 12 involves an asymmetric output waveform having a voltage V in one direction and a voltage V+ΔV in the other direction as shown in
Also, a capacitor may be connected in parallel to each of the primary windings Np1 to Npn of the inverter transformers TR1 to TRn, whereby the resonance frequency of a resonance circuit at the secondary side is regulated so as to stabilize a lamp current, and at the same time the harmonic component of the output voltage of the inverter means 12 is more effectively rejected so that the waveform of the voltage applied to the primary windings Np1 to Npn can be made into a substantially sinusoidal waveform.
Number | Date | Country | Kind |
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2004-374097 | Dec 2004 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2005/023159 | 12/16/2005 | WO | 00 | 5/24/2007 |
Publishing Document | Publishing Date | Country | Kind |
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WO2006/068054 | 6/29/2006 | WO | A |
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