Patent Publication Number: US-2009230877-A1

Title: Discharge lamp lighting apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a discharge lamp lighting apparatus for lighting a discharge lamp, in particular, a cold cathode fluorescent lamp (CCFL) of, for example, a liquid crystal display. 
     2. Description of the Related Art 
     An example of a discharge lamp lighting apparatus that applies voltages of opposite phases to both ends of a straight discharge lamp and thereby lights the discharge lamp is disclosed in Japanese Unexamined Patent Application Publication No. 2006-221985. The apparatus of this related art includes a master lighting unit and a slave lighting unit, wherein the units output voltages of opposite phases to both ends of a straight discharge lamp to turn on the discharge lamp. 
       FIG. 1  is a schematic view illustrating the discharge lamp lighting apparatus of the above-mentioned related art and  FIG. 2  is a circuit diagram illustrating the apparatus of  FIG. 1 . 
     In  FIG. 1 , a power source unit  11   a  is centrally arranged on a bottom face  1  of a panel. On the left side of the power source unit  11   a , the master lighting unit  12   a  that is an inverter is arranged. On the right side of the power source unit  11   a , the slave lighting unit  12   b  that is an inverter is arranged. The power source unit  11   a  supplies a source voltage Vcc to the master and slave lighting units  12   a  and  12   b.    
     The master lighting unit  12   a  includes a controller (controller IC)  13   a  ( 13   a - 1 ,  13   a - 2 ), a MOS switch  14   a  consisting of four MOSFETs arranged in a bridge configuration and serving as switching elements, and a resonant circuit  15  containing a transformer Ta. The resonant circuit  15  provides an AC voltage that is applied through a high-voltage output line  22   a  to a first end of the discharge lamp  3 . 
     The slave lighting unit  12   b  includes a controller  13   b  that operates in response to signals from the controller  13   a - 1  of the master lighting unit  12   a , a MOS switch  14   b  consisting of four MOSFETs arranged in a bridge configuration and serving as switching elements, and a resonant circuit  16  containing a transformer Tb. The resonant circuit  16  provides an AC voltage that is applied through a high-voltage output line  22   b  to a second end of the discharge lamp  3 . 
     Each of the MOS switches  14   a  and  14   b  includes the four MOSFETs arranged in a bridge configuration and serving as switching elements. The four MOSFETs are a p-type FET Qp 1  and an n-type FET Qn 1  that form a series circuit and a p-type FET Qp 2  and an n-type FET Qn 2  that form a series circuit. 
     The controller  13   a - 1  of the master lighting unit  12   a  compares a voltage of a secondary winding S of the transformer Ta rectified through a diode and a voltage of a secondary winding S of the transformer Tb rectified through a diode with a reference voltage and provides an error voltage. Further, the controller  13   a - 1  compares the error voltage with a triangular signal, generates a pulse signal whose pulse width corresponds to the error voltage, and supplies the pulse signal to the controller  13   a - 2 . The pulse signal is also supplied through terminals  17   a  and  17   b  and a signal line  18  to the controller  13   b.    
     Based on the pulse signal from the controller  13   a - 1 , each of the controllers  13   a - 2  and  13   b - 1  generates first to fourth drive signals and applies them to the p- and n-type FETs Qp 1 , Qn 1 , Qp 2 , and Qn 2 , respectively, in such a way as to alternately form an ON period in which the FETs Qp 1  and Qn 2  simultaneously turn on and an ON period in which the FETs Qp 2  and Qn 1  simultaneously turn on, thereby generating an AC voltage on a primary winding P of the transformer Ta (Tb). 
     The polarity of the transformer Tb is opposite to the polarity of the transformer Ta, and therefore, an output voltage from the master lighting unit  12   a  and an output voltage from the slave lighting unit  12   b  have opposite phases that are applied to the ends of the discharge lamp  3 , respectively, to light the discharge lamp  3 . 
     SUMMARY OF THE INVENTION 
     The discharge lamp lighting apparatus of the above-mentioned related art mounts the master lighting unit  12   a  on a first circuit board and the slave lighting unit  12   b  on a second circuit board and separately arranges the first and second circuit boards in the vicinities of the ends of the discharge lamp  3 . Mounting the two lighting units on the separate two circuit boards complicates the structure of the discharge lamp lighting apparatus and increases the cost thereof. 
     According to the present invention, a discharge lamp lighting apparatus that is capable of applying voltages of opposite phases to both ends of a discharge lamp with a simple configuration and at low cost can be provided. 
     According to a first aspect of the present invention, provided is a discharge lamp lighting apparatus mounted on a single circuit board, for converting a direct current into an alternating current, applying voltages of opposite phases to both ends of a discharge lamp, and thereby lighting the discharge lamp. The apparatus includes two resonant circuits each including a transformer, a resonant reactor, and a resonant capacitor, output terminals of the two resonant circuits being connected to the ends of the discharge lamp, to apply the voltages of opposite phases to the ends of the discharge lamp, respectively. Resonant characteristics of the two resonant circuits are equalized with each other by providing a difference between values of the resonant reactors of the two resonant circuits according to values of stray capacitances determined by the lengths of high-voltage output lines extended from the two resonant circuits to the ends of the discharge lamp. 
     A second aspect of the present invention provides a discharge lamp lighting apparatus mounted on a single circuit board, for converting a direct current into an alternating current, applying voltages of opposite phases to both ends of a discharge lamp, and thereby lighting the discharge lamp. The apparatus includes two resonant circuits each including a transformer, a resonant reactor, and a resonant capacitor, output terminals of the two resonant circuits being connected to the ends of the discharge lamp, to apply the voltages of opposite phases to the ends of the discharge lamp, respectively. Resonant characteristics of the two resonant circuits are equalized with each other by providing a difference between values of the resonant capacitors of the two resonant circuits according to values of stray capacitances determined by the lengths of high-voltage output lines extended from the two resonant circuits to the ends of the discharge lamp. 
     According to a third aspect of the present invention, the resonant capacitor includes a capacitor having a conductor pattern formed on a top face of the circuit board and a conductor pattern formed on a bottom face of the circuit board. 
     According to a fourth aspect of the present invention, the high-voltage output line having a flexible substrate employing the capacitor. 
     According to a fifth aspect of the present invention, the resonant characteristic of each of the resonant circuits is a resonant frequency that is determined by the resonant reactor, resonant capacitor, and stray capacitance related to the resonant circuit. 
     According to a sixth aspect of the present invention, the resonant reactor is a leakage inductance between primary and secondary windings of the transformer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a discharge lamp lighting apparatus according to a related art; 
         FIG. 2  is a circuit diagram illustrating the discharge lamp lighting apparatus of  FIG. 1 ; 
         FIG. 3  is a schematic view illustrating a discharge lamp lighting apparatus according to Embodiment 1 of the present invention; 
         FIG. 4  is a circuit diagram illustrating the discharge lamp lighting apparatus of  FIG. 3 ; 
         FIG. 5A  is a view illustrating an example 1 of a high-voltage output line arranged between a resonant circuit and a discharge lamp in a discharge lamp lighting apparatus according to Embodiment 2 of the present invention; 
         FIG. 5B  is a sectional view illustrating a location depicted by a chain line VB in  FIG. 5A ; 
         FIG. 6A  is a view illustrating an example 2 of the high-voltage output line arranged between the resonant circuit and the discharge lamp in the discharge lamp lighting apparatus according to Embodiment 2; and 
         FIG. 6B  is a sectional view illustrating a location depicted by a chain line VIB in  FIG. 6A . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Discharge lamp lighting apparatuses according to embodiments of the present invention will be explained in detail with reference to the drawings. The discharge lamp lighting apparatus according to each embodiment is arranged on a single circuit board that is arranged on a bottom face of a panel in the vicinity of an end of a discharge lamp. The apparatus is capable of applying voltages of opposite phases to ends of the discharge lamp with a simple configuration and at low cost. 
     Embodiment 1 
       FIG. 3  is a schematic view illustrating a discharge lamp lighting apparatus according to Embodiment 1 of the present invention and  FIG. 4  is a circuit diagram illustrating the same. 
     In  FIG. 3 , a power source unit  11  is centrally arranged on a bottom face  1  of a panel. On the left side of the power source unit  11 , there is arranged a single circuit board  12  on which the discharge lamp lighting apparatus serving as an inverter is mounted. The power source unit  11  supplies a source voltage Vcc to the discharge lamp lighting apparatus on the circuit board  12 . 
     The discharge lamp lighting apparatus includes a controller (controller IC)  13  ( 13 - 1 ,  13 - 2 ), a MOS switch  14 , a resonant circuit  15  containing a transformer Ta, and a resonant circuit  16  containing a transformer Tb. The resonant circuit  15  outputs an AC voltage to be supplied through a high-voltage output line  21   a  to a first end of a discharge lamp  3 . The resonant circuit  16  outputs an AC voltage to be supplied through a high-voltage output line  21   b  to a second end of the discharge lamp  3 . The discharge lamp  3  is a cold cathode fluorescent lamp (CCFL). 
     The MOS switch  14  is four MOSFETs arranged in a bridge configuration and serving as switching elements. The four MOSFETs are a high-side p-type FET Qp 1  and a low-side n-type FET Qn 1  that form a series circuit connected between the source voltage Vcc and the ground and a high-side p-type FET Qp 2  and a low-side n-type FET Qn 1  that form a series circuit connected between the source voltage Vcc and the ground. 
     Between a connection point of the p- and n-type FETs Qp 1  and Qn 1  and a connection point of the p- and n-type FETs Qp 2  and Qn 2 , there is connected a series circuit including a capacitor C 3   a  and a primary winding P of the transformer Ta. The series circuit of the capacitor C 3   a  and the primary winding P of the transformer Ta is connected in parallel with a series circuit including a capacitor C 3   b  and a primary winding P of the transformer Tb. The capacitor C 3   a  is connected to a winding start of the primary winding P of the transformer Ta and the capacitor C 3   b  is connected to a winding end of the primary winding P of the transformer Tb. 
     Sources of the p-type FETs Qp 1  and Qp 2  receive the source voltage Vcc. A gate of the p-type FET Qp 1  is connected to a terminal HDRV 1  of the controller  13 - 2 , a gate of the p-type FET Qp 2  is connected to a terminal HDRV 2  of the controller  13 - 2 , a gate of the n-type FET Qn 1  is connected to a terminal LDRV 1  of the controller  13 - 2 , and a gate of the n-type FETQn 2  is connected to a terminal LDRV 2  of the controller  13 - 2 . 
     A first end of a secondary winding S of the transformer Ta is connected through the high-voltage output line  21   a  to a first electrode of the discharge lamp  3 . A resonant reactor L 1  is a leakage inductance component of the transformer Ta. A second end of the secondary winding S of the transformer Ta is connected to a cathode of a diode D 1   a  and an anode of a diode D 2   a . The diodes D 1   a  and D 2   a  and a resistor R 3   a  form a lamp current detector to detect a current passing through the secondary winding S and supply a voltage proportional to the detected current to a terminal FB of the controller  13 - 1 . 
     Between the first end of the discharge lamp  3  and the ground, there is connected a series circuit including capacitors C 9   a  and C 4   a . A connection point of the capacitors C 9   a  and C 4   a  is connected to a cathode of a diode D 6   a  and an anode of a diode D 7   a . The diodes D 6   a  and D 7   a , a resistor R 10 , and a capacitor C 10  form a rectifying-smoothing circuit that detects a voltage proportional to an output voltage and supplies the detected voltage to a terminal OVP of the controller  13 - 1 . The capacitors C 9   a  and C 4   a  form a resonant capacitor. 
     A first end of a secondary winding S of the transformer Tb is connected through the high-voltage output line  21   b  to a second electrode of the discharge lamp  3 . A resonant reactor L 2  is a leakage inductance component of the transformer Tb. A second end of the secondary winding S of the transformer Tb is connected to a cathode of a diode D 1   b  and an anode of a diode D 2   b . The diodes D 1   b  and D 2   b  and a resistor R 3   b  form a lamp current detector to detect a current passing through the secondary winding S and supply a voltage proportional to the detected current to the terminal FB of the controller  13 - 1 . As mentioned above, the terminal FB is also connected to the output of the lamp current detector having the diodes D 1   a  and D 2   a  and resistor R 3   a.    
     Between the second end-of the discharge lamp  3  and the ground, there is connected a series circuit including capacitors C 9   b  and C 4   b . A connection point of the capacitors C 9   b  and C 4   b  is connected to a cathode of a diode D 6   b  and an anode of a diode D 7   b . The diodes D 6   b  and D 7   b , the resistor R 10 , and the capacitor C 10  form a rectifying-smoothing circuit that detects a voltage proportional to an output voltage and supply the detected voltage to the terminal OVP of the controller  13 - 1 . This output is connected to the output of the rectifying-smoothing circuit including the diodes D 6   a  and D 7   a  and these outputs are combined together. The capacitors C 9   b  and C 4   b  form a resonant capacitor. 
     The controller  13 - 1  compares the voltage of the secondary winding S of the transformer Ta rectified through the diodes and the voltage of the secondary winding S of the transformer Tb rectified through the diodes witha reference voltage, provides an error voltage according to a result of the comparison, compares the error voltage with a triangular signal, generates a pulse signal whose pulse width corresponds to the error voltage, and outputs the pulse signal to the controller  13 - 2 . 
     Based on the pulse signal outputted from the controller  13 - 1 , the controller  13 - 2  generates first to fourth drive signals and applies them to the p- and n-type FETs Qp 1 , Qn 1 , Qp 2 , and Qn 2 , respectively, in such a way as to alternately form an ON period in which the FETs Qp 1  and Qn 2  simultaneously turn on and an ON period in which the FETs Qp 2  and Qn 1  simultaneously turn on, thereby generating AC voltages on the primary windings P of the transformers Ta and Tb. 
     A connection point of the source of the p-type FET Qp 1  and the drain of the n-type FET Qn 1  is connected through the capacitor C 3   a  to the winding start of the primary winding P of the transformer Ta and is connected through the capacitor C 3   b  to the winding end of the primary winding P of the transformer Tb, so that the voltages generated by the transformers Ta and Tb have opposite phases. Namely, a voltage from the resonant circuit  15  and a voltage from the resonant circuit  16  have opposite phases and are applied to the ends of the discharge lamp  3 , respectively, to light the discharge lamp  3 . 
     According to the present embodiment, the two resonant circuits  15  and  16  should have the same resonant characteristic to apply the voltages of opposite phases to the ends of the discharge lamp  3 . That is, values of the resonant reactors L 1  and L 2  of the resonant circuits  15  and  16  are determined according to values of stray capacitances Cs 1  and Cs 2  those are dependent on a length of the high-voltage output line  21   a  extended from the resonant circuit  15  to the first end of the discharge lamp  3  and a length of the high-voltage output line  21   b  extended from the resonant circuit  16  to the second end of the discharge lamp  3 , respectively. These lengths are not the same with each other, and therefore, the values of the resonant reactors L 1  and L 2  to be determined will have a difference between them. 
     The stray capacitance Cs 1  appears between the high-voltage output line  21   a  and the ground and the stray capacitance Cs 2  appears between the high-voltage output line  21   b  and the ground. 
     The resonant characteristic of each resonant circuit is, for example, a resonant frequency. A resonant frequency f 1  of the resonant circuit  15  is determined by the resonant reactor L 1 , resonant capacitors C 9   a  and C 4   a , and stray capacitance Cs 1 . A resonant frequency f 2  of the resonant circuit  16  is determined by the resonant reactor L 2 , resonant capacitors C 9   b  and C 4   b , and stray capacitance Cs 2 . 
     More precisely, the resonant frequency f 1  of the resonant circuit  15  is determined by 
         f 1=1/{2π√( L 1×( Ca+Cs 1))}  (1). 
     The resonant frequency f 2  of the resonant circuit  16  is determined by 
         f 2=1/{2π√( L 2×( Cb+Cs 2))}  (2), 
       where 
         Ca =( C 4 a×C 9 a )/( C 4 a+C 9 a ), and 
         Cb =( C 4 b×C 9 b )/( C 4 b+C 9 b ). 
     To equalize the resonant frequencies f 1  and f 2  with each other on the basis that values of the resonant capacitors Ca and Cb are equal to each other, the resonant reactor L 2  should satisfy a relationship of 
         L 2= L 1×( Ca+Cs 1)/( Ca+Cs 2)   (3). 
     If the high-voltage output line  21   b  is long, the stray capacitance Cs 2  is large, and if the same is short, the stray capacitance Cs 2  is small. If the line  21   b  is longer than the line  21   a , the value of the resonant reactor L 2  of the line  21   b  is decreased or the value of the resonant reactor L 1  of the line  21   a  is increased, to provide a difference between the values of the resonant reactors L 1  and L 2  so as to satisfy the expression (3). 
     Instead of manipulating the resonant reactors L 1  and L 2 , manipulating the resonant capacitors Ca and Cb can also equalize the resonant frequencies f 1  and f 2  of the resonant circuits  15  and  16 . Namely, a difference is created between the values of the resonant capacitors Ca and Cb according to the values of the stray capacitances Cs 1  and Cs 2 , the value of Cs 1  being dependent on the length of the high-voltage output line  21   a  extended from the resonant circuit  15  to the first end of the discharge lamp  3  and the value of Cs 2  being dependent on the length of the high-voltage output line  21   b  extended from the resonant circuit  16  to the second end of the discharge lamp  3 . 
     That is, to equalize the resonant frequencies f 1  and f 2  with each other on the basis that the values of the resonant reactors L 1  and L 2  are equal to each other, the resonant capacitor Cb should satisfy a relationship of 
         Cb=Ca +( Cs 1− Cs 2)   (4). 
     If the high-voltage output line  21   b  is long, the stray capacitance Cs 2  is large, and if the same is short, the stray capacitance Cs 2  is small. If the line  21   b  is longer than the line  21   a , the value of the resonant capacitor Ca is increased or the value of the resonant capacitor Cb is decreased, to provide a difference between the values of the resonant capacitors Ca and Cb so as to satisfy the expression (4). 
     In this way, the discharge lamp lighting apparatus according to the embodiment arranges the apparatus on the single circuit board  12  and equalizes the resonant characteristics of the two resonant circuits  15  and  16  with each other by compensating a difference between the values of the stray capacitances Cs 1  and Cs 2  determined by the lengths of the high-voltage output lines  21   a  and  21   b  extended from the resonant circuits  15  and  16  to the discharge lamp  3 . That is, the embodiment differs the values of the resonant reactors L 1  and L 2  or the values of the resonant capacitors Ca and Cb of the resonant circuits  15  and  16  from each other. As results, the apparatus of the embodiment is simple and inexpensive to correctly apply voltages of opposite phases to the ends of the discharge lamp  3 . 
     Embodiment 2 
       FIGS. 5A and 5B  are views illustrating an example 1 of a high-voltage output line arranged from a resonant circuit to a discharge lamp in a discharge lamp lighting apparatus according to Embodiment 2 of the present invention. 
     In  FIG. 5A , the high-voltage output line  21   a  ( 21   b ) includes a flexible printed substrate  21  that is freely bendable. In FIG.  5 B, the flexible printed substrate  21  includes a base  32 , a conductor pattern  33   a  formed on a top face of the base  32 , a conductor pattern  33   b  formed on a bottom face of the base  32 , and covers  31   a  and  31   b  covering the conductor patterns  33   a  and  33   b.    
     The conductor pattern  33   a  on the top face of the base  32  and the conductor pattern  33   b  on the bottom face of the base  32  form a capacitor Cc. The capacitor Cc may serve as the resonant capacitor Ca (Cb) of Embodiment 1. 
       FIGS. 6A and 6B  are views illustrating an example 2 of the high-voltage output line arranged from the resonant circuit to the discharge lamp in the discharge lamp lighting apparatus according to Embodiment 2. 
     In  FIG. 6A , the example 2 arranges transformers Ta and Tb on a circuit board  42 . The transformers Ta and Tb are provided with conductor pattern portions  40   a  and  40   b , respectively. As illustrated in an enlarged sectional view of  FIG. 6B , the conductor pattern portion  40   a  has a conductor pattern  43   a  substantially having a square shape on a top face of the circuit board  42  and a conductor pattern  43   b  substantially having a square shape on a bottom face of the circuit board  42 . An output line (not illustrated) of the transformer Ta (Tb) is connected to one (for example,  43   a ) of the conductor patterns  43   a  and  43   b.    
     The conductor pattern  43   a  on the top face of the circuit board  42  and the conductor pattern  43   b  on the bottom face of the circuit board  42  form a capacitor. The capacitor may serve as the resonant capacitor Ca (Cb) of Embodiment 1. 
     The present invention is not limited to the above-mentioned Embodiments 1 and 2. According to Embodiments 1 and 2, the discharge lamp  3  is a cold cathode fluorescent lamp (CCFL). The discharge lamp to which the present invention is applied is not limited to the CCFL. For example, external electrode fluorescent lamps (EEFLs) that are connected in parallel are adoptable for the present invention. 
     The present invention is also applicable to an equivalent EEFL consisting of a CCFL with capacitors connected to each end of the CCFL in series. If the present invention is applied to discharge lamps that have positive impedance characteristics and are connected in parallel, the discharge lamps will collectively be considered as a single discharge lamp. 
     In summary, the discharge lamp lighting apparatus provided by the present invention is mounted on a single circuit board, to simplify the structure thereof and reduce the cost thereof. According to the values of stray capacitances that are depending on the lengths of high-voltage output lines from two resonant circuits to a discharge lamp, the apparatus provides a difference between the values of resonant reactors or resonant capacitors of the two resonant circuits and outputs voltages of opposite phases to both ends of the discharge lamp. 
     This application claims benefit of priority under 35USC §119 to Japanese Patent Application No. 2008-064625, filed on Mar. 13, 2008, the entire content of which is incorporated by reference herein. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the teachings. The scope of the invention is defined with reference to the following claims.