Patent Publication Number: US-7719211-B2

Title: Lamp current balancing topologies

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 11/253,918 filed on Oct. 19, 2005, now U.S. Pat. No. 7,372,213, the teachings of which are herein incorporated by reference. 

   FIELD 
   The present disclosure relates to lamp current balancing topologies. 
   BACKGROUND 
   In conventional circuits driving multiple lamps, there are two primary configurations. Both configurations may include an inverter controller capable of receiving feedback and fault protection signals, switches, ballast and resonant capacitors to amplify the signal, multiple lamps to be driven by the amplified signal and fault protection circuitry to generate the fault protection signal. The first configuration drives the lamps with a single transformer. In the conventional arrangement, when the lamps are driven by a single transformer, the current flowing through each individual lamp is not balanced, creating difficulties in acquiring a reliable feedback signal. An unreliable feedback signal may allow some of the lamps to stay off during the ignition process and may present safety issues. The second configuration drives each lamp with a dedicated transformer. Driving each lamp individually solves the feedback issue, but introduces a new component which increases cost of production and requires physical space in the device. Therefore, an inexpensive way to drive multiple lamps while maintaining a reliable feedback signal is needed. 
   SUMMARY 
   One system embodiment described herein may provide a transformer capable of generating, at least in part, AC voltage and current and a striking voltage. The system may also include at least two lamp loads coupled in parallel to the transformer. The system may also include current balancing circuitry coupled to the plurality of lamp loads, the current balancing circuitry is capable of balancing AC current supplied by the transformer through the at least two lamp loads, the current balancing circuitry is also capable providing simultaneous striking voltage, supplied by the transformer, to the at least two lamps loads. 
   A method according to on embodiment may include generating AC voltage and current and a striking voltage. The method of this embodiment may also include generating striking voltage and steady-state voltage for at least two lamp loads. The method of this embodiment may also include coupling at least two lamp loads in parallel. The method of this embodiment may also include coupling current balancing circuitry to the at least two lamp loads and providing, by the current balancing circuitry simultaneous striking voltage to the at least two lamps loads. The method of this embodiment may also include balancing, by the current balancing circuitry, AC current through the at least two lamp loads. 
   One apparatus embodiment may include current balancing circuitry coupled to a plurality of lamp loads and a transformer capable of generating, at least in part, AC voltage and current and a striking voltage. The current balancing circuitry may be capable of balancing AC current supplied by the transformer through the at least two lamp loads. The current balancing circuitry may also be capable of providing simultaneous striking voltage, supplied by the transformer, to the at least two lamps loads. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of embodiments of the claimed subject matter will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and in which: 
       FIG. 1  illustrates one exemplary system embodiment of the claimed subject matter; 
       FIG. 2  illustrates another exemplary system embodiment of the claimed subject matter; 
       FIG. 3  illustrates another exemplary system embodiment of the claimed subject matter; and 
       FIG. 4  illustrates another exemplary system embodiment of the claimed subject matter. 
   

   Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly, and be defined only as set forth in the accompanying claims. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates one exemplary system embodiment  100  of the claimed subject matter. System  100 , in this embodiment, may include inverter controller circuitry  101 , DC/AC inverter circuitry  102 , current balancing circuitry  116  and one or more loads  112 ,  114 . In this embodiment, loads  112  and  114  comprise lamp loads, for example cold cathode fluorescent lamps, as may be used in a liquid crystal display (LCD) panel. DC/AC inverter circuitry  102  may comprise, for example, a plurality of switches (not shown) arranged in a full-bridge, half-bridge, push-pull, active clamp, and/or Class D topology and/or other conventional and/or custom inverter topology. Inverter controller circuitry  101  may control the switches of the inverter circuitry  102  to generate a rectangular AC signal from a DC source. System  100  may also comprise a transformer  104 . Transformer  104  may receive the rectangular AC signal and, in conjunction with one or more resonant capacitors  108 , generate a smooth sinusoidal (or quasi-sinusoidal) AC signal to supply power to the lamp loads  112  and  114 . In this embodiment, current balancing circuitry  116  may be capable of balancing current supplied to each lamp load, in a manner described below. 
   As used in any embodiment herein, “circuitry” may comprise, for example, singly or in any combination, hardwired circuitry, programmable circuitry, state machine circuitry, and/or firmware that stores instructions executed by programmable circuitry. For ease of understanding, the drawings herein depict inverter controller circuitry  101 , DC/AC inverter circuitry  102  and current balancing circuitry  116  as separate components. However, it should be understood that inverter controller circuitry  101 , DC/AC inverter circuitry  102  and current balancing circuitry  116  may, in whole or in part, or collectively or individually, be comprised in one or more integrated circuits. As used in any embodiment herein, an “integrated circuit” means a semiconductor device and/or microelectronic device, such as, for example, a semiconductor integrated circuit chip. 
   The inverter controller circuitry  101  may be capable of controlling DC/AC inverter circuitry  102  to generate an AC voltage and current from a DC signal. The inverter controller circuitry  101  may also be capable of controlling the DC/AC inverter circuitry  102  to generate striking voltage and steady-state voltage for at least two lamp loads  112  and  114 . Striking voltage may include, for example, sufficient voltage to ignite the lamp. If the lamp is a CCFL, for example, a striking voltage on the order of 1500 Volts may be used to ignite the lamp. Once struck, the lamp may require less voltage during steady-state operation. If the lamp is a CCFL, for example, a steady state voltage on the order of a few hundred volts may be used. 
   Current balancing circuitry  116  may include respective ballast capacitors  106   a  and  106   b  for each respective lamp load  112 ,  114 . The lamp loads may be coupled in parallel with each other and to the transformer  104 . Current balancing circuitry  116  may also include a balancing inductor  110  disposed between each lamp  112  and  114 , for example, between the high voltage sides  118  and  120 , respectively, of the lamps, and each respective ballast capacitors  106   a  and  106   b . Ballast capacitors  106   a  and  106   b  and inductor  110  may operate to balance current through each lamp load IL 1  and IL 2 . “Balance”, as used in any embodiment herein with reference to the current in each lamp load, may be defined as approximately equal, or, alternatively, as a condition where the current in one lamp is a multiple of the current in another lamp load. Current balancing circuitry  116  may be capable of supplying lamp striking voltage to each lamp in the system  100  simultaneously. “Simultaneously”, as used in any embodiment herein with reference to striking voltage, may mean that each lamp is provided striking voltage at the same time. Lamp striking voltage may last for several seconds, and the aforementioned definition of “simultaneously” means that at least a portion of the time intervals for the striking voltage for each lamp overlap (which does not necessarily mean that the time intervals overlap completely). 
   For example, the current in each lamp load may be balanced when the respective currents IL 1  and Il 2  are approximately equal. “Approximately equal”, as used herein with reference to current, may be defined as within a selected and/or pre-defined tolerance and/or within a tolerance that may be defined by component values. In this embodiment, ballast capacitors  106   a  and  106   b  may be given a common value Cb, then according to Kirchhoff&#39;s voltage law, a proper current balance may be achieved at a certain operating frequency fo if the balancing inductor Lb  110  and the ballast capacitors Cb  106   a  and  106   b  are given values according to:
 
w 2 LbCb=2; where w=2πfo
 
   Since the inductor  110  is located on the high voltage sides ( 118  and  120 ) of the lamps  112  and  114 , the safety and voltage ratings of the inductor  110  may be considered. 
     FIG. 2  illustrates another exemplary system embodiment  200  of the claimed subject matter. System  200  is similar to system  100 , except that the current balancing circuitry  224  comprises differential mode inductor circuitry  216 , . . . ,  218  and this embodiment is generalized for n-number of lamp loads, i.e., lamps L 1  ( 210 ), L 2  ( 212 ), . . . , Ln( 214 ). 
   The low voltage terminals of the n lamps  210 ,  212 ,  214  may be connected by n−1 differential mode inductors  216 ,  218 . Thus, for example, if system  200  includes 3 lamps, than system  200  may also include two differential mode inductors  216  and  218 . Each differential mode inductor may include a primary winding  220  and  228  and a secondary winding  222  and  230 . Each lamp load  210 ,  212 , . . . ,  214  may be coupled to a respective primary winding  220  or  228  or a respective secondary winding  222  or  230 . In this embodiment, each differential mode inductor  216 , . . . ,  218  may operate as 1-1 current transformers, meaning the current through the primary windings is approximately equal to the current through the secondary windings. 
   The high voltage side of each lamp may be coupled in series to respective ballast capacitors  204 ,  206 , . . . ,  208 . Each lamp and ballast capacitor may be coupled in parallel to each other and to transformer  104 . 
   The inductance values for the differential mode inductors  216 ,  218  may be calculated using Kirchhoff&#39;s laws. The differential mode inductors  216 ,  218  may be arranged so that the low voltage terminal of lamp L 1   210  is connected to the primary winding of differential mode inductor DM 1   216 , the low voltage terminal of lamp L 2   212  is connected to the secondary winding of differential mode inductor DM 1   216 , the low voltage terminal of lamp Ln  214  is connected to the secondary winding of differential mode inductor DMn  218 . The differential mode inductors  216 ,  218  may be further arranged so the secondary winding of differential mode inductor DM 1   216  is in series with the primary winding of differential mode inductor DMn  218 . 
   As with the previous embodiment, current balancing circuitry  224  coupled to the lamp loads may operate to balance current through each lamp load. Also, current balancing circuitry  224  may be capable of supplying lamp striking voltage to each lamp in the system  200  simultaneously. 
     FIG. 3  illustrates another exemplary embodiment  300  of the claimed subject matter. This particular embodiment may include inverter controller circuitry  101 ′, DC/AC inverter circuitry  102 , current balancing circuitry  350 , and one or more loads  112 ,  114 . In this embodiment, loads  112  and  114  comprise lamp loads, for example cold cathode fluorescent lamps (CCFLs), as may be used in a liquid crystal display (LCD) panel. DC/AC inverter circuitry  102  may comprise, for example, a plurality of switches (not shown) arranged in a full-bridge, half-bridge, push-pull, active clamp, and/or Class D topology and/or other conventional and/or custom inverter topology. Inverter controller circuitry  101 ′ may control the switches of the inverter circuitry  102  to generate a rectangular AC signal from a DC source. 
   System  300  may also comprise transformer  104 . Transformer  104  may receive the rectangular AC signal and, in conjunction with one or more resonant capacitors  302 , generate a smooth sinusoidal (or quasi-sinusoidal) AC signal to supply power to the lamp loads  112  and  114 . In this embodiment, current balancing circuitry  350  may be capable of balancing current supplied to each lamp load, in a manner described below. 
   The inverter controller circuitry  101 ′ may have all of the capabilities of the inverter controller circuitry  101 ′ described above with reference to  FIGS. 1 and 2 , and, in this embodiment may also comprise soft start circuitry  322 , fault protection circuitry  326  and a feedback comparator  320 . During initial power on, after reset, and/or at other time periods, soft start circuitry  322  may be capable of controlling the switches  102  of the DC/AC inverter circuitry to generate a nominal or minimal current value applied to transformer T 1   104 . Soft start circuitry  322  may also be capable of ramping up power delivered to the CCFL loads, based on, for example, a user-defined and/or programmable interval. 
   The soft start circuitry  322  may increase the signal VS  324  which may cause the inverter controller circuitry  101 ′ to control the DC/AC inverter circuitry  102 . If the signal VS  324  reaches a threshold voltage VT  328 , the soft start circuitry  322  may enable the fault protection circuitry  326 . The fault protection circuitry  326  may also be enabled by a fault protection signal FP  314 . Fault protection circuitry  326  may control switches  102  to reduce, minimize, and/or shut-off power delivered to one or more lamp loads (for example, as may be desirable if an open-lamp condition is detected). The inverter controller circuitry  101 ′ may control the DC/AC inverter circuitry  102  based on, at least in part, lamp current feedback information. 
   To that end, inverter controller circuitry  101 ′ may also include a feedback comparator  320 . The feedback comparator  320  may compare lamp current from one or more loads  112 ,  114  to a reference signal ADJ  318 , for example, a signal proportional to the brightness setting in a liquid crystal display (LCD) panel. Inverter controller circuitry  101 ′ may also receive voltage feedback indicative of the voltage across one or more lamp loads, and may also control switches  102  based on, at least in part, voltage feedback information. Lamp current information for lamp  112  may be generated by voltage divider circuitry, for example a voltage divider comprising resistor  354  and  356 . Voltage divider resistors  354  and  356  may generate a current feedback signal indicative of, or proportional to, the current in lamp  112 . Similarly, lamp current information for lamp  114  may be generated by voltage divider circuitry, for example a voltage divider comprising resistor  358  and  360 . Voltage divider resistors  358  and  360  may generate a current feedback signal indicative of, or proportional to, the current in lamp  114 . 
   Current balancing circuitry  350  may include respective ballast capacitors  304   a  and  304   b  for each respective lamp load  112 ,  114 . Current balancing circuitry may also include differential mode inductor circuitry  306  electrically coupled to the high voltage side of the lamp load. The ballast capacitors  304   a  and  304   b  and the differential mode inductor  306  may operate to balance the current, as defined above, through each lamp load IL 1  and IL 2 . Additionally, current balancing circuitry may be capable of supplying lamp striking voltage to each lamp in the system  300  simultaneously, as may defined above. 
   In this particular embodiment, ballast capacitors  304   a  and  304   b  may be given a common value C. The inductance of the primary and secondary windings of the differential mode inductor  306  may be given a common value L and contain a mutual inductance M. The lamp loads may be replaced with loads RL 1  and RL 2  for the purposes of calculations and the inductance of the differential mode inductor may be found using Kirchhoff&#39;s Laws:
 
 RL 1 2   −RL 2 2 =(4 L/C )*(1 −K ), where  K=M/L.  
 
   For the purpose of calculation, the values of RL 1  and RL 2  may be assumed to be a worst case scenario, for example 20% apart with values of 120 KΩ and 100 KΩ respectively, and the inductance L may be calculated from those assumed values. When the RL 1 /RL 2  differential is near the assumed differential, the current balancing circuitry  350  will correct the current imbalance. When the RL 1 /RL 2  differential is near zero, the current will be naturally balanced because there will be no lamp impedance differential. The range may be picked realistically because the current balance may suffer when the RL 1 /RL 2  differential is between the assumed value and zero. 
   When the above formula is followed, the differential mode inductor  306  may act as a 1-1 current transformer and balance the current, IL 1  and IL 2 , flowing to each lamp  112  and  114 , respectively. However, it is equally contemplated herein that inductor  306  may be configured to operate in other modes, for example, a m to n current transformer, where m does not equal n. The ballast capacitors  304   a  and  304   b  and the inductance of the primary and secondary windings of the differential mode conductor do not have to be equal for the differential mode inductor  306  to balance the lamp currents IL 1  and IL 2 , and may have unequal capacitance and inductance values respectively. If the values are unequal, the differential mode inductor  306  may be chosen according to Kirchhoff&#39;s laws. Since the differential inductor  306  is located on the high voltage side of the lamp loads  112  and  114 , the safety and voltage ratings of the differential mode inductor  306  may be considered. 
   The fault protection signal generating circuitry  310  may generate a fault protection signal FP  314  that may be used by the inverter controller circuitry  101 ′. The fault protection generating circuitry  310  may comprise switches  330  and  332  that may process the output voltages of each individual lamp  112 ,  114 . In this embodiment, there may be a transistor  332 ,  330  for each individual lamp  112 ,  114  that may be gated by the output voltage of respective lamp  112 ,  114  output voltage. The transistors  332 ,  330  may be connected in series in such a manner that when all transistors  332 ,  330  are turned on the fault protection signal FP  314  is pulled to ground. If one of the lamps  112 ,  114  stops producing voltage, the associated transistor  332 ,  330  may be turned off causing the fault protection signal  314  to be pulled to approximately VDD  312 . This may operate to trigger the fault protection circuitry  326  in the inverter controller  101 ′ to reduce and/or shut off power delivered to the lamp loads (via switches  102 ). 
     FIG. 4  illustrates another exemplary embodiment  400  for the claimed subject matter. This particular embodiment may include inverter controller circuitry  101 ′, DC/AC inverter circuitry  102 , current balancing circuitry  422 , and one or more loads  112  and  114 . The inverter controller circuitry  101 ′, DC/AC inverter circuitry  102 , and current balancing circuitry  422  may have similar capabilities and configurations to their respective counterparts in system  300  except that the current balancing circuitry  422  in system  400  may comprise a differential mode inductor  408  located on the low voltage side of the lamp loads. Shifting the differential mode inductor  408  to the low voltage side of the lamp may reduce the safety hazards created by providing the differential mode inductor  408  with too much voltage, while still maintaining a current balance between the respective lamps. 
   The fault protection signal generating circuitry  424  may generate a fault protection signal FP  314  that may be used by the inverter controller circuitry  101 ′. The fault protection signal generating circuitry  424  may comprise transistor logic  412  and  414  that processes the output voltages of each individual lamp  112  and  114 . The output voltages of each lamp may be sensed and OR-ed together, by diodes D 1  and D 2   420 , to create a signal VB  410 . The signal VB  410  may be used to gate switch Q 1   412 , controlling the signal VD  416 . The signal VD  416  controls switch Q 2   414 , thereby controlling the fault protection signal FP 314 . When the lamps  112  and  114  are functioning in a normal manner, the signal VB  410  may be small and unable to trigger the switch Q 1   412 . If the switch Q 1   412  is not turned on, the signal VD  416  may be pulled to approximately VDD  418 . If VD  416  is large, the switch Q 2   414  may be turned on, pulling the fault protection signal  314  to ground. However, if the lamps  112  and  114  are removed or malfunctioning, VB  410  may be large and may enable the switch Q 1   412 . If the switch Q 1   412  is enabled, the signal VD  416  may be pulled to ground, the switch Q 2   414  may be turned off. If the switch Q 2   414  is turned off, then the fault protection signal  316  may be approximately VDD  418 . If the fault protection signal output  314  is high, then the fault protection circuitry  326  may be enabled. 
   The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described (or portions thereof), and it is recognized that various modifications are possible within the scope of the claims. Other modifications, variations, and alternatives are also possible. Accordingly, the claims are intended to cover all such equivalents.