# Patent

## Title
Circuit structure for driving a plurality of cold cathode fluorescent lamps

## Abstract

A DC/AC converter circuit structure for driving a plurality of cold cathode fluorescent lamps is described. A common-mode choke is used between the cold cathode fluorescent lamps. The common-mode choke balances the currents respectively flowing through the cold cathode fluorescent lamps.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. Nonprovisional application Ser. No. 11/685,607 filed Mar. 13, 2007, now U.S. Pat. No. 7,345,431 which itself is a continuation of U.S. Nonprovisional application Ser. No. 10/924,585 filed Aug. 24, 2004, now U.S. Pat. No. 7,190,123, which itself is a continuation of U.S. Nonprovisional application Ser. No. 10/383,277 filed Mar. 7, 2003, now U.S. Pat. No. 6,781,325, the teachings all of which are herein incorporated by reference.


FIELD OF THE INVENTION
The present invention relates to a driver circuit, and more specifically, to a circuit for driving cold cathode fluorescent lamps.
BACKGROUND OF THE INVENTION
Both the notebook computers and the portable electronic apparatus use the cold cathode fluorescent lamp as a backlight because this lamp has the best illumination efficiency. Therefore, the cold cathode fluorescent lamp has quickly been adopted for use as the backlight in PDAs, notebook computers and portable electronic apparatus. The quality requirement of the converter for the cold cathode fluorescent lamp is also increased.
A high voltage DC/AC converter is required to drive the cold cathode fluorescent lamp because this lamp uses a high AC operation voltage. However, with the increasing size of the LCD panel, the panel requires multiple lamps to provide the necessary illumination. Therefore, an effective converter is required to drive multiple cold cathode fluorescent lamps. The driving technique requires careful treatment.
FIG. 1 shows a schematic drawing of a circuit structure for an DC/AC converter used to drive two cold cathode fluorescent lamps in accordance with the prior art. DC power 100 provides DC power to the full bridge circuit 102. DC power 100 is connected to a primary winding 104 of a transformer through the full bridge circuit 102. The secondary winding 106 of a transformer is coupled to two cold cathode fluorescent lamps 112 and 114 through two high voltage capacitors 108 and 110, respectively. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 102. However, this circuit structure does not ensure that each cold cathode fluorescent lamp connected with the circuit structure is ignited successfully. The characteristics of the cold cathode fluorescent lamp is negative resistance and the voltage needed to ignite the lamp is different under various conditions such as aging of the lamp, temperature of the lamp and parasitic coupling between and lamp and the metal chassis. For example, one of the two cold cathode fluorescent lamps connected in this circuit structure is severely aged, the circuit cannot ignite the lamp due to the voltage at the transformer decreases once the other lamp has ignited. This, in turn, decreases the life-span of the cold cathode fluorescent lamps.
FIG. 2 shows a schematic drawing of another circuit structure schematic drawing for a DC/AC converter that used to drive two cold cathode fluorescent lamps in accordance with the prior art. DC power 100 provides DC power to the full bridge circuit 102. DC power 100 is connected to a primary winding 104 of a transformer through the full bridge circuit 102. The secondary winding 106 of a transformer is coupled to two cold cathode fluorescent lamps 112 and 114 through an inductor 116 and two high voltage capacitors 108 and 110, respectively. A half-bridge, a push-pull or a Royer circuit can be used to replace the full bridge circuit 102. However, this circuit structure uses an inductor 116 between the secondary winding 106 and two high voltage capacitors 108 and 110, which may cause this circuit structure to be affected easily by an operation frequency associated with a DC/AC power converter. The variation of operating frequency may cause different AC currents to flow through the two cold cathode fluorescent lamps 112 and 114, respectively. In addition, this circuit structure is also sensitive to load variations. Therefore, if this circuit structure is used to drive multiple cold cathode fluorescent lamps, it is difficult to balance the current flowing through each lamp. Moreover, circuit design is difficult and complicated.
FIG. 3 shows a schematic drawing of a circuit structure of a plurality of transformers that are used to drive a plurality of cold cathode fluorescent lamps in accordance with the prior art. It is used to solve the problems described in the two circuit structures shown in FIG. 1 and FIG. 2. DC power 100 provides DC power to the full bridge circuit 102. DC power 100 is connected to two primary windings 104a and 104b through the full bridge 102. The secondary windings 106a and 106b are coupled to two cold cathode fluorescent lamps 112 and 114 through two high voltage capacitors 122 and 124, respectively. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 102. Although this circuit structure increases the reliability and stability, structural formation of this kind of DC/AC converter for driving a cold cathode fluorescent lamp is expensive. Furthermore, a DC/AC converter with this circuit structure is bulky.
SUMMARY OF THE INVENTION
In accordance with the foregoing description, there are many drawbacks in the conventional DC/AC converters when driving a plurality of cold cathode fluorescent lamps. For example, the first circuit structure depicted in the FIG. 1 cannot ensure that each lamp is ignited. The second circuit structure depicted in the FIG. 2 is easily affected by the operating frequency. Moreover, it is difficult to balance the current flowing through each lamp. Further, the technique of using a plurality of DC/AC converters to drive a plurality of cold cathode fluorescent lamps as depicted in the FIG. 3 is expensive and large in size.
Therefore, the main purpose of the present invention is to provide a circuit structure for driving a plurality of cold cathode fluorescent lamps to solve the problems existing in the prior arts.
Another purpose of the present invention is to provide an DC/AC converter for driving a plurality of cold cathode fluorescent lamps that is not affected by the variation of the back-light module including the chassis and the cold cathode fluorescent lamps.
Another purpose of the present invention is to provide a DC/AC converter structure for driving a plurality of cold cathode fluorescent lamps that is not affected by operating frequency of a DC/AC power converter. Therefore, the circuit structure may balance the current flowing through each lamp.
The present invention provides a DC/AC converter structure for driving a plurality of cold cathode fluorescent lamps. This structure utilizes a common-mode choke between the load that is connected to the secondary winding of a transformer in the DC/AC converter. This common-mode choke balances the current flowing through each lamp so that each lamp provides same amount of luminance. Moreover, this circuit structure is not affected by the operating frequency of the DC/AC power converter.
In accordance with the circuit structure, one exemplary circuit is to drive three or more loads. The circuit adds an additional common-mode choke between the third load and the first load. The current flowing through these loads are balanced via the characteristics of the common-mode choke. Such a circuit structure realizes an DC/AC converter that drives a plurality of loads and the current flowing through these loads are equal. Moreover, the balance of the current among the loads is not affected by the number of the loads.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated and better understood by referencing the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a schematic drawing of a circuit structure for an DC/AC converter used to drive two cold cathode fluorescent lamps in accordance with the prior art;
FIG. 2 is a schematic drawing of another circuit structure for an DC/AC converter that is used to drive two cold cathode fluorescent lamps in accordance with the prior art, wherein an inductor is used to connect the load;
FIG. 3 is a schematic drawing of another circuit structure for a plurality of transformers used to drive a plurality of cold cathode fluorescent lamps in accordance with the prior art;
FIG. 4 is a schematic drawing of a common-mode choke in accordance with the present invention;
FIG. 5A is a schematic drawing where the common-mode choke is applied in an DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the first embodiment of the present invention;
FIG. 5B is a schematic drawing of the common-mode choke applied in an DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the second embodiment of the present invention;
FIG. 6 is a schematic drawing comparing the current flowing through two cold cathode fluorescent lamps when applying the DC/AC converter to the two cold cathode fluorescent lamps in accordance with the first embodiment of the present invention;
FIG. 7A is a schematic drawing of the DC/AC converter circuit structure of the first embodiment used to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention;
FIG. 7B is a schematic drawing of the DC/AC converter circuit structure of the second embodiment applied to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention;
FIG. 8A is a schematic drawing of the common-mode choke applied in an DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the third embodiment of the present invention;
FIG. 8B is a schematic drawing of the common-mode choke applied in an AC/DC converter to drive two cold cathode fluorescent lamps in accordance with the fourth embodiment of the present invention;
FIG. 8C is a schematic drawing of the circuit structure of the third embodiment used to calculate the inductance of the common-mode choke in accordance with the present invention;
FIG. 9 is a schematic drawing comparing the current flowing through the two cold cathode fluorescent lamps when applying the DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the third embodiment of the present invention;
FIG. 10A is a schematic drawing of the DC/AC converter circuit structure of the third embodiment to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention;
FIG. 10B is a schematic drawing of the DC/AC converter circuit structure of the fourth embodiment used to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention;
FIG. 11A to FIG. 11D respectively are schematic drawings of measurements of the current at the output of the common-mode choke in the FIG. 5B in accordance with the present invention;
FIG. 11E to FIG. 11H are schematic drawings for comparing the frequency and the current at the output of the common-mode choke in the FIG. 5B in accordance with the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Without limiting the spirit and scope of the present invention, the circuit structure proposed in the present invention is illustrated with four preferred embodiments. One with ordinary skill in the art, upon acknowledging the embodiments, can apply the circuit structure of the present invention to various converter topologies. The circuit structure of the present invention allows uniform and simultaneous illumination of all lamps. The present invention also balances current among all lamps by using of common-mode chokes in the circuit structure. Additionally, the present invention only requires a secondary winding of a transformer to drive a plurality of cold cathode fluorescent lamps. Therefore, the size of the transformer is reduced. The application of the present invention is not limited by the preferred embodiments described in the following.
The present invention provides a DC/AC converter circuit structure for driving a plurality of cold cathode fluorescent lamps. This circuit structure uses a common-mode choke between the loads that is connected to the secondary winding of a transformer in the DC/AC converter structure. This common-mode choke balances the current flowing through the loads. FIG. 4 is a common-mode choke schematic drawing in accordance with the present invention. The current flowing through the primary winding N1 in the common-mode choke is I1. The current flowing through the secondary winding N2 in the common-mode choke is I2. The following is a basic formula in accordance with the principle of the transformer.

N1×I1−N2×I2=0

I1/I2=N2/N1 

Current I1 and current I2 are equal when the primary winding N1 and the secondary winding N2 are designed to have the same number of turns and reversed polarity. Therefore, the common-mode choke ensures that the currents flowing through the cold cathode fluorescent lamps are equal by designing the common-mode choke having the same number of turns N1 and N2 where the primary winding N1 and the secondary winding N2 in the common-mode choke are connected to cold cathode fluorescent lamps respectively.
FIG. 5A is a schematic drawing of the common-mode choke 300 applied in a DC/AC converter to drive cold cathode fluorescent lamps in accordance with the first embodiment of the present invention. A DC power 200 provides a DC power to the full bridge circuit 202. This DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to two cold cathode fluorescent lamps 212 and 214 through two high voltage capacitors 208 and 210, respectively. The two cold cathode fluorescent lamps 212 and 214 are connected to the first winding N1 and the second winding N2 of the common-mode choke 300 of the present invention respectively. The cold cathode fluorescent lamp 214 is connected to the first winding N1 and the cold cathode fluorescent lamp 212 is connected to the second winding N2. The output of the common-mode choke 300 is connected to a dual diode 220 to feed back the current on the output of the full bridge circuit 202. This feedback signal is received and the controller in the full bridge circuit 202 regulates the power to the output. A half-bridge circuit, a push-pull circuit or a Royer circuit can replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. The material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core.
FIG. 5B is a schematic drawing of the common-mode choke 300 applied to a DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the second embodiment of the present invention. A DC power 200 provides DC power to the full bridge circuit 202. DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to the two input ends of the common-mode choke 300 of the present invention through two high voltage capacitors 208 and 210, respectively. The two output ends of the common-mode choke 300 are respectively connected to the two cold cathode fluorescent lamps 212 and 214. The cold cathode fluorescent lamp 214 is connected to the first winding N1 and the cold cathode fluorescent lamp 212 is connected to the second winding N2. The other end of the cold cathode fluorescent lamp 214 is connected to a dual diode 220 to feed back the current on the output end of the cold cathode fluorescent lamp 214 to the full bridge circuit 202. A half-bridge circuit, a push-pull circuitor a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. The material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core.
In other words, the common-mode choke 300 of the present invention can be located on the high voltage side or the low voltage side of the cold cathode fluorescent lamp. The common-mode choke 300 balances the current flowing through the first winding N1 and the current flowing through the second winding N2 by the design of the common-mode choke 300.
The inductor value in the common-mode choke 300 used in the FIG. 5A can be solved by the method described in the following. In calculations, two loads R1 and R2 are used to replace the two cold cathode fluorescent lamps 212 and 214 because the cold cathode fluorescent lamp possesses a negative resistance characteristics. Therefore, the voltage difference between the input end and the output end of the cold cathode fluorescent lamp 212 is VR1. The voltage difference between the input end and the output end of the cold cathode fluorescent lamp 214 is VR2. The following formulas are obtained in accordance with Kirchhoff's Law:

VO=V208+VR1+VL1  (1)

VO=V210+VR2−VL2  (2)

VO is the output voltage of the secondary winding 206 of the transformer. V208 is the voltage value between the two ends of the high voltage capacitor 208. VL1 is the voltage value of the first winding N1 of the common-mode choke 300. VL2 is the voltage value of the second winding N2 of the common-mode choke 300.
Next, a complex number is used to replace the inductor and capacitor value. The capacitance of both the high voltage capacitor 208 and 210 is C. The inductance of both the first winding N1 and the second winding N2 of the common-mode choke 300 is L. The coupling coefficient of the common-mode choke 300 is K. The following formula is obtained by calculating equations (1) and (2).




  
    
      
        
          (
          
            
              R
              1
              2
            
            -
            
              R
              2
              2
            
          
          )
        
        =
        
          
            
              4
              ⁢
              L
            
            C
          
          ⁢
          
            (
            
              1
              -
              K
            
            )
          
        
      
    
    
      
        (
        3
        )
      
    
  




Therefore, the inductance of the common-mode choke can be obtained from equation (3). For example, the inductance of both the first winding N1 and the second winding N2 of the common-mode choke are 409 mH when resistor R1 has a resistance of 120 K ohm, resistor R2 has a resistance of 90 K ohm, the coupling coefficient of the common-mode choke is 0.85 and the capacitance values of both the high voltage capacitors are 39 Pf.
FIG. 6 is a drawing comparing the current flowing through the two cold cathode fluorescent lamps when the DC/AC converter is used to drive two cold cathode fluorescent lamps in accordance with the first embodiment of the present invention. In accordance with the comparison drawing, the current flowing through the two cold cathode fluorescent lamps are almost equal. Obviously, the circuit structure of the present invention balances the currents respectively flowing through the two cold cathode fluorescent lamps.
FIG. 7A is a schematic drawing of the DC/AC converter circuit structure of the first embodiment used to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention. DC power 200 provides DC power to the full bridge circuit 202. DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to a plurality of high voltage capacitors C1 to Cn. Each high voltage capacitor is connected to a corresponding cold cathode fluorescent lamp CCFL1 to CCFLn. Any adjacent two cold cathode fluorescent lamps are connected to a common-mode choke. In other words, when applying the DC/AC converter circuit structure of the present invention to drive a plurality of cold cathode fluorescent lamps, the number of common-mode chokes used is less than the number of cold cathode fluorescent lamps driven by one. Therefore, the number of the used common-mode choke is (N−1) if the number of the driven cold cathode fluorescent lamps is N.
On the other hand, the common-mode choke CC1 balances the current flowing through the cold cathode fluorescent lamp CCFL1 and the current flowing through the cold cathode fluorescent lamp CCFL2. The common-mode choke CC2 balances the current flowing through the cold cathode fluorescent lamp CCFL2 and the current flowing through the cold cathode fluorescent lamp CCFL3 Similarly, the common-mode choke CCn-1 balances the current flowing through the cold cathode fluorescent lamp CCFLn-1 and the current flowing through the cold cathode fluorescent lamp CCFLn. Therefore, the current flowing through the cold cathode fluorescent lamp CCFL1 to CCFLn will be balanced by adding these common-mode chokes disclosed by the present invention to the DC/AC converter structure.
The output end of the common-mode choke CCn-1 is connected to a dual diode 220 to feed back the current at the output end to the full bridge circuit 202. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. The material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core.
Moreover, as shown in FIG. 7A, one of the two output ends of the any common-mode choke is grounded and the other output end is connected to one of the two output ends of the adjacent common-mode choke. For example, one of the two output ends of the common-mode choke CCM is grounded and the other output end of the common-mode choke CCM is connected to one of the two output ends of the adjacent common-mode choke CCM-1, and M=2, 3, . . . N−1. It is noted that the grounded output ends of these common-mode chokes can also be connected together to connect to the dual diode 220 to feed back the current at the output ends to the full bridge circuit 202.
FIG. 7B is a schematic drawing of the DC/AC converter circuit structure of the second embodiment used to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention. DC power 200 provides DC power to the full bridge circuit 202. DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to a plurality of high voltage capacitors C1 to Cn. Any adjacent two high voltage capacitors are respectively connected to the two input ends of a corresponding common-mode choke. The two output ends of each common-mode choke are respectively connected to the corresponding cold cathode fluorescent lamp CCFL1 to CCFLn. In other words, when using the DC/AC converter circuit structure of the present invention to drive a plurality of cold cathode fluorescent lamps, the number of common-mode chokes used is less than the number of the driven cold cathode fluorescent lamps by one. Therefore, the number of the used common-mode choke is (N−1) if the number of the driven cold cathode fluorescent lamps is N.
On the other hand, the common-mode choke CC1 balances the current flowing through the cold cathode fluorescent lamp CCFL1 and the current flowing through the cold cathode fluorescent lamp CCFL2. The common-mode choke CC2 balances the current flowing through the cold cathode fluorescent lamp CCFL2 and the current flowing through the cold cathode fluorescent lamp CCFL3. Similarly, the common-mode choke CCn-1 balances the current flowing through the cold cathode fluorescent lamp CCFLn-1 and the current flowing through the cold cathode fluorescent lamp CCFLn. Therefore, the current flowing through the cold cathode fluorescent lamp CCFL1 to CCFLn will be balanced by adding these common-mode chokes disclosed by the present invention to the DC/AC converter structure.
The output end of the cold cathode fluorescent lamp CCFLn is connected to a dual diode 220 to feed back the current on the output end of the lamp CCFLn to the full bridge circuit 202. This feedback signal modifies the full bridge circuit 202 to output the required energy. A half-bridge circuit, a push-pull circuitor a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. The material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core.
Moreover, as shown in FIG. 7B, the output end of cold cathode fluorescent lamps CCFL1 to CCFLn are connected together to connect to the dual diode 220 to feed back the current on the output ends of these lamps to the full bridge circuit 202. It is noted in the structure where the cold cathode fluorescent lamp CCFLn is the only lamp connected to the dual diode 220 to feed back the current on the output end of the lamp CCFLn to the full bridge circuit 202. Simple structure as it is, it achieves the goal of the present invention. On the other hand, the output ends of the rest of cold cathode fluorescent lamps CCFL1 to CCFLn-1 are grounded.
FIG. 8A is a schematic drawing of the common-mode choke 300 applied in a DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the third embodiment of the present invention. DC power 200 provides DC power to the full bridge circuit 202. DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to the two high voltage capacitors 208 and 210, in which the high voltage capacitor 210 is coupled with the common-mode choke 300 of the present invention. The two output ends of the common-mode choke 300 are connected to the two cold cathode fluorescent lamps 212 and 214 respectively. The cold cathode fluorescent lamp 214 is connected to the first winding and the cold cathode fluorescent lamp 212 is connected to the second winding. The output ends of the two cold cathode fluorescent lamps 212 and 214 are connected together and connected to a dual diode 220 to feed back the currents on the output end of the cold cathode fluorescent lamp 212 and 214 to the full bridge circuit 202. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. On the other hand, the material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core. The main difference between the third embodiment and the second embodiment is that only common-mode choke 300 is coupled with one high voltage capacitor 210.
FIG. 8B is a schematic drawing of the common-mode choke 300 applied in an DC/AC converter to drive two cold cathode fluorescent lamps in accordance with the fourth embodiment of the present invention. A DC power 200 provides a DC power to the full bridge circuit 202. This DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to two high voltage capacitors 208 and 210, wherein the high voltage capacitor 210 is connected to the input ends of the two cold cathode fluorescent lamps 212 and 214. The output ends of the two cold cathode fluorescent lamps 212 and 214 are respectively connected to the first winding and the second winding of the common-mode choke 300 of the present invention. The cold cathode fluorescent lamp 214 is connected to the first winding and the cold cathode fluorescent lamp 212 is connected to the second winding. One of the output ends of the common-mode choke 300 is connected to a dual diode 220 to feed back the current on the output end to the full bridge circuit 202. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. On the other hand, the material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core. The main difference between the first embodiment and the fourth embodiment is that only the common-mode choke 300 is coupled with one high voltage capacitor 210.
Similarly to the first and second embodiments, the common-mode choke 300 of the third and fourth embodiments of the present invention can be located on the high voltage side or the low voltage side of the cold cathode fluorescent lamp. The common-mode choke 300 balances the current flowing through the first winding N1 and the current flowing through the second winding N2 by the design of the common-mode choke 300.
The inductance in the common-mode choke 300 used in the FIG. 8A can be calculated by the method described in the following. When calculating, one resistor and one capacitor in parallel are first used to replace the cold cathode fluorescent lamp because the cold cathode fluorescent lamp possesses the negative resistance characteristics and the parasitic capacitance of the cold cathode fluorescent are included. Next, the one resistor and one capacitor are changed from in parallel to in series, as shown in the FIG. 8C. The two groups (R1, C1) and (R2, C2), each group composed of one resistor and one capacitor in series, are respectively used to replace the two cold cathode fluorescent lamps 212 and 214 the FIG. 8C. Therefore, in accordance with FIG. 8C, the voltage difference between the input end and the output end of the cold cathode fluorescent lamp 214 is (VR1+VC1). The voltage difference between the input end and the output end of the cold cathode fluorescent lamp 212 is (VR2+VC2). The end voltage of the first winding 300a of the common-mode choke 300 is VO1. The end voltage of the second winding 300b of the common-mode choke 300 is VO2. The following equations are obtained in accordance with Kirchhoff's Voltage Law:

VT=VO1+VR1+VC1  (4)

VT=−VO2+VR2+VC2  (5)

VT is the voltage between the capacitor 210 and the common-mode choke 300.
Next, the impedance of the capacitor will be expressed in the complex domain for calculations. The current flowing through the first winding 300a of the common-mode choke 300 is I1. The current flowing through the second winding 300b of the common-mode choke 300 is I2. Then, equations (4) and (5) yield in:

VT=VO1+I1×R1+I1×(1/jωC1)  (6)

VT=−VO2+I2×R2+I2×(1/jωC2)  (7)

The current I1 flowing through the first winding 300a and the current I2 flowing through the second winding 300b are equal. The inductance of both the first winding 300a and the second winding 300b of the common-mode choke 300 is L. The coupling coefficient of the common-mode choke 300 is K. Then, the following equation is obtained from equations (6) and (7)




  
    
      
        L
        =
        
          
            1
            
              2
              ⁢
              
                (
                
                  1
                  -
                  K
                
                )
              
            
          
          [
          
            
              
                (
                
                  
                    R
                    1
                    2
                  
                  -
                  
                    R
                    2
                    2
                  
                
                )
              
              
                
                  1
                  /
                  
                    C
                    1
                  
                
                +
                
                  1
                  /
                  
                    C
                    2
                  
                
              
            
            +
            
              
                1
                
                  ω
                  2
                
              
              ⁢
              
                (
                
                  
                    1
                    
                      C
                      1
                    
                  
                  -
                  
                    1
                    
                      C
                      2
                    
                  
                
                )
              
            
          
          ]
        
      
    
    
      
        (
        8
        )
      
    
  




Therefore, the inductance of the common-mode choke can be obtained from equation (8). For example, the inductance of both the first winding 300a and the second winding 300b of the common-mode choke 300 are 650 mH when resistor R1 has a resistance of 120 K ohm, resistor R2 has a resistance of 90 K ohm, the coupling coefficient of the common-mode choke is 0.85 and the frequency is selected 50 KHz.
FIG. 9 is a drawing comparing the current flowing through the two cold cathode fluorescent lamps when the DC/AC converter is used to drive two cold cathode fluorescent lamps in accordance with the third embodiment of the present invention. In accordance with the comparison drawing, the current flowing through the two cold cathode fluorescent lamps are almost equal. Obviously, the circuit structure of the present invention balances the current flowing through the two cold cathode fluorescent lamps respectively.
FIG. 10A is a schematic drawing of the DC/AC converter circuit structure of the third embodiment used to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention. A DC power 200 provides a DC power to the full bridge circuit 202. This DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to two high voltage capacitors 208 and 210. The high voltage capacitor 210 is connected to a plurality of common-mode chokes CC1 to CCn. The output ends of each common-mode choke is coupled with the corresponding cold cathode fluorescent lamps CCFL1 to CCFLn. In other words, when the DC/AC converter circuit structure of the present invention is used to drive a plurality of cold cathode fluorescent lamps, the number of the common-mode chokes used is less than the number of the driven cold cathode fluorescent lamps by one. Therefore, the number of common-mode chokes used is (N−1) if the number of the driven cold cathode fluorescent lamps is N.
On the other hand, the common-mode choke CC1 balances the current flowing through the cold cathode fluorescent lamp CCFL1 and the current flowing through the cold cathode fluorescent lamp CCFL2. The common-mode choke CC2 balances the current flowing through the cold cathode fluorescent lamp CCFL2 and the current flowing through the cold cathode fluorescent lamp CCFL3. The rest can be deduced by analogy. The common-mode choke CCn-1 balances the current flowing through the cold cathode fluorescent lamp CCFLn-1 and the current flowing through the cold cathode fluorescent lamp CCFLn. Therefore, these currents respectively flowing through the cold cathode fluorescent lamp CCFL1 to CCFLn are balanced by adding these common-mode chokes disclosed by the present invention to the DC/AC converter structure.
The output ends of the cold cathode fluorescent lamps CCFL1 to CCFLn are connected to a dual diode 220 to feed back the current on the output ends of the lamps to the full bridge circuit 202. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. On the other hand, the material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core.
Moreover, as shown in FIG. 10A, the output end of theses cold cathode fluorescent lamp CCFL1 to CCFLn are connected together to connect to the dual diode 220 to feed back the current on the output ends of these lamps to the full bridge circuit 202. In the structure, here the cold cathode fluorescent lamp CCFLn is the only lamp connected to the dual diode 220 to feed back the current at the output end of the lamp CCFLn of the full bridge circuit 202. It also satisfies the goals of the present invention. On the other hand, the output ends of the rest cold cathode fluorescent lamps CCFL1 to CCFLn-1 are grounded.
FIG. 10B is a schematic drawing of the DC/AC converter circuit structure of the fourth embodiment used to drive a plurality of cold cathode fluorescent lamps in accordance with the present invention. DC power 200 provides DC power to the full bridge circuit 202. DC power 200 is connected to a primary winding 204 of a transformer through the full bridge circuit 202. The secondary winding 206 of a transformer is coupled to two high voltage capacitors 208 and 210. The high voltage capacitor 210 is connected to a plurality of the cold cathode fluorescent lamp CCFL1 to CCFLn. Any adjacent two cold cathode fluorescent lamps are connected to a corresponding common-mode choke CC1 to CCn. In other words, when the DC/AC converter circuit structure of the present invention is used to drive a plurality of cold cathode fluorescent lamps, the number of used common-mode chokes used is less than the number of the driven cold cathode fluorescent lamps by one. Therefore, the number of common-mode chokes used is (N−1) if the number of the driven cold cathode fluorescent lamps is N.
On the other hand, the common-mode choke CC1 balances the current flowing through the cold cathode fluorescent lamp CCFL1 and the current flowing through the cold cathode fluorescent lamp CCFL2. The common-mode choke CC2 balances the current flowing through the cold cathode fluorescent lamp CCFL2 and the current flowing through the cold cathode fluorescent lamp CCFL3. Similarly, the common-mode choke CCn-1 balances the current flowing through the cold cathode fluorescent lamp CCFLn-1 and the current flowing through the cold cathode fluorescent lamp CCFLn. Therefore, the current flowing through the cold cathode fluorescent lamp CCFL1 to CCFLn are balanced by adding these common-mode chokes disclosed by the present invention to the DC/AC converter structure.
The output end of the common-mode choke CCn-1 is connected to a dual diode 220 to feed back the current at the output end to the full bridge circuit 202. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 202. The structure of the common-mode choke is similar to the structure of a transformer. On the other hand, the material of the common-mode choke 300 is MPP Powder Core, Micrometals Powdered Iron Core, Ferrite EE-core, Pot-Core or Toroid core.
Moreover, as shown in the FIG. 10B, one of the two output ends of the any common-mode choke is grounded and the other output end is connected to one of the two output ends of the adjacent common-mode choke. For example, one of the two output ends of the common-mode choke CCM is grounded and the other output end of the common-mode choke CCM is connected to one of the two output ends of the adjacent common-mode choke CCM-1, and M=2, 3, . . . N−1. It is noted that the grounded output ends of these common-mode chokes can also be connected together to connect to the dual diode 220 to feed back the current on the output ends to the full bridge circuit 202.
FIGS. 11A to 11D are measurement drawings of the currents at the output ends of the common-mode choke 300 in the FIG. 5B in accordance with the present invention. The current flowing through the first winding is IO1. The current flowing through the second winding is IO2. The test conditions and the test result are shown as follows.
Test Conditions:
Ambient temperature: 25° C.
Current probe: Tektronix P6022, S/N: 011-0161-00
Power supply: GW GPC-3030D
Multi-meter: HP 34401A
Test Result:













IO1
IO2
Diff. between IO1 and IO2







8.15 mA
8.11 mA
0.04 mA
FIG. 11A


6.80 mA
6.86 mA
0.06 mA
FIG. 11B


5.60 mA
5.53 mA
0.07 mA
FIG. 11C


3.91 mA
3.88 mA
0.03 mA
FIG. 11D









From the above table, the differential between the current IO1 flowing through the first winding and the current IO2 flowing through the second winding is very small.
FIGS. 11E to 11H are measurement drawings when comparing the frequency and the currents on the output ends of the common-mode choke 300 in FIG. 5B in accordance with the present invention. The current flowing through the first winding is IO1. The current flowing through the second winding is IO2. The test results are shown as follows.
Test Result:
















Frequency
IO1
IO2









60 Khz
8.13 mA
8.10 mA
FIG. 11E



55 Khz
8.14 mA
8.10 mA
FIG. 11F



50 Khz
8.12 mA
8.10 mA
FIG. 11G



47 Khz
8.14 mA
8.10 mA
FIG. 11H










From the above table, the frequency does not affect currents IO1 and IO2.
In accordance with the foregoing description and the test result, the circuit structure of the present invention provides the following advantages. First, this circuit structure balances the currents flowing through the multiple cold cathode fluorescent lamps when using a transformer to drive a plurality of cold cathode fluorescent lamps. On the other hand, the number and the structure of the cold cathode fluorescent lamps do not affect the balance of the current in accordance with the present invention. Second, this circuit structure does not require a plurality of transformers when driving a plurality of cold cathode fluorescent lamps. It reduces the number of components. Therefore, this circuit structure is smaller in size and lower in cost.
As is understood by a person skilled in the art, the foregoing descriptions of the preferred embodiment of the present invention are an illustration of the present invention rather than a limitation thereof. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. While a preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

## Claims

1. A DC/AC converter comprising:
power delivery circuitry configured to convert external DC power to AC power to be supplied to at least two loads; and
at least one common mode choke coupled to said at least two loads for balancing currents flowing through said at least two loads;
wherein said at least one common mode choke has a primary winding and a secondary winding, wherein said primary winding and said secondary winding of said at least one common mode choke are coupled to said at least two loads in series, respectively.

2. The DC/AC converter of claim 1, further comprising:
a feedback signal circuit for providing a feedback signal indicative of an electrical status of one of said at least two loads.

3. The DC/AC converter of claim 2, wherein said feedback signal circuit comprises dual diode circuitry for providing said feedback signal.

4. The DC/AC converter of claim 1, wherein said power delivery circuitry further comprises:
a switch network coupled to said external DC power; and
a transformer coupled to said switch network for supplying said AC power to said at least two loads.

5. The DC/AC converter of claim 4, wherein said switch network comprises a full bridge circuit.

6. The DC/AC converter of claim 4, wherein said switch network comprises a half-bridge circuit.

7. The DC/AC converter of claim 4, wherein said switch network comprises a push-pull circuit.

8. The DC/AC converter of claim 4, wherein said switch network comprises a Royer circuit.

9. The DC/AC converter of claim 4, wherein said transformer of said power delivery circuitry further comprises:
a primary winding coupled to said switch network; and
a secondary winding coupled to said at least two loads for supplying said AC power to said at least two loads at a predetermined voltage.

10. The DC/AC converter of claim 9, further comprising:
a capacitor coupled to said secondary winding in parallel.

11. The DC/AC converter of claim 9, further comprising:
a capacitor coupled to said secondary winding in series.