Abstract:
A system for driving a plurality of lamps is disclosed to balance currents flowing through the lamps by means of impedance matching. According to the multi-lamp driving system of the present invention, a balancing controller is employed to match the impedance of each lamp path so as to regulate currents flowing through the lamps to be substantially the same.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a lamp driving system. More particularly, the present invention relates to a multi-lamp driving system in the application of the backlight module of a liquid crystal display. 
     2. Description of the Related Art 
     A discharge lamp used to backlight an LCD panel such as a cold cathode fluorescent lamp (CCFL) has terminal voltage characteristics that vary depending upon the immediate history and the frequency of a stimulus (AC signal) applied to the lamp. Until the CCFL is struck or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage, e.g., the terminal voltage must be equal to or greater than 1500 Volts. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ the value of the strike voltage over a relatively wide range of input currents. For example, the run voltage could be 500 Volts over a range of 500 microAmps to 6 milliAmps for a CCFL that has a strike voltage of 1,500 Volt. Usually, the CCFL is driven by AC signals having frequencies that range from 30 KiloHertz to 100 KiloHertz. 
     The discharge lamp exhibits a negative impedance characteristic that the equivalent impedance is decreased upon an increase of input power. Therefore, a circuit for providing the lamp with power, such as an inverter, should be configured with a controllable alternating current power supply and a feedback loop for monitoring the current flowing through the lamp to ensure stable operation and make load regulation as well. 
     Referring to FIG. 1, a conventional lamp driving system is schematically depicted. The system of FIG. 1 has only one feedback loop used for controlling the total current flowing through a lamp or lamps, but not used for controlling or balancing those currents flowing througheach lamp. If the current through one lamp is significantly larger than others, the lamp will be shortened in lifetime and the LCD panel will be degraded in brightness uniformity. 
     Referring to FIG. 2, another conventional lamp driving system is schematically depicted. However, the system of FIG. 2 is configured with two sets of control circuits resulting in an increase of cost and space. 
     Referring to FIG. 3, further another conventional driving system is schematically depicted. However, the system of FIG. 3 has two transformers also resulting in an increase of cost and space. Moreover, the transformers are configured with secondary coils connected in parallel to be adverse to high-voltage processing. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a lamp driving system for controlling the balance of currents at load end, which can be extensively applied to a system with single feedback loop and multiple loads. 
     It is another object of the present invention to provide a lamp driving system with cost efficiency, compact space and simplified manufacturing. 
     It is further another object of the present invention to provide a lamp driving system for controlling the balance of currents precisely. 
     To achieve aforementioned objects, the present invention provides a multi-lamp driving system comprising: an inverter for generating an AC power, a lamp set having a first lamp and a second lamp, and a balancing controller coupled with the inverter and the lamp set for balancing currents flowing through the first lamp and the second lamp. The balancing controller comprises: a first load coupled with the first lamp and the inverter, a second load coupled with the second lamp and the inverter, and a third load coupled with the first load and the second load, wherein the impedance ratio of the third load to the first load is negative. 
     Moreover, the present invention provides a multi-lamp driving system comprising: an inverter for generating an AC power, a lamp set having a plurality of lamps, and a balancing controller coupled with the lamp set and the inverter for balancing currents flowing through the plurality of lamps. The balancing controller comprises: a plurality of loads, each of which is coupled with one of the plurality of lamps and the inverter; and a load choke coupled with the plurality of loads to balance currents flowing through the plurality of lamps. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The following detailed description, given by way of examples and not intended to limit the invention to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which: 
     FIG. 1 schematically depicts a conventional lamp driving system; 
     FIG. 2 schematically depicts another conventional lamp driving system; 
     FIG. 3 schematically depicts further another conventional lamp driving system; 
     FIG. 4 schematically depicts the first preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIG. 5 schematically depicts the balancing controller of FIG. 4; 
     FIG. 6 schematically depicts the second preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIG. 7 schematically depicts the third preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIG. 8 schematically depicts the fourth preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIG. 9 schematically depicts the fifth preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIGS. 10A to  10 C schematically depict various circuit configurations of the balancing controller according to the present invention; 
     FIG. 11 schematically depicts the sixth preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIG. 12 schematically depicts the seventh preferred embodiment of a lamp driving system in accordance with the present invention; 
     FIGS. 13A to  13 B schematically depict two embodiments with their balancing controllers provided at the high-voltage end and low-voltage end respectively; and 
     FIG. 14 schematically depicts the eighth preferred embodiment of a lamp driving system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 4, the first preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. In FIG. 4, the lamp driving system comprises an inverter  70 , a lamp set  80  and a balancing controller  50 . The inverter  70  is provided with a power driving device  10 , a transformer T 1  and a PWM controller  30 . The lamp set  80  is constituted by lamps Lp 1  and Lp 2 , and the balancing controller  50  is constituted by loads Zb, Zc and Zd. 
     The power driving device  10  is employed to convert a DC power Vin into an AC power which is stepped up by the transformer T 1  and then provided for the lamp set  80 . The PWM controller  30  controls the power driving device in response to a feedback signal generated from the lamp set  80 . According to the first preferred embodiment of the present invention, the balancing controller  50  is used to regulate currents flowing through the lamps Lp 1  and Lp 2  to be substantially the same. The loads Za, Zb, Zc and Zd can be the combinations of resistors, capacitors, inductors, transistors or integrated circuits. The operation of the balancing controller  50  is based upon the impedance regulation of the loads Zb, Zc and Zd for the purpose of balancing load currents. The impedance regulation can be made in a linear or digital manner. 
     FIG. 5 is utilized to explain the operations of the balancing controller  50  of FIG.  4 . 
     Assume that Za=0, Zb=Zc, Lp 1 =Z 1  and Lp 2 =Z 2 , V 12 =I 1 Z 1 −I 2 Z 2 , and 
     
       
           Iz =(1 /Zd )( I   1   Z   1 − I   2   Z   2 )  (1) 
       
     
     
       
           V   0 = I   1 ( Z   1 + Zc )+ IzZc   (2) 
       
     
     
       
           V   0 = I   2 ( Z   2 + Zc )− IzZc   (3) 
       
     
     From equations (2) and (3), 
     
       
           I   1 ( Z   1 + Zc )+ IzZc=I   2 ( Z   2 + Zc )− IzZc   
       
     
     
       
           I   1 ( Z   1 + Zc )+2 IzZc=I   2 ( Z   2 + Zc ) 
       
     
     
       
           I   1 ( Z   1 + Zc )+(2 Zc/Zd ) ( I   1   Z   1 − I   2   Z   2 )= I   2 ( Z   2 + Zc ) 
       
     
     
       
           I   1 ( Z   1 + Zc +2 Z   1   Zc/Zd )= I   2 ( Z   2 + Zc +2 Z   2   Zc/Zd ) 
       
     
     
       
         If ( Z   1 + Zc +2 Z   1   Zc/Zd )=( Z   2 + Zc +2 Z   2   Zc/Zd ),  I   1 = I   2   
       
     
     
       
         (2 Zc/Zd )( Z   1 − Z   2 )= Z   2 − Z   1   
       
     
      2 Zc/Zd =(−1) 
     Thus, I 1 =I 2  can conform to the requirement of current balancing if Zc/Zd is chosen to be(−½). 
     If capacitance C and inductance L are utilized to the loads Zc and Zd respectively, at an operating frequency ω, 
     
       
           Zc/Zd =(1 /jωC )/( jωL )=−1/(ω 2   LC )=−{fraction ( 1 / 2 )} 
       
     
     
       
         1 /LC=ω   2 /2 
       
     
     In other words, the purpose of current balancing can be attained if 1/LC=ω 2 /2. 
     According to the present invention, current balancing can be achieved by means of impedance matching. In addition, the balancing controller  50  of FIG. 4 can be replaced by the the combination of capacitors and an inductor as shown in FIG.  10 A. Moreover, the balancing controller  50  can be the combination of capacitors, an inductor and a resistor as shown in FIG.  10 B. Furthermore, other example of the balancing controller  50  can be the combinations of inductors and a capacitor as shown in FIG.  10 C. Accordingly, those currents flowing through the lamps Lp 1  and Lp 2  can be substantially the same when the ratio Zc/Zd is (−½). 
     Moreover, as long as the equivalent impedance ratio Zc/Zd is properly designed to be a negative ratio, the current difference between the lamps can be effectively reduced. As an example, 
     
       
           I   1 ( Z   1 + Zc +2 Z   1   Zc/Zd )= I   2 ( Z   2 + Zc +2 Z   2   Zc/Zd ) 
       
     
     
       
           I   1 / I   2 =( Z   2 + Zc +2 Z   2   Zc/Zd )/( Z   1 + Zc +2 Z   1   Zc/Zd ) 
       
     
     Assume Z 1 =10, Z 2 =11, Zc=−10j and Zd=15j (Zc/Zd=−1/1.5), 
     
       
           I   1 / I   2 =(11−10 j +2*11*(−10 j )/15 j )/(10−10 j +2*10*(−10 j )/15 j )=(11−10 j −14.67)/(10−10 j −13.33)=(3.67+10 j )/(3.33+10 j )=(10.65∠θ1)/(10.54∠θ2) 
       
     
     Accordingly, the current error (I 1 -I 2 )/I 1  can be reduced by 1% (without taking phase into account). Thus, current difference between the lamps can be effectively reduced as long as the equivalent impedance ratio Zc/Zd is negative. 
     Referring to FIG. 6, the second preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. The circuit of FIG. 6 is similar to that of FIG. 5, but having difference in current feedback signals provided for the PWM controller  30 . In FIG. 5, the feedback signal provided for the PWM controller  30  is responsive to the current flowing through the lamp Lp 1 , but the feedback signal provided for the PWM controller  30  is responsive to the currents flowing through the lamps Lp 1  and Lp 2 . 
     Referring to FIG. 7, the third preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. In this embodiment, the balancing controller  50  is connected between the lamp set  80  and the PWM controller  30 . 
     Referring to FIG. 8, the fourth preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. In this case, a load choke  60  is employed to regulate load balance of whole circuitry. According to the fourth embodiment of the present invention, the balancing controller  50   a  comprises loads Zp 1 , Zp 2 , . . . , Zpm and the load choke  60 . The impedance relation of the load choke  60  and the loads Zp 1 , Zp 2 , . . . , Zpm can be found upon the formulae as mentioned above. 
     Referring to FIG. 9, the fifth preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. A load choke  60  of Figure functions as that of FIG.  6 . However, the fifth preferred embodiment could be applied to an inverter provided with multiple transformers. 
     Referring to FIG. 11, the sixth preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. The circuit of FIG. 11 is similar to that of FIG. 4 except that the balancing controller  50  in FIG. 4 is replaced by the balancing controller  50 ′ in FIG. 11 by performing a Δ-Y transform. Specifically, the Δ-circuit consisting of loads Zb, Zc and Zd in the balancing controller  50  of FIG. 4 is transformed to a Y-circuit consisting of loads Zb, Zc and Zd in the balancing controller  50 ′ of FIG. 11 by applying a Δ-Y transform. The Load Ze is seriesly connected to the load Za while the loads Zf and Zg are seriesly connected to the lamps Lp 1  and Lp 2  respectively. 
     Recalling that, as was mentioned earlier, it is assumed Za=Zc, Zc/Zd and Zb/Zd are both negative, and preferably Zc/Zd=Zb/Zd=(−½) in the embodiment of FIG. 4, the values of loads Ze, Zf and Zg in the equivalent circuit can therefore be derived from the following equations: 
     
       
           Ze=ZbZc /( Zb+Zc+Zd )= Zc   2 /2 Zc+Zd   
       
     
     
       
           Zf=ZbZd /( Zb+Zc+Zd )= ZcZd /2 Zc+Zd   
       
     
     
       
           Zg=ZcZd /( Zb+Zc+Zd )= ZcZd /2 Zc+Zd   
       
     
     Accordingly, the load Zf is substantially the same as load Zg, and the impedance ratio of the load Ze to the load Zf is preferably equal to (−½). Thereby, the equivalent Y-type balancing controller  50 ′ exhibits the same operation characteristics as the Δ-type balancing controller  50 . Accordingly to the sixth preferred embodiment of the invention, the balancing controller  50 ′ preferably comprises the combination of capacitors and an inductor as shown in FIG.  13 A. 
     Similarly, the balancing controller  50  of the second preferred embodiment in FIG. 6 may also be replaced by the balancing controller  50 ′ in FIG. 7, the description of which is omitted for simplification. 
     Referring to FIG. 12, the seventh preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. In the lamp driving system of the sixth preferred embodiment in FIG. 11, the balancing controller  50 ′ is provided at the high-voltage end of the lamp set. However, according to the seventh preferred embodiment of the invention, the balancing controller  50 ′ may also be provided at the low-voltage end of the lamp set to form the configuration as show in FIG. 12 with the loads Zf and Zg seriesly coupled to the low voltage ends of the lamps Lp 1  and Lp 2  respectively. The balancing controller  50 ′ of the seventh preferred embodiment preferably comprises the combination of capacitors and an inductor as shown in FIG.  13 B. 
     Referring to FIG. 12, the eighth preferred embodiment of a lamp driving system in accordance with the present invention is schematically depicted. The balancing controller  50 ″ of the eighth preferred embodiment comprises loadx Ze, Zp 1 , Zp 2 , . . . , Zpn, wherein impedances of the loads Zp 1 , Zp 2 , . . . , Zpn are substantially the same and the impedance ratio of the load Ze to the load Zp 1  is negative, for balancing the current passing through each of the lamps Lp 1 , Lp 2 , . . . , Lpn. For example, the load Ze is a capacitor and the loads Zp 1 , Zp 2 , . . . , Zpn are inductors. Although the loads Zp 1 , Zp 2 , . . . , Zpn shown are coupled to the high-voltage end of the lamps Lp 1 , Lp 2 , . . . , Lpn; however, similar to the seventh preferred embodiment of FIG. 12, the loads may also be coupled to the low-voltage end of the lamp set in an application having more lamps. 
     While the invention has been described with reference to various illustrative embodiments, the description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those person skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as may fall within the scope of the invention defined by the following claims and their equivalents.