ARRANGEMENT SUITABLE FOR DRIVING FLOATING CCFL BASED

BACKLIGHT

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the field of cold cathode fluorescent lamp based lighting and more particularly to an arrangement in which balancing transformers are supplied at each end of the lamp.

[0002] Fluorescent lamps are used in a number of applications including, without limitation, backlighting of display screens, televisions and monitors. One particular type of fluorescent lamp is a cold cathode fluorescent lamp (CCFL). Such lamps require a high starting voltage (typically on the order of 700 to 1 ,600 volts) for a short period of time to ionize a gas contained within the lamp tubes and fire or ignite the lamp. This starting voltage may be referred to herein as a strike voltage or striking voltage. After the gas in a CCFL is ionized and the lamp is fired, less voltage is needed to keep the lamp on. [0003] In liquid crystal display (LCD) applications, a backlight is needed to illuminate the screen so as to make a visible display. Backlight systems in LCD or other applications typically include one or more CCFLs and an inverter system to provide both DC to AC power conversion and control of the lamp brightness. Even brightness across the panel and clean operation of inverters with low switching stresses, low EMI, and low switching losses is desirable. [0004] The lamps are typically arranged with their longitudinal axis proceeding horizontally. In general, even brightness involves two dimensions: uniform brightness in the vertical dimension, i.e. among the various lamps; and uniform brightness along the longitudinal axis of each of the various lamps in the horizontal dimension. Brightness uniformity in the vertical dimension is largely dependent on matching the lamp currents which normally requires a certain type of balancing technique to maintain an even lamp current distribution. U.S. Patent S/N 7,242,147 issued July 10, 2007 to Jin, entitled "Current Sharing Scheme for Multiple CCFL Lamp Operation", the entire contents of which is incorporated herein by reference, is addressed to a ring balancer comprising a plurality of balancing

transformers which facilitate current sharing in a multi-lamp backlight system thus providing even lamp current distribution.

[0005] Brightness uniformity in the horizontal dimension is impacted by the existence of parasitic capacitance between the CCFLs and the chassis. As a result of the parasitic capacitance, leakage current exists along the length of the lamps and such leakage further results in diminishing brightness along the lamps' longitudinal axis towards the cold end in a single ended drive architecture. The term single ended drive architecture refers to a backlight arrangement in which the high voltage drive power is applied from only one side of the lamp, which is usually called the 'hot' end, and the other side of the lamp is normally at ground potential and referred as the 'cold' end. With the increasing size of LCD televisions and monitors, increases in lamp length, wire length and operating voltage associated with the resultant large backlighting systems make the leakage effect more significant, and consequently uniform horizontal brightness across lamps arranged in a single ended drive architecture is more difficult to achieve. In order to obtain even horizontal brightness for each of the CCF lamps, i.e. that the lamps should not exhibit a light gradient along its longitudinal axis, energy has to be alternatively driven into each end of the lamp. Thus, most large backlight inverter systems are configured to support 'floating' lamp structures, in which both lamp terminals are connected to a high voltage driving source, with a 180° phase shift to each other, and floating in relation to the chassis ground plane.

[0006] As described above, a factor in achieving even brightness over a CCFL is the ability to symmetrically power the lamp alternatively at both ends. This is more difficult to achieve as the length of the lamp increases. Among the conventional inverter topologies, a phase shifted full-bridge topology and a resonant full-bridge topology are most commonly used for CCFL inverter applications because of their ability to produce symmetric lamp current waveforms and clean switching operations. [0007] U.S. Patent S/N 7,187,139 issued March 6, 2007 to Jin, entitled "Split

Phase Inverters for CCFL Backlight System", the entire contents of which is incorporated herein by reference, is addressed to an inverter arrangement in which the switching elements are split into two inverter arms that are deployed at separate terminals of a floating lamp structure. Such a concept provides even brightness across the longitudinal dimension of the lamps with lower cost compared with the

conventional approach of deploying a full bridge circuit at each end of the lamps, while maintaining the advantages of soft switching operation of the full bridge. Unfortunately, separate inverter circuits are still needed to develop driving power at both ends of the lamp, and in addition, wiring of power cables and control signals could lead to potential electromagnetic interference issues, in particular as high voltage signals traversing the chassis length exhibitive capacitive coupling to the chassis. Often, a reflective material is disposed behind the lamps, typically based on metal, the metal based reflective material further adding to the capacitive coupling. [0008] What is further desired, and not provided by the prior art, is a backlighting arrangement that can provide even luminance across each lamp in the system, preferably with only one inverter circuit, and further preferably where there is no high voltage or high switching current wiring across the horizontal length of the panel.

SUMMARY OF THE INVENTION

[0009] Accordingly, it is a principal object of the present invention to overcome at least some of the disadvantages of the prior art. This is provided in certain embodiments by a backlighting arrangement in which pairs of balancing transformers are provided, each associated with a particular luminaire. The primary winding of each of the balancing transformers is coupled in series with a respective end of the associated luminaire. The secondary windings of the balancing transformers are connected in a single closed loop, and arranged to be in-phase. [00010] In one exemplary embodiment, the luminaires each comprise a pair of lamps, and an additional pair of balancing transformers is provided associated with each pair of lamps. The primary windings of the additional pair are coupled in series and between the lamps. The secondary windings of the additional pair are connected in-phase within the single closed loop. The luminaire is connected across an AC power source, such as an inverter or a single ended AC power source, and the nexus of the pair of lamps not directly connected to the AC power source receives energy via the balancing transformers thereby providing even brightness.

[00011] The present embodiments enable a backlighting arrangement comprising: a first lead and a second lead arranged to receive and return an alternating current; at least one luminaire; and at least one first balancing transformer

pair each of the transformer pair associated with a particular one of the at least one luminaire, the primary winding of a first balancing transformer of each of the first balancing transformer pair serially coupled between the first lead and a first end of the associated at least one luminaire, and the primary winding of a second balancing transformer of each of the first balancing transformer pair serially coupled between the second lead and a second end of each of the associated at least one luminaire, wherein the secondary windings of all of the at least one first balancing transformer pair are serially connected in a closed in-phase loop. [00012] In one embodiment at least one of the at least one luminaire comprises a serially connected pair of linear lamps. In another embodiment at least one of the at least one luminaire comprises a U-shaped lamp.

[00013] In one embodiment at least one of the at least one luminaire comprises a single linear lamp. In another embodiment, the backlighting arrangement further comprises a differential alternating current source arranged to supply power to the at least one luminaire via the first and second leads. In yet another embodiment the at least one luminaire comprises a plurality of luminaires.

[00014] In one embodiment the backlighting arrangement further comprises a single ended alternating current source arranged to supply power to the at least one luminaire via the first and second leads, wherein the first lead is connected to the single ended alternating current source, and the second lead is connected to a ground connection. In yet another embodiment, the backlighting arrangement further comprises a sense resistor serially connected within the serially connected closed in- phase loop arranged to present a voltage drop representation of the current flowing through the closed in-phase loop. [00015] In one embodiment the backlighting arrangement further comprises at least one second balancing transformer pair each of the second transformer pair associated with a particular one of the at least one luminaire and wherein each of the at least one luminaire comprises a pair of linear lamps each exhibiting a far end removed from each of the first and second ends of the luminaire, the primary windings of the second balancing transformer pair being arranged in series and serially coupled between the far ends of associated pair of linear lamps, the secondary windings of the second balancing transformer pair being serially connected in-phase in the closed in-phase serial loop. In one further embodiment the backlighting

arrangement further comprises a differential alternating current source arranged to supply power to the at least one luminaire via the first and second leads. In another further embodiment the backlighting arrangement further comprises a single ended alternating current source arranged to supply power to the at least one luminaire via the means for first and second leads, wherein the first lead is connected to the single ended alternating current source, and the second lead is connected to a ground connection. In yet another further embodiment the backlighting arrangement further comprises a sense resistor serially connected within the serially connected closed in- phase loop arranged to present a voltage drop representation of the current flowing through the closed in-phase loop.

[00016] In one embodiment each of the at least one luminaire comprises a pair of linear lamps each exhibiting a far end removed from each of the first and second ends of the luminaire, the arrangement further comprising at least one second balancing transformer each associated with a particular one of the pair of linear lamps, the primary windings of each of the second balancing transformer being coupled between the far ends of the associated pair of linear lamps, the secondary windings of the second balancing transformer being serially connected in-phase in the closed in- phase serial loop. In one further embodiment the at least one pair of linear lamps are arranged substantially in parallel to backlight a display, and wherein the serially connected closed in-phase loop exhibits a single twisted wire pair connecting a portion of the closed in-phase loop associated with a first end of the display to a portion of the closed in-phase loop associated with a second end of the display opposing the first end of the display. [00017] The present embodiments independently provide for a method of driving at least one luminaire, comprising: receiving an alternating current; providing at least one luminaire; and providing a first balancing transformer pair associated with each of the provided at least one luminaire, the primary winding of a first transformer of the respective balancing transformer pair associated with a first end of the associated luminaire, and the primary winding of a second transformer of the particular balancing transformer pair associated with a second end of the associated luminaire; coupling the received alternating current via the primary windings of the first balancing transformer pair to each end of the provided at least one luminaire; and

arranging the secondary windings of all of the provided at least one first balancing transformer pair in a serially connected closed in-phase loop.

[00018] In one embodiment at least one of the provided at least one luminaire comprises a serially connected pair of linear lamps. In another embodiment, at least one of the provided at least one luminaire comprises a U-shaped lamp.

[00019] In one embodiment at least one of the provided at least one luminaire comprises a single linear lamp. In another embodiment the method further comprises sensing a current flowing through the closed in-phase loop. [00020] In one embodiment each of the provided at least one luminaire comprises a pair of linear lamps each exhibiting a far end removed from each of the first and second ends of the luminaire, the method further comprising: providing at least one second balancing transformer pair, each balancing transformer of the pair associated with a particular one of the provided at least one luminaire; arranging the primary windings of the second balancing transformer pair in series and serially connecting the series arranged primary windings between the far ends of the associated pair of linear lamps; and arranging the secondary windings of the provided at least one second balancing transformer pair in the serially connected closed in- phase loop. In one further embodiment the method further comprises sensing a current flowing through the closed in-phase loop. [00021] In one embodiment each of the provided at least one luminaire comprises a pair of linear lamps each exhibiting a far end removed from each of the first and second ends of the luminaire, the method further comprising: providing at least one second balancing transformer; serially connecting the primary winding of one of the provided at least one second balancing transformer between the far ends of the associated pair of linear lamps; and arranging the secondary windings of the provided at least one second balancing transformer in the serially connected closed in- phase loop.

[00022] The present embodiment independently provide for a backlighting arrangement comprising: a means for receiving an alternating current exhibiting a first lead and a second lead; a plurality of luminaires; and a plurality of first balancing transformer pairs each associated with a particular one of the plurality of luminaires, the primary winding of a first balancing transformer of each of the first balancing transformer pair serially coupled between the first lead of the means for receiving an

alternating current and a first end of the associated luminaire, and the primary winding of a second balancing transformer of each of the first balancing transformer pair serially coupled between the second lead of the means for receiving an alternating current and a second end of the associated luminaire, wherein the secondary windings of all of the at least one first balancing transformer pair are serially connected in a closed in-phase loop.

[00023] In one embodiment the backlighting arrangement further comprises a plurality of second balancing transformer pairs each associated with a particular one of the plurality of luminaires and wherein each of the plurality of luminaires comprises a pair of lamps each exhibiting a far end removed from each of the first and second ends of the luminaire, the primary windings of the associated second balancing transformer pair being arranged in series and serially connected between the far ends of the pair of linear lamps, the secondary windings of the second balancing transformer pair being serially connected in-phase in the closed in-phase serial loop.

[00024] Additional features and advantages of the invention will become apparent from the following drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS [00025] For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout. [00026] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:

[00027] Fig. IA illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement comprising a luminaire constituted of a single lamp;

[00028] Fig. IB illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement comprising a luminaire constituted of a pair of lamps;

[00029] Fig. 2 illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement comprising a plurality of luminaires, each constituted of a serially connected linear lamp pair, and a differential AC source in which energy is supplied to the far side of each of the lamps by a balancing network;

[00030] Fig. 3 illustrates a high level block diagram of an exemplary embodiment of a lighting arrangement comprising a plurality of luminaires, each constituted of a single linear lamp, and a single ended AC source; [00031] Fig. 4 illustrates a high level block diagram of an exemplary embodiment of a lighting arrangement comprising a plurality of luminaires, each constituted of a U-shaped lamp, and a single ended AC source;

[00032] Fig. 5 illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement comprising a plurality of luminaires, each constituted of a pair of serially coupled linear lamps, and a differential AC source;

[00033] Fig. 6 illustrates a high level block diagram of an exemplary embodiment of a lighting arrangement comprising a plurality of luminaires, each constituted of a pair of serially coupled linear lamps, and a single ended AC source; [00034] Fig. 7 illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement comprising a plurality of luminaires, each constituted of a U-shaped lamp, and a differential AC source; and [00035] Fig. 8 illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement comprising a plurality of luminaires, each constituted of a linear lamp pair, each of the linear lamp pairs sharing a single balancing transformer at the far end, and a differential AC source, in which energy is supplied to the far side of each of the lamp pairs by a balancing network.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[00036] Certain of the present embodiments enable a backlighting arrangement in which pairs of balancing transformers are provided, each associated with a particular luminaire. The primary winding of each of the balancing transformers is coupled in series with a respective end of the associated luminaire. The secondary windings of the balancing transformers are connected in a single closed loop, and arranged to be in-phase.

[00037] In one exemplary embodiment, the luminaires each comprise a pair of lamps, and an additional pair of balancing transformers is provided associated with each pair of lamps. The primary windings of the additional pair are coupled in series and between the lamps. The secondary windings of the additional pair are connected in-phase within the single closed loop. The luminaire is connected across an AC power source, such as an inverter or a single ended AC power source, and the nexus of the pair of lamps not directly connected to the AC power source receives energy via the balancing transformer thereby providing even brightness.

[00038] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

[00039] Fig. IA illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement 10 comprising a single luminaire, constituted of a lamp 20, arranged to backlight a display 30. Display 30 is typically constituted of a metal based chassis. Floating lighting arrangement 10 further comprises: a driver 40; a driving transformer 50 exhibiting a first output 60 and a second output 70; a first and a second balancing transformer 80; and a twisted wire pair 90. The outputs of driver 40 are connected to both ends of the primary winding of driving transformer 50. The first end of the secondary winding of driving transformer 50, denoted first output 60, is connected to the first end of the primary winding of first balancing transformer 80. The second end of the primary winding of first balancing transformer 80 is connected to the first end of lamp 20. The second

end of lamp 20 is connected to the first end of the primary winding of second balancing transformer 80, and the second end of the primary winding of second balancing transformer 80 is connected the second end of the secondary winding of driving transformer 50, denoted second output 70. The secondary windings of first and second balancing transformers 80 are connected in a closed serial loop, the serial loop further comprising a sense resistor RS. The polarity of the secondary windings of first and second balancing transformers 80 are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, the wires of the closed loop connecting the secondary windings of first and second balancing transformers 80 are arranged via a twisted wire pair 90.

[00040] Preferably, the first end of lamp 20 is in physical proximity of driving transformer 50, e.g. on the same side of display 30 typically constituted of a metal based chassis, as driving transformer 50, and in physical proximity of first balancing transformer 80, and preferably generally define a first plane. Preferably, lamp 20, typically constituted of a linear lamp, generally extends axially away from the proximity of driving transformer 50, and generally defines a second plane, further preferably orthogonal to the first plane.

[00041] In operation, driver 40, which in one embodiment comprises a direct drive backlight driver as described in U.S. Patent S/N 5,930,121 issued July 27, 1999 to Henry, entitled "Direct Drive Backlight System", the entire contents of which is incorporated herein by reference, provides a differential AC source via driving transformer 50. In one further embodiment the secondary of driving transformer 50 is allowed to float. For simplicity, we designate first output 60 as AC+ and second output 70 as AC-, which is appropriate for 1/2 the drive cycle. During the second half of the drive cycle, polarity is reversed and the direction of current flow is reversed.

[00042] A current Il is developed through the primary winding of first balancing transformer 80, responsive to AC+ at first output 60, and driven through lamp 20. Current Il proceeds via the primary winding of second balancing transformer 80 and is returned to AC- at second output 70. Current 12 is developed in the secondary of first balancing transformer 80, responsive to II, and flows via sense resistor RS and a first wire of twisted wire pair 90 to the secondary of second balancing transformer 80. The voltage developed across the secondary of second balancing transformer 80 is in phase in the closed loop with the voltage developed

across the secondary of first balancing transformer 80, and thus current 12 continues through the secondary of second balancing transformer 80 and is returned via a second wire of twisted wire pair 90.

[00043] Advantageously, in a preferred embodiment the turns ratio of each of first and second balancing transformers 80 are such that twisted wire pair 90 exhibits low voltage and high current, thereby reducing any capacitive coupling to the constituent chassis of display 30. The use of twisted wire pair 90, exhibiting similar current and voltage with reverse polarity in each of the constituent wires further reduces any electromagnetic interference caused by twisted wire pair 90 traversing the length of display 30.

[00044] As described above, the secondary windings of first and second balancing transformers 80 are serially connected in a closed loop, and thus the current circulating in each of the secondary winding is substantially equal. If the magnetizing currents of the balancing transformers are neglected, the following relationship can be established for each of the balancing transformers :

Npi -H = Ns _I -H; N _P2 -I1 = N _S2 -I2; EQ. 1

[00045] Npi and Il denote the primary turns and primary current respectively of first balancing transformer 80; Ns ₁ and 12 denote the secondary turns and secondary current respectively of first balancing transformer 80; Np ₂ and Il denote the primary turns and primary current respectively of second balancing transformer 80; and Ns ₂ and 12 denote the secondary turns and secondary current respectively of second balancing transformer 80. Solving for Il and 12 of EQ. 1 results in:

Il = (Nsi/N _P1 )-I2 = (N _S2 /N _P2 )-I2 EQ. 2

Thus, in accordance with EQ. 2, the secondary current sensed by the voltage drop across sense resistor RS,is a function of the primary current and the turns ratio of the balancing transformers 80. Sense resistor RS is advantageously not connected to the high voltage associated with first and second outputs 60, 70, and thus may be connected to a low voltage controller to sense the current through lamp 20.

[00046] Current 12 connected via the closed loop of the secondary windings, ensures that the current Il entering the first end of lamp 20 is substantially equal to current Il leaving the second end of lamp 20.

[00047] Fig. IB illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement 100 constituted of a pair of linear lamps 20, arranged to backlight a display 30. Floating lighting arrangement 100 further comprises: a driver 40; a driving transformer 50 exhibiting a first output 60 and second output 70; a first and a second balancing transformer 80; a first and a second balancing transformer 85; and a twisted wire pair 90. Balancing transformers 80 and 85 may be of identical type without exceeding the scope of the invention.

[00048] The outputs of driver 40 are connected to both ends of the primary winding of driving transformer 50. The first end of the secondary winding of driving transformer 50, denoted first output 60, is connected to the first end of the primary winding of first balancing transformer 80. The second end of the primary winding of first balancing transformer 80 is connected to the first end of first lamp 20. The second end of first lamp 20 is connected to the first end of the primary winding of first balancing transformer 85, and the second end of the primary winding of first balancing transformer 85 is connected the first end of the primary winding of second balancing transformer 85. The second end of the primary winding of second balancing transformer 85 is connected to the first end of second lamp 20. The second end of second lamp 20 is connected to the first end of the primary winding of second balancing transformer 80 and the second end of the primary winding of second balancing transformer 80 is connected to the second end of the secondary winding of driving transformer 50, denoted second output 70. [00049] The secondary windings of first and second balancing transformers 80 and the secondary windings of first and second balancing transformers 85 are connected in a single closed serial loop via a sense resistor RS. The polarity of the secondary windings of the first and second balancing transformers 80 and the secondary windings of the first and second balancing transformers 85 are arranged so that voltages induced in the secondary windings are in phase and add within the serial closed loop. Optionally, the wires of the closed loop connecting the respective ends of the secondary windings of the first and second balancing transformers 80 to

respective ends of the secondary windings of the first and second balancing transformers 85 are arranged via a twisted wire pair 90.

[00050] Preferably, the first end of first lamp 20 and the second end of second lamp 20 are in physical proximity of driving transformer 50, e.g. on the same side of display 30 typically constituted of a metal based chassis, as driving transformer 50, and in physical proximity of first and second balancing transformers 80, and preferably generally define a first plane. Preferably, first and second lamps 20, each typically constituted of a linear lamp, generally extend axially away from the proximity of driving transformer 50, and generally define a second plane, further preferably orthogonal to the first plane.

[00051] In operation, driver 40 provides a differential AC source via driving transformer 50. In one further embodiment the secondary of driving transformer 50 is allowed to float. For simplicity, we designate first output 60 as AC+ and second output 70 as AC-, which is appropriate for 1/2 the drive cycle. During the second half of the drive cycle, polarity is reversed and the direction of current flow is reversed.

[00052] A current Il is developed through the primary winding of first balancing transformer 80, responsive to AC+ at first output 60, and driven through first lamp 20. Current Il proceeds through the primary winding of first balancing transformer 85, through the primary winding of second balancing transformer 85, through second lamp 20, through the primary winding of second balancing transformer 80 and is returned to AC- at second output 70. As described above, the secondary windings of first and second balancing transformers 80 and first and second balancing transformers 85 are serially connected in a closed loop, and thus current 12 circulating in each of the secondary windings is substantially equal. If the magnetizing currents of the balancing transformers are neglected, the following relationship can be established for each of the balancing transformers:

Np ₁ -Ip ₁ = N _S1 -Isi; Np ₂ -Ip ₂ = N _S 2-Is2; Np ₃ -Ip ₃ = N _S 3-Is3; Np ₄ -Ip ₄ = N _S 4-Is4; EQ. 3

Npi and Ip ₁ of EQ. 3 denote the primary turns and primary current respectively of first balancing transformer 80; Ns ₁ and Isi denote the secondary turns and secondary current respectively of first balancing transformer 80; Np ₂ and Ip ₂ denote the primary turns and primary current respectively of first balancing transformer 85; Ns ₂ and Is ₂

denote the secondary turns and secondary current respectively of first balancing transformer 85; Np ₃ and Ip ₃ denote the primary turns and primary current respectively of second balancing transformer 85; Ns3 and Is3 denote the secondary turns and secondary current respectively of second balancing transformer 85; Np ₄ and Ip ₄ denote the primary turns and primary current respectively of second balancing transformer 80; and Ns ₄ and Is ₄ denote the secondary turns and secondary current respectively of second balancing transformer 80. Solving for each of the primary currents results in:

Ipi = (N _S1 /N _P1 )-I _S1 ; Ip ₂ = (Ns ₂ ZN _P2 )-Is ₂ ; I _P3 = (Ns ₃ ZN _P3 yi _S3 ; Ip ₄ = (N _S4 ZN _P4 )-I _S4 EQ. 4

[00053] From EQ. 4 it is obvious that the primary current and hence the lamp current conducted by the respective lamps can be controlled proportionally with the turns ratio (Ns ₁ ZNp ₁ , Ns ₂ ZNs ₂ A NskZNpk) of the balancing transformers. Physically, if any current in a particular balancing transformer deviates from the relationships defined in EQ. 4, the resulting magnetic flux from the error ampere turns will induce a corresponding correction voltage in the primary winding to force the primary current to follow the balancing condition of EQ. 4. A balanced lamp current condition between first lamp 20 and second lamp 20 can be thus obtained by using the same primary to secondary turns ratio for all the balancing transformers 80, 85. [00054] Further, because the secondary loop current is proportional to the primary side lamp current according to EQ. 4, lamp current can also be detected by sense resistor RS in the secondary winding loop and measured responsive to voltage drop across sense resistor RS. Because the secondary windings of balancing transformers 80, 85 are isolated from the lamp high voltage side, the signal from sense resistor RS can be fed to a low voltage controller circuit directly for regulation and monitoring purposes. Such application is especially useful with a floating lamp configuration, such as floating lighting arrangement 100, where no ground potential node is available in the lamp circuit for direct current sensing. [00055] Coupling the secondary windings of the balancing transformers 80, 85 in a closed loop also couples energy between balancing transformers 80, 85 through the circulating current in the secondary winding loop. The energies needed to drive the far end of first and second lamps 20 are coupled by this mechanism through balancing transformers 85. Under such circumstances the balancing error of the lamp

current is related to the lamp operating voltage and the magnetizing inductance of the balancing transformer as described below under steady state operating condition:

δI = V/(ωLm) EQ. 5

Where δI represents the balancing error, i.e. the difference of the lamp current from the lamp terminals, ω is the angular frequency of the AC source, Lm is the magnetizing inductance from the primary side of the balancer, and V is the lamp operating voltage. [00056] With such an arrangement, there is no requirement for an inverter circuit, or inverter arms, driving the far ends of first and second lamp 20, resulting in a significant cost savings since the driving current is supplied via the secondary winding loop. Advantageously, there are only two wires extending across display 30, in line with the longitudinal axes of first and second lamps 20, to form the loop connection of the balancer secondary windings. Because current 12 flowing in the two wires has equal amplitude and opposite direction, the two wires can be brought to one edge of display 30 and twisted together to yield minimum electro-magnetic field interference, as illustrated by twisted wire pair 90. Further, because the voltage in secondary windings of transformer balancers 20 may be set to be very low responsive to an appropriate turns ratio, the twisted wire pair does not produce any high capacitive leakage current and associated interference.

[00057] Fig. 2 illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement 200 arranged to backlight a display 30 comprising a plurality of luminaires 205A ... 205K, each constituted of a pair of serially arranged linear lamps 20Al, 20A2 ...20Kl, 20K2, and a differential AC source in which energy is supplied to the far side of each of the lamps by a balancing network. Floating lighting arrangement 200 further comprises: a driver 40; a driving transformer 50 exhibiting a first output 60 and a second output 70; a plurality of balancing transformers 80; a plurality of balancing transformers 85; and a wire pair 210A, 210B. Each luminaire 205 A, ..., 205K has associated therewith a balancing transformer 80 associated with a first end thereof and a balancing transformer 80 associated with a second end thereof. Each luminaire 205 A, ..., 205K has further

associated therewith a pair of balancing transformers 85 serially connected between the far ends of the constituent linear lamps 20Al, 20A2 ...20Kl, 20K2. [00058] The outputs of driver 40 are connected to both ends of the primary winding of driving transformer 50. The first end of the secondary winding of driving transformer 50, denoted first output 60, is connected through the primary winding of a respective balancing transformer 80 to a first end of first linear lamp 20Al, ...,20Kl of each of the respective luminaires 205 A, ..., 205K. The nexus of the second end of first linear lamp 20Al, ..., 20Kl and the first end of second linear lamp 20A2, ..., 20K2 of each luminaire 205 A, ..., 205K, is connected through the primary windings of the respective associated pair of balancing transformers 85 arranged in series. The second end of each second linear lamp 20A2 ... 20K2 is connected through the primary winding of a respective associated balancing transformer 80 to the second end of the secondary winding of driving transformer 50, denoted second output 70. [00059] The secondary windings of the balancing transformers 80, 85 are connected in a closed loop, in which the polarity of the secondary windings are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor RS is inserted within the loop to detect current flow. Optionally, the wires of the closed loop connecting across the length of the linear lamps, denoted 210A, 210B, are arranged in a twisted wire pair. For clarity, and to further illustrate the phase relationship of the secondary transformers, lighting arrangement 200 is illustrated with first output 60 exhibiting AC+ and second output 70 exhibiting AC-, which is appropriate for 1/2 the drive cycle. During the second half of the drive cycle, polarity is reversed and the direction of current flow is reversed. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12.

[00060] Preferably, the first end of each first linear lamp 20Al, ..., 20Kl and the second end of each second linear lamp 20A2, ..., 20K2 are in physical proximity of driving transformer 50, e.g. on the same side of display 30 typically constituted of a metal based chassis, as driving transformer 50, and in physical proximity of first balancing transformers 80, and preferably generally define a first plane. Preferably, first linear lamps 20Al, ..., 20Kl and second linear lamps 20A2, ..., 20K2 generally extend axially away from the proximity of driving transformer 50, and generally define a second plane, further preferably orthogonal to the first plane.

[00061] In operation lighting arrangement 200 operates in all respects similar to the operation of lighting arrangement 100, with power for the side of all lamps not directly connected to driving transformer 50, i.e. the far or cold end, supplied by the closed loop of the secondary windings of balancing transformers 80, 85. Power is thus alternately driven into each end of each lamp 20.

[00062] Fig. 3 illustrates a high level block diagram of an embodiment of a lighting arrangement 300 arranged to backlight a display 30 in accordance with a principle of the invention comprising a plurality of luminaires, each constituted of a single linear lamps 2OA, 2OB, ... 2OK, 2OL and a single ended high voltage AC source, exhibiting a common return which is typically connected to chassis ground plane, in which energy is supplied to the far end of each of the linear lamps 2OA, 2OB, ... 2OK, 2OL by a balancing network. Grounded lighting arrangement 300 further comprises a plurality of balancing transformers 80 each associated with one end of a particular linear lamp 2OA, 2OB,... 2OK, 2OL. The number of lamps is shown as being divisible by 2, however this is not meant to be limiting in any way and an odd number of lamps 20 may be supplied without exceeding the scope of the invention. There are twice as many balancing transformers 80 as linear lamps. [00063] The high voltage AC input is connected in parallel through the primary winding of a respective balancing transformer 80 to a first end of each linear lamp 2OA, 2OB,...20L, 2OK. The second end of each linear lamp 2OA, 2OB,...20L, 2OK is connected through the primary winding of the respective associated balancing transformer 80 to the common return.

[00064] The secondary windings of the balancing transformers 80 are connected in a closed loop, in which the polarity of the secondary windings are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor (not shown) is inserted within the loop to detect current flow. Optionally, the wires of the closed loop connecting across the length of the linear lamps are arranged in a twisted wire pair. For clarity, and to further illustrate the phase relationship of the secondary transformers, the direction of current flow is illustrated when a positive voltage appears at the high voltage AC input, denoted HVAC. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12. Current flows in the opposite

direction for each of Il and 12 when a negative voltage, with respect to the common return, appears at HVAC.

[00065] Preferably, the first end of each linear lamp 2OA, 2OB,...2OL, 2OK is in physical proximity of a source driving transformer providing the HVAC, e.g. on the same side of display 30 typically constituted of a metal based chassis, as the driving transformer, and in physical proximity of the associated balancing transformers 80, and preferably generally define a first plane. Preferably, each linear lamp 2OA, 2OB,...2OL, 2OK generally extend axially away from the proximity of the source driving transformer providing the HVAC, and generally define a second plane, further preferably orthogonal to the first plane.

[00066] In operation, lighting arrangement 300 operates in all respects similar to the operation of lighting arrangement 200, except that all the lamps are driven with the same voltage from their hot side, i.e. the side connected to HVAC. Driving energy is coupled to the far or cold side by the closed loop of the secondary winding when a negative voltage with respect to the common return appears at input HVAC. Power is thus alternately driven into each end of each lamp 20.

[00067] Fig. 4 illustrates a high level block diagram of an embodiment of a exemplary lighting arrangement 400 arranged to backlight a display 30 comprising a plurality of luminaires, each constituted of a U-shaped lamp 410A, ..., 410K, and a single ended AC source, exhibiting a common return which is typically connected to chassis ground plane, in accordance with a principle of the invention, in which energy is supplied to the side of each of the lamp pairs connected to the common return by a balancing network. Grounded lighting arrangement 400 further comprises a plurality of balancing transformers 80 each associated with one end of a particular U-shaped lamp 410A, ..., 410K. There are twice as many balancing transformers 80 as U- shaped lamps 410.

[00068] The high voltage AC input is connected in parallel through the primary winding of a respective balancing transformer 80 to a first end of each U-shaped lamp 410A, ..., 410K. The second end of each U-shaped lamp 410A, ..., 410K is connected through the primary winding of a respective balancing transformer 80 to the common return.

[00069] The secondary windings of the balancing transformers 80 are connected in a closed loop, in which the polarity of the secondary windings are

arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor (not shown) is inserted within the loop to detect current flow. For clarity, and to further illustrate the phase relationship of the secondary transformers, the direction of current flow is illustrated when a positive voltage appears at the high voltage AC input, denoted HVAC. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12.

[00070] Preferably, the first end and second ends of each U-shaped lamp 410A,

... 410K are in physical proximity of a source driving transformer providing the single ended high voltage AC input, e.g. on the same side of display 30 typically constituted of a metal based chassis, as the driving transformer, and in physical proximity of the associated balancing transformers 80, and preferably generally define a first plane. Preferably, each U-shaped lamp 410A, ... 410K generally extends axially away from the proximity of the source driving transformer providing the high voltage AC input, and generally define a second plane, further preferably orthogonal to the first plane.

[00071] In operation lighting arrangement 400 operates in all respects similar to the operation of lighting arrangement 300, with the far or cold end of the lamps 410 appearing on the same vertical plane as the hot end by the U-shape lamp arrangement. The drive power for the cold end is derived through the closed secondary winding loop as described above in relation to arrangement 300. Power is thus alternately driven into each end of each lamp 410.

[00072] Fig. 5 illustrates a high level block diagram of an exemplary embodiment of a floating lighting arrangement 500 arranged to backlight a display 30 comprising a plurality of luminaires 510A, ..., 510K, each constituted of a pair of serially coupled linear lamps, and a differential AC source. Floating lighting arrangement 500 further comprises a plurality of balancing transformers 80 each associated with one end of a particular luminaire 510A, ..., 510K. The number of balancing transformers is twice the number of luminaires 510. [00073] One end of the differential driving AC voltage, denoted AC+, is connected in parallel through the primary winding of a respective balancing transformer 80 to a first end of each of the luminaires 510A, ..., 510K. The second end of each luminaire 510A, ..., 510K is connected through the primary winding of

the respective associated balancing transformer 80 to the second end of the differential driving AC voltage, denoted AC-.

[00074] The secondary windings of the balancing transformers 80 are connected in a closed loop, in which the polarity of the secondary windings are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor (not shown) is inserted within the loop to detect current flow. For clarity, and to further illustrate the phase relationship of the secondary transformers, the direction of current flow is illustrated when a positive voltage appears at AC+. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12.

[00075] Preferably, the first end and second ends of each luminaire 510A, ...,

510K are in physical proximity of a source driving transformer providing the differential high voltage AC input, e.g. on the same side of display 30 typically constituted of a metal based chassis, as the driving transformer, and in physical proximity of the associated balancing transformers 80, and preferably generally define a first plane. Preferably, each luminaire 510A, ..., 510K generally extends axially away from the proximity of the source driving transformer providing the differential high voltage AC input, and generally define a second plane, further preferably orthogonal to the first plane. [00076] In operation lighting arrangement 500 operates in all respects similar to the operation of lighting arrangement 400 and is therefore not further detailed. Disadvantageously, power is not directly driven into the far, or cold, end of each of the linear lamps of the luminaries 510A, ..., 510K. [00077] Fig. 6 illustrates a high level block diagram of an embodiment of a grounded lighting arrangement 600 arranged to backlight a display 30 in accordance with a principle of the invention comprising a plurality of luminaires 51 OA, ... , 51 OK, each constituted of a pair of serially coupled linear lamps, and a single ended high voltage AC source, exhibiting a common return which is typically connected to a chassis ground plane. Grounded lighting arrangement 600 further comprises a plurality of balancing transformers 80 each associated with one end of a particular luminaire 510A, ..., 510K. The number of balancing transformers is twice the number of luminaires 510.

[00078] The input of the single ended high voltage AC source is connected in parallel through the primary winding of a respective balancing transformer 80 to a first end of each of luminaires 510A, ..., 510K. The second end of each luminaire 510A, ..., 510K is connected through the primary winding of the respective associated balancing transformer 80 to the common return.

[00079] The secondary windings of the balancing transformers 80 are connected in a closed loop, in which the polarity of the secondary windings are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor (not shown) is inserted within the loop to detect current flow. For clarity, and to further illustrate the phase relationship of the secondary transformers, the direction of current flow is illustrated when a positive voltage appears at the high voltage AC input, denoted HVAC. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12. [00080] Preferably, the first end and second ends of each luminaire 510A, ...,

510K are in physical proximity of a source driving transformer providing the single ended high voltage AC input, e.g. on the same side of display 30 typically constituted of a metal based chassis, as the driving transformer, and in physical proximity of the associated balancing transformers 80, and preferably generally define a first plane. Preferably, each luminaire 510A, ..., 510K generally extends axially away from the proximity of the source driving transformer providing the differential high voltage AC input, and generally define a second plane, further preferably orthogonal to the first plane. [00081] In operation lighting arrangement 600 operates in all respects similar to the operation of lighting arrangement 500 and is therefore not further detailed.

[00082] Fig. 7 illustrates a high level block diagram of an embodiment of a floating lighting arrangement 700 arranged to backlight a display 30 in accordance with a principle of the invention comprising a plurality of luminaires, each constituted of a U-shaped lamp 410A, ..., 410K, and a differential AC source. Lighting arrangement 700 further comprises a plurality of balancing transformers 80 each associated with one end of a particular U-shaped lamp 410A, ..., 410K. There are twice as many balancing transformers 80 as U-shaped lamps 410.

[00083] A first end of the differential AC input, denoted AC+, is connected in parallel through the primary winding of a respective balancing transformer 80 to a first end of each U-shaped lamp 410A, ..., 410K. The second end of each U-shaped lamp 410A, ..., 410K is connected through the primary winding of the respective associated balancing transformer 80 to the second end of the differential AC input, denoted AC-.

[00084] The secondary windings of the balancing transformers 80 are connected in a closed loop, in which the polarity of the secondary windings are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor (not shown) is inserted within the loop to detect current flow. For clarity, and to further illustrate the phase relationship of the secondary transformers, the direction of current flow is illustrated when a positive voltage appears at first input AC+. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12. [00085] Preferably, the first end and second ends of each U-shaped lamp 410A,

... 410K are in physical proximity of a source driving transformer providing the differential AC input, e.g. on the same side of display 30 typically constituted of a metal based chassis, as the driving transformer, and in physical proximity of the associated balancing transformers 80, and preferably generally define a first plane. Preferably, each U-shaped lamp 410A, ... 410K generally extends axially away from the proximity of the source driving transformer providing the differential AC input, and generally define a second plane, further preferably orthogonal to the first plane. [00086] In operation lighting arrangement 700 operates in all respects similar to the operation of lighting arrangement 400 and is therefore not further detailed. [00087] Fig. 8 illustrates a high level block diagram of an embodiment of a floating lighting arrangement 800 in accordance with a principle of the invention comprising a plurality of luminaries 205 A, ..., 205K, each constituted of a serially arranged linear lamp pair, 20Al, 20A2 ...20Kl, 20K2, and a differential AC source in which energy is supplied to the far end of each of the lamp pairs by a balancing network Floating lighting arrangement 800 comprises: a driver 40; a driving transformer 50 exhibiting a first output 60 and a second output 70; a plurality of balancing transformers 80; a plurality of balancing transformers 85; and a wire pair 210A, 210B. Each luminaire 205 A, ..., 205K has associated therewith a balancing

transformer 80 associated with a first end thereof and a balancing transformer 80 associated with a second end thereof. A single balancing transformer 85 serially connects the far ends of the lamps of each linear lamp pair 20Al, 20A2 ...20Kl, 20K2. [00088] The outputs of driver 40 are connected to both ends of the primary winding of driving transformer 50. The first end of the secondary winding of driving transformer 50, denoted first output 60, is connected through the primary winding of a respective balancing transformer 80 to a first end of first lamp 20Al, ...,20Kl of each of the respective luminaires 205 A, ..., 205K. The nexus of the second end of the respective first lamp 20Al, ..., 20Kl and the first end of the respective second lamp 20A2, ..., 20K2 of each luminaire 205 A, ..., 205K, is connected through the primary winding of the respective associated balancing transformer 85. The second end of each second lamp 20A2 ... 20K2 is connected through the primary winding of the respective associated balancing transformer 80 to the second end of the secondary winding of driving transformer 50, denoted second output 70.

[00089] The secondary windings of the balancing transformers 80, 85 are connected in a closed loop, in which the polarity of the secondary windings are arranged so that voltages induced in the secondary windings are in phase and add within the closed loop. Optionally, a sense resistor RS is inserted within the loop to detect current flow. Optionally, the wires of the closed loop connecting across the length of the linear lamps, denoted 210A, 210B, are arranged in a twisted wire pair. For clarity, and to further illustrate the phase relationship of the secondary transformers, lighting arrangement 200 is illustrated with first output 60 exhibiting AC+ and second output 70 exhibiting AC-, which is appropriate for 1/2 the drive cycle. During the second half of the drive cycle, polarity is reversed and the direction of current flow is reversed. Current flow in the primary windings is illustrated as II, and current flow in the secondary loop is illustrated as 12.

[00090] Preferably, the first end of each first linear lamp 20Al, ..., 20Kl and the second end of each second linear lamp 20A2, ..., 20K2 are in physical proximity of driving transformer 50, e.g. on the same side of display 30 typically constituted of a metal based chassis, as driving transformer 50, and in physical proximity of first balancing transformers 80, and preferably generally define a first plane. Preferably, first linear lamps 20Al, ..., 20Kl and second linear lamps 20A2, ..., 20K2, typically

constituted of linear lamps, generally extend axially away from the proximity of driving transformer 50, and generally define a second plane, further preferably orthogonal to the first plane.

[00091] In operation lighting arrangement 800 is in all respects similar to lighting arrangement 200, with a single balancing transformer shared between the linear lamp pairs of each luminaire 205. Arrangement 800 reduces the amount of balancing transformers required at the far end. Disadvantageously, the driving voltage developed at the far end of the lamps is half of that supplied by arrangement 200 if the same type of balancing transformer is used. There is no requirement that the same balancing transformers be utilized, and balancing transformers 85 of arrangement 800 may be supplied with double the turns ratio to compensate for the reduced driving voltage.

[00092] Arrangement 800 exhibits a drive at each of the lamps 20, as contrasted with arrangement 500 in which drive for the nexus of the serially connected lamps is not supplied.

[00093] Thus certain of the present embodiments enable a backlighting arrangement in which pairs of balancing transformers are provided, each associated with a particular luminaire. The primary winding of each of the balancing transformers is coupled in series with a respective end of the associated luminaire. The secondary windings of the balancing transformers are connected in a single closed loop, and arranged to be in-phase.

[00094] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

[00095] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein.

[00096] All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. [00097] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and subcombinations of the various features described hereinabove as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.