fluorescent lamp in lcd panel and inverter circuit factory

IProducts: Products: Power IC, Control IC, Hall IC, Bipolar Transistor, MOS FET, IGBT, Thyristor, Rectifier Diode, LED (Light Emitting Diode), CCFL (Cold Cathode Fluorescent Lamp),

Switching Power Supply, UPS (Uninterruptible Power Supply), DC Power Supply, Inverter, Universal Airway Beacon System and Other Power Supplies and Equipments

Also, as purchaser of these products/technology, you agree to follow the procedures for the export or transfer of these products/technology, under the Foreign Exchange and Foreign

Manufacturer of DC to AC inverters for cold cathode fluorescent lamps (CCFL). Types include single and dual output inverters with fixed and dimmable options. Available in 5 to 24 V rating and 140 to 720 mA amperage. Suitable for use in backlighting ATM, cockpit instrumentation, and medical information display devices. Industries served ranges from automotive and aviation OEM"s to engineering, architectural, and special effects firms, film market, and computer, POP, and gaming manufacturers. RoHS compliant.

This application claims priority to Korean Patent Application No. 2005-115610, filed on Nov. 30, 2005, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference.

Recently, information processing devices have rapidly advanced to have various shapes and functions. Information processed by such information processing devices is in the form of an electrical signal. Therefore, users require a display device to visually recognize the information processed by the information processing devices.

An example of the display device is a flat panel display device such as a liquid crystal display (“LCD”). An LCD displays an image using liquid crystal. When compared to other display devices, an LCD is thin and lightweight and also has a low power consumption and a low driving voltage. Therefore, an LCD is widely used in various fields.

Such an LCD including a liquid crystal panel displaying an image thereon and a backlight assembly providing light to the liquid crystal panel is described in Japanese Patent Publication No. 2005-49747, for example.

Referring to FIG. 1, the conventional backlight assembly includes eight cold cathode fluorescent lamps (“CCFLs”) 810 and a balance circuit 820. As a liquid crystal panel increases in size, the conventional backlight assembly requires a plurality of CCFLs to provide uniform brightness in the liquid crystal panel.

Sinusoidal voltages are applied from an external inverter 800 to the CCFLs 810, and thus sinusoidal currents flow through the CCFLs 810. If the CCFLs 810 are driven by sinusoidal voltages with the same polarity, the sinusoidal voltages with the same polarity cause an interference with a driving circuit of a liquid crystal panel by generating noise of interference patterns on the liquid crystal panel. To prevent the interference, the CCFLs 810 are divided into two groups as illustrated in FIG. 1, and the two groups are driven respectively by high sinusoidal voltages with opposite polarities. That is, the inverter 800 is configured to output both a high positive voltage is and a high negative voltage. The odd-numbered CCFLs 810 and even-numbered CCFLs 810, starting from the top, are driven by the high positive voltage and the high negative voltage, respectively.

The CCFLs 810 have a negative resistance and are connected in parallel to one another. Therefore, when a current starts to flow through a given one of the CCFLs 810, a resistance of the given CCFL decreases and thus a current easily flows through the given CCFL. This causes concentration of the current at the given CCFL. To prevent the current concentration, the balance circuit 820 is connected in series to the CCFLs 810, as illustrated in FIG. 1.

The balance circuit 820 includes four balance transformers. Each of the balance transformers includes: a primary coil 821 connected directly between neighboring CCFLs receiving the high positive voltage and the high negative voltage, respectively; and a respective secondary coil 822 installed adjacent to the primary coil 821. When a current flows through any one of the CCFLs 810, a current flows through the primary coil 821 and thus a current flows through the respective secondary coil 822. Since the respective secondary coils 822 are connected in series to one another, the current flowing through the secondary coil 822 causes a current to flow through the corresponding primary coil 821. As a result, currents flowing through the respective CCFLs 810 become equal to one another.

In this configuration, a balancing voltage of each balance transformer that is necessary to balance the CCFLs 810 is obtained by grounding one point of the series connected secondary coils 822 and detecting a voltage between the grounded point and a detection node 830 remote from the grounded point. In a normal state, the balancing voltage is in the range of, for example, 1 V to 2 V.

This balancing voltage varies with a distribution of resistances including the negative resistances of the CCFLs 810. Using this property, a short or open circuit due to a failure in the CCFLs 810 can be detected. That is, when a short circuit occurs due to a failure in the CCFLs 810, a voltage (e.g., 5˜6 V) higher than the balancing voltage in the normal state is detected at the detection node 830 as a result of the balancing operation of the balance transformers.

Referring to FIG. 2, the CCFLs 810 are arranged horizontally in a vertically-standing protection structure 920. The rear surface of the protection structure 920 is covered with a reflection plate 910, and the front surface of the protection structure 920 is covered with a diffusion plate 900. In the conventional backlight assembly, temperature increases upward due to heat by light emitted from the CCFLs 810, resulting in a temperature gradient.

The CCFLs 810 each have a temperature-dependent resistance. Therefore, due to the temperature gradient, the CCFLs 810 have a resistance that is lowered as the CCFLs 810 are further spaced apart from a lower portion of protection structure 920. In other words, the CCFLS 810 have an increase in voltage due to the lowered resistance based on the temperature gradient from bottom to top of the protection structure 920 (e.g., the temperature increases from bottom to top of the protection structure 920).

To eliminate the resistance difference between the CCFLs 810, the balance transformers operate to balance the CCFLs 810. Accordingly, a voltage of, for example, about 3V is induced at the detection node 830. However, when an increase in voltage is detected at the detection node 830 in the conventional backlight assembly, it is impossible to determine whether a resistance difference between the CCFLs 810 or a short circuit due to a failure in the CCFLs 810 has caused the voltage increase. Accordingly, it is difficult to accurately trouble-shoot a failure in the CCFLs 810.

One aspect of the present invention is to provide an inverter circuit to easily troubleshoot a failure in CCFLs, as well as providing a backlight assembly and a liquid crystal display having the above backlight assembly.

In exemplary embodiment of the present invention, an inverter circuit includes first and second electrodes supplying voltage of opposite polarities, respectively, among an even number of CCFLs disposed in one direction, to even-numbered CCFLs and odd-numbered CCFLs, and a balance circuit controlling currents flowing through the CCFLs. The CCFLs are halved into a first group higher in temperature and a second group lower in temperature than the first group, and the balance circuit includes a plurality of primary coils directly connected between at least one of the first group and at least one of the second group and a plurality of secondary coils corresponding to the primary coils and connected to each other to form a loop.

Each of the primary coils is directly connected between at least one of the odd-numbered CCFLs in the first group and at least one of the even-numbered CCFLs in the second group, or between at least one of the even-numbered CCFLs in the first group and at least one of the odd-numbered CCFLs in the second group.

One point of the secondary coils that are serially connected in the loop is grounded, and the balance circuit further includes a voltage detector detecting a voltage between the grounded point and a detection node remote from the grounded point.

The CCFLs are each disposed horizontally and arranged vertically with respect to one another. Each of the primary coils may be directly connected between at least one pair of the CCFLs located at vertically symmetrical positions. Alternatively, each of the primary coils may be directly connected between a CCFL at a highest position and a CCFL at a lowest position of the vertically arranged CCFLs and between two adjacent CCFLs among remaining CCFLs.

In another exemplary embodiment of the present invention, a backlight assembly includes an even number of CCFLs disposed in one direction. The backlight assembly includes first and second electrodes supplying voltage of opposite polarities, respectively, to even-numbered CCFLs and odd-numbered CCFLs among the CCFLs, and a balance circuit controlling currents flowing through the CCFLs. The CCFLs are halved into a first group higher in temperature and a second group lower in temperature than the first group, and the balance circuit includes a plurality of primary coils directly connected between at least one of the first group and at least one of the second group, and a plurality of secondary coils corresponding to the primary coils. The secondary coils are serially connected to each other to form a loop.

Each of the primary coils is directly connected between at least one of the odd-numbered CCFLs in the first group and at least one of the even-numbered CCFLs in the second group, or between at least one of the even-numbered CCFLs in the first group and at least one of the odd-numbered CCFLs in the second group.

One point of the secondary coils serially connected in the loop is grounded, and the balance circuit further includes a voltage detector detecting a voltage between the grounded point and a detection node remote from the grounded point.

The CCFLs are each disposed horizontally and arranged vertically with respect to one another. Each of the primary coils may be directly connected between at least one pair of the CCFLs located at vertically symmetrical positions. Alternatively, each of the primary coils may be directly connected between a CCFL at a highest position and a CCFL at a lowest position of the CCFLs and between two adjacent CCFLs among remaining CCFLs.

The CCFLs are disposed in a vertically-standing protection structure, and the protection structure has a rear surface covered with a reflection plate and a front surface covered with a diffusion plate.

In yet another exemplary embodiment of the present invention, a liquid crystal display includes a liquid crystal panel receiving a light to display an image and a backlight assembly. The backlight assembly includes an even number of cold cathode fluorescent lamps disposed in one direction to provide the liquid crystal panel with the light, a first electrode supplying a first voltage having a polarity to even-numbered cold cathode fluorescent lamps among the cold cathode fluorescent lamps, a second electrode supplying a second voltage having an opposite polarity to the first voltage to odd-numbered cold cathode fluorescent lamps among the cold cathode fluorescent lamps, and a balance circuit controlling currents flowing through the cold cathode fluorescent lamps. The cold cathode fluorescent lamps are halved into a first group higher in temperature and a second group lower in temperature than the first group, and the balance circuit includes a plurality of primary coils directly connected between at least one of the first group and at least one of the second group, and a plurality of secondary coils corresponding to the primary coils. The secondary coils are serially connected to each other to form a loop.

The above and other aspects, advantages and features of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 5 is a schematic circuit diagram of an inverter used in the liquid crystal display according to an exemplary embodiment of the present invention;

FIG. 6 is a schematic circuit diagram of an inverter used in the liquid crystal display according to another exemplary embodiment of the present invention;

FIG. 7 is a schematic circuit diagram of a voltage detector used in the liquid crystal display according to an exemplary embodiment of the present invention;

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms, “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings. However, the present invention is not limited to the exemplary embodiments illustrated herein after, and the exemplary embodiments herein are rather introduced to provide easy and complete understanding of the scope and spirit of the present invention.

Inverter circuits, backlight assemblies, and liquid crystal displays (“LCDs”) with the same, according to exemplary embodiments of the present invention, will now be described with reference to FIGS. 3 through 9.

The display unit 170 includes a liquid crystal panel 171 displaying an image, and a data driving circuit 172 and a gate driving circuit 173 that supply driving signals to the liquid crystal panel 171. The data driving circuit 172 is connected to the liquid crystal panel 171 through a data tape carrier package (“TCP”) 174, and the gate driving circuit 173 is connected to the liquid crystal panel 171 through a gate TCP 175.

The liquid crystal panel 171 includes a thin film transistor (“TFT”) substrate 176, a color filter substrate 177 facing the TFT substrate 176 and a liquid crystal layer 178 interposed between the TFT substrate 176 and the color filter substrate 177.

The TFT substrate 176 may be, for example, a transparent glass substrate where switching TFTs are arranged in a matrix configuration. Each of the TFTs has a source terminal connected to a data line, a gate terminal connected to a gate line, and a drain terminal connected to a transparent conductive pixel electrode (not illustrated).

The color filter substrate 177 is, for example, a substrate where red, green and blue (“RGB”) color pixels (not illustrated) are formed thereon by a thin film process. A transparent conductive common electrode (not illustrated) is formed on the color filter substrate 177.

The receiving container 180 includes a bottom plate 181 and sidewalls 182 formed on edges of the bottom plate 181 to form a receiving space. The receiving container 180 fixes the backlight assembly 110 and the liquid crystal panel 171 thereto.

The bottom plate 181 has a sufficient surface area for receiving the backlight assembly 110. The bottom plate 181 and the backlight assembly 110 may have a same shape. In this exemplary embodiment, the bottom plate 110 and the backlight assembly may have a substantially square plate-like shape. The sidewalls 182 extend from the edge of the bottom plate 181 in a direction approximately perpendicular to the bottom plate 181.

The inverter 160 is disposed outside the receiving container 180 to generate a discharge voltage to drive the backlight assembly 110. The discharge voltage from the inverter 160 is applied to the backlight assembly 110 through a first power supply line 163 and a second power supply line 164. The first and second power supply lines 163 and 164 are connected to first and second electrodes 140 aand 140 bthat are formed at both sides of the backlight assembly 110, respectively. The first and second power supply lines 163 and 164 may be directly connected to the first and second electrodes 140 aand 140 b, respectively. Alternatively, the first and second power supply lines 163 and 164 may be indirectly connected to the first and second electrodes 140 aand 140 b, respectively, using a separate connection member (not illustrated).

The LCD 100 also further includes a top chassis 190. The top chassis 190 is coupled to the receiving container 180 while surrounding an edge portion of the liquid crystal panel 171. The top chassis 190 prevents the liquid crystal panel 171 from being damaged by an external impact and from being separated from the receiving container 180.

The LCD 100 may further include at least one optical sheet 195 to enhance characteristics of light emitted from the backlight assembly 110. The optical sheet 195 may include a diffusion sheet to diffuse the light and a prism sheet to condense the light.

A sinusoidal voltage from the inverter 160 of FIG. 3 is applied to the CCFLs 210. This causes sinusoidal currents to flow through the CCFLs 210. The CCFLs 210 are divided into two groups, and the two groups, for example odd-numbered CCFLs 210 and even-numbered CCFLs 210, are driven by high sinusoidal voltages of opposite polarities. That is, the inverter 160 can output both a high positive voltage and a high negative voltage, and the odd-numbered CCFLs 210 and the even-numbered CCFLs 210, starting from a top as illustrated in FIG. 4, are driven by the high positive voltage and the high negative voltage, respectively.

The CCFLs 210 may be, for example, a commercial CCFL. Although eight CCFLs 210 are illustrated in FIG. 2, the present invention is not limited thereto. That is, the number of the CCFLs 210 may be any even number.

Referring to FIG. 5, the inverter 160 includes two power sources 300 and 310 that output the high positive voltage and the high negative voltage, respectively, two primary coils 321 that are connected to the power sources 300 and 310, respectively, and two secondary coils 322 that are disposed adjacent to the primary coils 321, respectively.

Referring to FIG. 6, the inverter 160 includes one power source 400, one primary coil 421 connected to the power source 400 and two secondary coils 422 that are disposed adjacent to the primary coil 421. Here, the secondary coils 422 are configured such that their sinusoidal voltages have opposite polarities, thereby outputting both the high positive voltage and the high negative voltage.

Referring back to FIG. 4, the balance circuit 220 includes four balance transformers. The balance transformers include primary coils 221 connected, respectively, between the highest CCFL and the lowest CCFL (e.g., the two outboard CCFLs 210), between the second-highest CCFL and the second-lowest CCFL, between the third-highest CCFL and the third-lowest CCFL, and between the fourth-highest CCFL and the fourth-lowest CCFL (e.g., adjacent middle CCFLs 210), and secondary coils 222 disposed adjacent to the primary coils 221, respectively. When a current flows through one of the CCFLs 210, a current flows through a corresponding primary coil of the primary coils 221 and thus a current also flows through an adjacent secondary coil to the corresponding primary coil of the secondary coils 222. Since the secondary coils 222 are connected in series to one another, the currents flowing through the secondary coils 222 cause currents to flow through the corresponding primary coils 221. As a result, currents flowing through the CCFLs 210 are controlled such that amounts of the currents flowing through the CCFLs 210 become equal to one another. The primary and secondary coils 221 and 222 may have an inductance of, for example, but without limitation, between about 100 μH to about 700 μH.

In this configuration, a balancing voltage of each balance transformer that is necessary to balance the CCFLs 210 can be obtained by grounding one point of the secondary coils 222 and detecting a voltage between the grounded point and a detection node 230 remote from the grounded point. In a normal state, the balancing voltage is in the range of about 1 volt to about 2 volts.

The balancing voltage varies with a distribution of resistances including negative resistances of the CCFLs 210. Using this property, a short or open circuit due to a failure in the CCFL 210 can be detected. That is, when a short or open circuit occurs due to the failure in the CCFLs 210, a voltage (e.g., 5 V˜6 V) higher than the voltage in the normal state is detected at the detection node 230 as a result of the balancing operation of the balance transformers.

The voltage between the grounded point and the detection node 230 may be detected using any voltage detector that can detect a voltage between two points.

Referring to FIG. 7, the voltage detector includes a diode 500, a capacitor 510, a resistor 540 and a comparator 530. When a reference voltage 520 is applied to the comparator 530 and the voltage between the ground voltage at the grounded point and the detection node 230 is higher than the reference voltage 520, the comparator 530 outputs a signal “H”. On the contrary, when the voltage between the ground voltage at the grounded point and the detection node 230 is lower than the reference voltage 520, the comparator 530 outputs a signal “L”.

Referring to FIG. 8, the CCFLs 210 are arranged horizontally in a vertically-standing protection structure 620. The protection structure 620 has a rear surface covered with a reflection plate 610 and a front surface covered with a diffusion plate 600. Accordingly, temperature increases upward due to heat from light emitted from the CCFLs 210, resulting in a temperature gradient. The CCFLs 210 each has a temperature-dependent resistance. Therefore, due to the temperature gradient, the upper CCFL 210 has a lower resistance while the lower CCFL 210 has a higher resistance. To reduce the resistance difference between the CCFLs 210, the balance transformers operate to balance the CCFLs 210.

Referring again to FIG. 4, the primary coils 221 are disposed, respectively, between the highest CCFL 210 with the lowest resistance and the lowest CCFL 210 with the highest resistance (e.g., the two outboard CCFLs 210), between the second-highest CCFL 210 with the second-lowest resistance and the second-lowest CCFL 210 with the second-highest resistance, between the third-highest CCFL 210 with the third-lowest resistance and the third-lowest CCFL 210 with the third-highest resistance, and between the fourth-highest CCFL 210 with the fourth-lowest resistance and the fourth-lowest CCFL 210 with the fourth-highest resistance (e.g., adjacent middle CCFLs 210).

The sums of the resistances of the respective two CCFLs 210 connected to the respective balance transformers are averaged to reduce the unbalance thereof due to the temperature gradient. As a result, unlike in the conventional backlight assembly where the primary coil of the balance transformer is disposed between the two adjacent CCFLs 810, in the backlight assembly 110 of the present invention, the increase in the voltage at the detection node 230 caused by the voltage difference between the respective CCFLs 210 due to the temperature gradient can be detected. Accordingly, it can be determined that an increase in a voltage detected at the detection node 230 is caused by a short circuit due to a failure in a CCFL 210. Consequently, it is possible to easily troubleshoot the failure in the CCFL 210.

In the present exemplary embodiment of FIG. 9, a backlight assembly 110 is configured to have a same circuit configuration as that of the backlight assembly 110 of FIG. 2 with the exception that a balance circuit 220 of FIG. 9 is different in structure from the balance circuit 220 of FIG. 2.

Referring to FIG. 9, in the backlight assembly 110, primary coils 221 of balance transformers are disposed, respectively, between the highest CCFL 210 with the lowest resistance and the lowest CCFL 210 with the highest resistance (e.g., the two outbound CCFLs 210), between the second-highest CCFL 210 with the second-lowest resistance and the third-highest CCFL 210 with the third-lowest resistance, between the fourth-highest CCFL 210 with the fourth-lowest resistance and the fourth-lowest CCFL 210 with the fourth-highest resistance (e.g., the two adjacent middle CCFLs 210), and between the third-lowest CCFL 210 with the third-highest resistance and the second-lowest CCFL 210 with the second-highest resistance.

In order to balance the sums of the resistances of the respective two CCFLs 210 connected to the respective balance transformers, it is preferable that each of the primary coils 221 is disposed between the corresponding two CCFLs 210 located at vertically symmetrical positions, as illustrated in FIG. 4. However, as illustrated in FIG. 9, when at least one of the primary coils 221 is disposed between at least one of the odd-numbered upper disposed CCFLs 210 (lower in resistance) and at least one of the even-numbered lower disposed CCFLs 210 (higher in resistance), starting from the top, the distribution of the sums of the respective two CCFLs 210 connected to the respective balance transformers can be reduced when compared to the case of the conventional backlight assembly of FIG. 1. Consequently, when compared to the case of the conventional backlight assembly, it is possible to easily troubleshoot a failure in the CCFL 210. Likewise, when at least one of the primary coils 221 is disposed between at least one of the even-numbered upper disposed CCFLs 210 (higher in resistance) and at least one of the odd-numbered lower disposed CCFLs 210 (lower in resistance), starting from the top, it is possible to easily troubleshoot the failure in the CCFLs 210.

Consequently, the present invention can provide inverter circuits and backlight assemblies that make it possible to easily troubleshoot the failure in the CCFLs. The backlight assemblies according to the present invention can be applied very suitably to an LCD, and also to other devices.

As described above, the inverter circuits, the backlight assemblies, and the LCDs according to the present invention can reduce the unbalance of the sums of the resistances of respective two CCFLs connected to the respective primary coils, when compared to the conventional devices. Consequently, the present invention makes it possible to easily troubleshoot the failure in the CCFLs of the backlight assembly.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)

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Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)

H05B41/28—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters

H05B41/282—Circuit arrangements in which the lamp is fed by power derived from dc by means of a converter, e.g. by high-voltage dc using static converters with semiconductor devices

H05B41/3921—Controlling the intensity of light continuously using semiconductor devices, e.g. thyristor with possibility of light intensity variations

H05B41/3927—Controlling the intensity of light continuously using semiconductor devices, e.g. thyristor with possibility of light intensity variations by pulse width modulation

Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

An improved LCD back light panel lamp connecting structure comprises at least one set of cold cathode fluorescent lamps (CCFL), each having its high voltage end and feedback end arranged alternately, and the feedback ends on both sides of the CCFL respectively coupled to two return boards. Such two return boards are coupled to a pulse width modulation (PWM) control unit, so that the two return boards feed back the current to the PWM control unit. The high voltage end of the CCFL is coupled to a transformer.

The present invention relates to an improved LCD back light panel lamp connecting structure, more particularly to a plurality of cold cathode fluorescent lamps (CCFL), each having its high voltage end and feedback end arranged alternately. [0001] BACKGROUND OF THE INVENTION

A traditional LCD TV or touch screen of a LCD display requires a high brightness to compensate the visual requirements. In general, a cold cathode fluorescent lamp (CCFL) is lit by high voltage; the larger the current, the brighter is the lamp. Therefore several CCFL lamps are generally used to compensate the brightness and evenness, and it is the most important issue is to keep the current of the lamp even and minimize the error. The installation of several sets of loading also increases the number of control units for the lighting and the area of the circuit board, and thus making the manufacturing more complicated and the cost higher. In FIG. 1, it shows a driving device that lights up a CCFL, and comprises a power supply unit [0002] 13, a pulse width modulation (PWM) control unit 14, a driving unit 15, a transformer 11, and a loaded cold cathode florescent lamp (FFCL) 12. When the input of the input voltage is initialized, the driving unit 15 immediately drives the transformer 11 to light up the CCFL 12 by the negative/positive voltage effect and the PWM control 14 detects the current of the CCFL lamp 12 through the current feedback 16 and outputs a resonant frequency. The average current of the CCFL lamp 12 can be controlled by means of the driving unit 15 and the transformer 11. Therefore, the light produced can be projected onto the back light panel of the LCD.

Please refer to FIG. 2 for the schematic circuit diagram of a plurality of lamps in accordance with a prior art. In the figure, the high voltage ends of a plurality of lamps [0003] 21 respectively connect to a connector 221 and a connector of a transformer 23, and said plurality of transformers 23 are integrated to a circuit board 26 to form an inverter 20, and the feedback end of the plurality of lamps 21 are mutually coupled and connected to the PWM control unit 25, so that the PWM control unit 25 can detect the current of the lamp 21 through the current feedback to output a resonant frequency and control the average current of the CCFL lamp 12.

1. Firstly, the feedback end of the prior-art multiple lamps generally makes the wiring job more complicated, not only increasing the size of the circuit board, but also making the manufacturing complicated, increasing the cost, and unnecessarily consuming higher voltage. [0005]

2. Secondly, since the high voltage ends of the cold cathode fluorescent lamps are installed on the same side of the lamp, therefore when the lamp is lit, the temperature at that side is usually too high and thus affecting the life of the lamp. [0006]

3. Thirdly, when several lamps are used to compensate the brightness and evenness, it generally causes uneven current and brightness between the lamps since there generally exists a discrepancy between lamps for their production. Thus, it becomes an issue of selecting lamps, or it may require more lamps to improve the brightness and evenness. Such arrangement will increase the cost, and make the manufacture more complicated and the adjustment more difficult. [0007] SUMMARY OF THE INVENTION

The primary objective of the present invention is to overcome the shortcomings and avoid the deficiencies of the prior art. The present invention alternately arranges the high voltage end and the feedback end of at least one set of cold cathode fluorescent lamps to save wire materials, average the current of the lamp, and enhance the stability of the current. [0008]

To achieve the above objective, the improved LCD back light panel lamp connection structure of the present invention comprises at least one set of cold cathode fluorescent lamps (CCFL), each having its high voltage end and feedback end arranged alternately, and the feedback ends on both sides of the CCFL respectively coupled to two return boards. Such two return boards are coupled to a pulse width modulation (PWM) control unit, so that the two return boards feed back the current to the PWM control unit. The high voltage end of the CCFL respectively couples to a transformer and drives the transformer to light up several sets of cold cathode fluorescent lamps, and the feedback end of the plurality of CCFLs feeds back the current through the two return boards to a PWM control unit. Such PWM control unit detects the current of the lamp to output a resonant frequency and control the average current of the several sets of CCFLs. [0009]

FIG. 4 is a schematic diagram of the transformer and power supply board of the present invention.[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer to FIGS. 3 and 4 for the block diagram of the circuit and the schematic diagram of the transformer and power supply board of the present invention respectively. In the figures, the improved LCD back light panel lamp connecting structure comprises at least one set of cold cathode fluorescent lamps (CCFL) [0014] 31, each having its high voltage end 32 and feedback end 33 arranged alternately, and the feedback ends 33 on both sides of the CCFL 31 respectively coupled to two return boards 34, 35. Such two return boards 34, 35 are coupled to a pulse width modulation (PWM) control unit 36, so that the two return boards 34, 35 feed back the current to the PWM control unit 36; the high voltage end 32 of the CCFL 31 couples to a connector 38 at an output end of a transformer 45 by a connector 37; the feedback ends 33 of the CCFL 31 mutually couples to the two return boards 34, 35; said transformer comprises at least one transformer or ceramic transformer being coupled to the connector 37 of the CCFL 31 by a connector 38, and each transformer 45 is integrated on a circuit board 43 to form a inverter 46, and then connected to a connector 40 on a power supply board 41 by a connector 44 on the circuit board 43. The power supply board 41 is coupled to a power supply unit 42 so that the power can be supplies to each transformer 45 on the inverter 46 via the power supply board 41, which can save wire materials and simplify the structure.

Two or four cold cathode fluorescent lamps could be used as the CCFL [0015] 31 in this preferred embodiment, so that the transformer 45 is driven to light up several sets of the CCFLs 31, and the feed back end 33 of each CCFL 31 feeds back the current to the PWM control unit 36 through the two return boards 34, 35, and the PWM control unit 36 detects the current of the CCFL 31 to output a resonant frequency, and control the average current of several sets of the CCFLs 31.

In view of the description above, the present invention definitely overcomes the shortcomings of the prior art and has the following advantages: [0016]

1. The present invention arranges the CCFLs alternately, such that the high voltage ends of the lamps are arranged alternately, and thus will not overheat one side of the lamp when the lamp is lit. [0017]

2. The feedback ends of several lamps of the present invention are connected in series, and all coupled to the two return boards, not only saving wire material, lowering the consumption of voltage for transmission, and increasing the stability of the circuit, but also making the manufacture easy and the cost lower. [0018]

3. The present invention arranges the high voltage end and the feedback end of several lamps alternately, so that the current of each CCFL can be more evenly distributed, and thus achieving the purpose of even lighting. [0019]

While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. [0020]

1. An improved LCD back light panel lamp connection structure, comprising at least one set of cold cathode fluorescent lamp with its high voltage end and feedback end alternately arranged.

2. The improved LCD back light panel lamp connection structure of claim 1, wherein said-feedback ends on both sides of said cold cathode fluorescent lamp are respectively coupled together and to two return boards.

3. The improved LCD back light panel lamp connection structure of claim 1, wherein said two return boards are coupled to a pulse width modulation control unit.

4. The improved LCD back light panel lamp connection structure of claim 1, wherein said high voltage ends of said plurality of cold cathode fluorescent lamps are respectively coupled to an output end of a transformer.

5. The improved LCD back light panel lamp connection structure of claim 4, wherein said transformer is coupled onto a circuit board to form an inverter, and said circuit board is coupled to a power supply board, and said power supply board is coupled to a power supply unit, such that the power supply unit supplies power to each transformer on the inverter directly through said power supply board.

A CORRECTIVE ASSIGNMENT TO CORRECT THE ASSIGNEE PREVIOUSLY RECORDED AT REEL 013742, FRAME 0283;ASSIGNORS:CHOU, CHIN-WEN;CHENG, EDDIE;REEL/FRAME:016872/0376

We then test leakage inductance between primary and secondary; we find that the leakage is roughly 25% of the primary inductance as this is probably a deliberate design factor in the operation of the output LC circuit.

As the secondary is made from fine wire, and subject to high voltages and even higher “strike” voltages to start the gas tube, we next use the SURG test to check for any Inter-winding coil weaknesses. This is performed by injecting high voltage pulses and measuring the characteristic ringing decay on the secondary winding. An empirical nominal from a known good transformer is selected for our limits. Any breakdown in the insulation will result in energy loss and hence a different decay response.

To check correct ratio of windings from secondary to primary. The winding with the most turns is energised as this is best practice for optimum accuracy.

As the Leakage Inductance is governed by turns ratio, and core response (already covered by TR and LS) and the physical positioning of the windings, some customers (using automatic winding methods) may have enough confidence in winding positioning to only audit this parameter occasionally, rather then on every part tested.

This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/501,502 filed on Sep. 9, 2003 and entitled “SPLIT PHASE INVERTERS FOR CCFL BACKLIGHT SYSTEM,” the entirety of which is incorporated herein by reference. BACKGROUND OF THE INVENTION

The present invention relates to power inverter circuits for driving fluorescent lamps such as cold cathode fluorescent lamps (CCFLs) or the like. More specifically, the present invention relates to a split phase topology for power inverter circuits.

Fluorescent lamps are used in a number of applications including, for example, backlighting for display screens, or the like. 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 the 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.

In liquid crystal display (LCD) applications, backlight is needed to illuminate the screen to make a visible display. Backlight systems in LCD or other applications typically include one or more lamps and an inverter system to provide DC to AC power conversion and to control the brightness of the lamps. Even brightness across the lamps and clean operation of inverters with low switching stresses, low EMI, and low switching losses are desirable. However, increases in lamp length, wire length and operating voltage associated with large backlighting systems make even luminance difficult to achieve. Even luminance across non-floating lamp structures is even more difficult. Thus, some backlight inverter systems are configured to support floating lamp structures.

The size of LCD display panels are increasing with the increasing size of large screen displays such as those associated with large screen TVs, desktop monitors, or the like. As the size of LCD display panels increase, the size of their backlighting systems also increase. The associated increase in power level of the backlight inverter systems exacerbates problems typically found in conventional half-bridge and push-pull inverter topologies. These problems may include, for example, switching spikes, high voltage/current stresses, switching losses, electromagnetic interference, combinations of the forgoing, or the like.

A number of conventional inverter topologies facilitate zero-voltage or zero current switching to reduce switching stresses and losses. These inverter topologies include, for example, an active clamping forward topology, a phase shifted full-bridge topology, a resonant full-bridge topology, an asymmetric half-bridge topology.

A factor in achieving even brightness over a lamp is the ability to symmetrically power the lamp at both ends. This is more difficult to achieve as the length of the lamp increases. Among the conventional inverter topologies, the phase shifted full-bridge topology and the resonant full-bridge topology are acceptable for CCFL inverter applications because of their ability to produce symmetric lamp current waveforms. However, there are some disadvantages associated with resonant type inverters including, for example, high amplitude of voltage or current excursion, variable operating frequency, or the like. These disadvantages are not desirable in many lamp applications.

FIG. 1 illustrates a conventional backlight system 100 for powering a lamp 102. The lamp 102 is coupled between the secondary winding of a first transformer 104 and the secondary winding of a second transformer 106. The primary winding of the first transformer 104 and the primary winding of the second transformer 106 are coupled to an switching network 110 through two switching power lines 112, 114. The switching network 110 comprises four power MOSFETS (metal oxide semiconductor field effect transistors) 120, 122, 124, 126 connected in a full-bridge topology to provide DC to AC conversion. The four power MOSFETS 120, 122, 124, 126 are coupled to DC power lines V+, V−. The disadvantage of the configuration shown in FIG. 1 is that high current or high voltage switching lines 112, 114 typically have to cross a display panel.

When the length of the lamp 102 increases with the panel size, the configuration shown in FIG. 2 may be used to avoid running long switching power lines across the length of the panel. The configuration shown in FIG. 2 powers a lamp 202 by using separate inverters 204, 206 at respective ends of the lamp 202. Both inverters 204, 206 use a full-bridge switching topology. Thus, as compared to the inverter shown in FIG. 1, the power devices and associated component count, including controllers 240, 242, is doubled. Increasing the number of components increases the cost and surface area of a printed circuit board using the product. Thus, the total size of the product is increased. SUMMARY OF THE INVENTION

The present invention proposes a split phase inverter to drive floating lamps symmetrically with zero-voltage switching operation and reduced device count. For example, a floating lamp structure can be driven by two power stages near the respective lamp terminals. In one embodiment, the split phase inverter enables zero-voltage switching with two-transistor power stages and uses phase shift modulation or pulse width modulation (PWM) to control the lamp current. Voltage/current stresses, electromagnetic interference, switching losses and component count are thereby minimized.

In one embodiment, a four-switch element inverter splits into two two-switch inverter arms to form a split phase inverter (or split inverter). The inverter arms (or split switching arms) are deployed separately at two respective separate terminals of a floating CCFL to achieve even light output. Each of the split switching arms is dedicated to driving a separate terminal of the CCFL. Both split switching arms are controlled by a common controller. Thus, the advantages of a full-bridge inverter circuit are achieved with half the number of components, including the inverter controller.

A novel regulation method facilitates full lamp current regulation for wide input range conditions. For example, the lamp current regulation can be achieved with fixed-frequency, zero-voltage switching operations by controlling the switching pattern of each dedicated two-switch network and the waveform relations between them (e.g., by utilizing a complementary switching strategy with an optimized dead time insertion). In one embodiment, an optimized dead time is inserted at switch over transitions between the two switches of a two-switch inverter arm to avoid shoot through conditions.

In one embodiment, a novel split inverter is used for backlight systems in large LCD display panel applications (e.g., 46″ LCD televisions, desktop monitor or the like). The split inverter includes two-transistor switching networks respectively dedicated to providing AC signals at separate lamp terminals. The novel split inverter provides very clean switching waveforms and high efficiency (e.g., approximately 91%). The novel split inverter advantageously uses half the number of MOSFET devices, has a smaller packaging size, and runs cooler than competitive inverters for the same application.

In one embodiment, the split inverter is configured to drive more than one floating lamp. For example, multiple lamps can be driven in parallel. The two-transistor switching networks (or split inverter arms) can be in a half bridge configuration, a push-pull configuration, a push-pull forward configuration, or the like.

In one embodiment, the switching operations of the split inverter arms are synchronized. The output connections of the two-switch inverter arms are arranged such that voltages applied to the two opposite terminals of the lamps are in an anti-phase relationship. The respective outputs of the split inverter arms add up differentially to drive the lamps. Lamp current regulation is achieved by controlling either the phase relationship or the pulse pattern between the outputs of the split inverter arms.

In one embodiment, the split inverter arms operate in a complementary switching pattern, and the regulation of the lamp current is fulfilled by adjusting the phase angle or the symmetric pulse width between the respective outputs of the split inverter arms. Since the transformers are substantially similar, and the split inverter arms share substantially the same input voltage, the output voltages of the split inverter arms cancel each other when their switching states are in phase. Similarly, the two output voltages stack up to drive the lamps when the switching states are out of phase (or anti-phase).

One inverter controller (i.e., a common controller) can generate the control signals for both split inverter arms because a symmetric pair of switching waveforms is used. In an exemplary embodiment, a phase shift modulation scheme is used in which the split inverter arms switch at near 50% duty cycle (e.g., at substantially 50% duty cycle with dead time insertions), and the effective voltage across the lamps varies substantially with the phase difference of the respective output voltages generated by the split inverter arms. By adjusting the phase difference between the split inverter arms, the voltage across the lamps and the corresponding lamp current can be regulated.

In an exemplary embodiment, a pulse width modulation scheme is used in which switching patterns for the split inverter arms have symmetric pulse widths but not necessarily at near 50% duty cycle. The effective lamp voltage varies with the pulse widths of the respective output voltages generated by the split inverter arms. The lamp current can be regulated by symmetrically adjusting the pulse widths.

For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention provides a technique for driving floating lamps symmetrically with reduced device count. In one embodiment, a floating lamp structure is driven by two dedicated power stages disposed proximate the respective lamp terminals. In one embodiment, the dedicated power stages are two-transistor switching networks. A novel pulse width modulation (PWM) scheme or phase shift modulation may be used to control the lamp current. The technique reduces voltage stresses, current stresses, electromagnetic interference, switching losses and component count.

In one embodiment, a four-switching element inverter is split into two inverter arms, which are deployed separately at two respective terminals of a floating cold cathode fluorescent lamp (CCFL) structure to achieve even light output. The advantages of a full-bridge inverter circuit are achieved with half the number of components, including the inverter controller. A novel regulation method facilitates full lamp current regulation for wide input range conditions. The lamp current regulation can be achieved with fixed frequency, zero-voltage switching operations by controlling the switching pattern of each arm and the waveform relations between the arms. As discussed in greater detail below, this is achieved by utilizing a complementary switching strategy with an optimized dead-time insertion.

In one embodiment, a split inverter is used in backlight systems for large display panels such as those associated with large screen televisions (e.g., having approximately a 46 inch LCD display or greater), desktop monitors or the like. The split inverter provides very clean switching waveforms and high efficiency (e.g., 91% efficiency or greater). The split inverter advantageously uses half the number of switching devices (e.g., MOSFETs), has a smaller packaging size and runs cooler than conventional inverters for the same applications.

In the following description, reference is made to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific embodiments or processes in which the invention may be practiced. Where possible, the same reference numbers are used throughout the drawings to refer to the same or like components. In some instances, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention, however, may be practiced without the specific details or with certain alternative equivalent components and methods to those described herein. In other instances, well-known components and methods have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

FIG. 3 is a block diagram illustrating a split inverter system 300 according to an embodiment of the invention. The split inverter system 300 includes a lamp structure 301 coupled between a first inverter arm 302 and a second inverter arm 304. In an exemplary embodiment, the lamp structure 301 includes one or more CCFLs (not shown) in a floating configuration. As used herein, “floating” refers to its normal and customary meaning and includes electrically isolating the lamp structure 301 from ground or any direct connection to the driving circuitry (i.e., the first inverter arm 302 and the second inverter arm 304). For example, the first inverter arm 302 and the second inverter arm 304 may include respective transformers used to isolate and inductively couple power to the lamp structure 301.

In one embodiment, the first inverter arm 302 and the second inverter arm 304 each comprise a two-switch network configured to convert a direct current (DC) voltage (represented by V+ and V−) to an alternating current (AC) signal. The first inverter arm 302 is configured to provide a first AC signal to a first terminal 312 of the lamp structure 301 and the second inverter arm 304 is configured to provide a second AC signal to a second terminal 314 of the lamp structure 301. The first AC signal and the second AC signal add up differentially to drive the lamp structure 301. Thus, the first inverter arm 302 and the second inverter arm 304 operate as complementary power stages at both ends of the lamp structure 301. As discussed in more detail below, the characteristics of the AC lamp voltage (Vlmp) are adjusted by synchronously modulating the first AC signal and the second AC signal.

The first inverter arm 302 is dedicated to and placed proximate the first terminal 312. Similarly, the second inverter arm 304 is dedicated to and placed proximate the second terminal 314. Thus, power switching lines do not cross the length of the lamp structure 301 or a substantial portion thereof. When the split inverter system 300 is used to provide backlighting for a display panel, for example, there are no power switching lines associated with the backlighting system crossing the display panel and causing high EMI, high switching stresses and high switching losses. Thus, the performance of the backlighting system and display panel is improved.

The split inverter system 300 also includes a common controller 306 coupled to the first inverter arm 302 and the second inverter arm 306 through respective signal lines 308, 310. Preferably, the signal lines 308, 310 have relatively low voltage, low EMI, and low losses as compared to the first terminal 312 and the second terminal 314 of the lamp structure 301. Although two separate controllers can be used to drive the first inverter arm 302 and the second inverter arm 304, in a preferred embodiment the common controller 306 is configured to drive both the first inverter arm 302 and the second inverter arm 304. Since a separate controller is not required for each inverter arm 302, 304, the total number of components and the cost of the split inverter system are reduced as compared, for example, to the conventional backlighting system 200 shown in FIG. 2.

The controller 306 comprises, by way of example, one or more processors, ASICs or other substrate configurations, hardware, program logic, or software capable of representing data and instructions which operate as described herein or similar thereto. The controller 306 may also comprise controller circuitry, processor circuitry, general purpose single-chip or multiple-chip microprocessors, digital signal processors, embedded microprocessors, microcontrollers, combinations of the foregoing, or the like. In operation, the controller 306 controls the first inverter arm 302 and the second inverter arm 304 such that the lamp structure 301 is driven with symmetrical AC signals at both ends. The controller 306 controls the brightness of the lamp structure 301 by modulating or adjusting the symmetrical AC signals relative to one another.

In one embodiment, the controller 306 is configured to synchronize the switching operations of the first inverter arm 302 and the second inverter arm 304. The output connections of the two inverter arms 302, 304 are arranged such that voltages applied to the two opposite terminals 312, 314 of the lamp structure 301 are in an anti-phase relationship. The lamp structure 310 is powered symmetrically at both terminals 312, 314 to obtain even brightness over substantially the whole lamp structure 310. Lamp current regulation is achieved by controlling either the phase relationship or the pulse pattern between the two outputs as explained in more detail below.

FIGS. 4A-6B are partial schematic diagrams illustrating components of exemplary split inverter systems, usable by the split inverter system 300 shown in FIG. 3, according to embodiments of the invention. FIGS. 4A and 4B illustrate a split inverter system 400 comprising a first half-bridge inverter arm 402 and a second half-bridge inverter arm 404 disposed at opposite ends of a corresponding lamp structure 406 (FIG. 4A) or 408 (FIG. 4B). The first half-bridge inverter arm 402 comprises a first output transformer 412 having a primary winding 410 and a secondary winding 414. The first half-bridge inverter arm 402 is configured to generate a first AC signal V21 across the secondary winding 414 of the first output transformer 412. The second half-bridge inverter arm 404 comprises a second output transformer 418 having a primary winding 416 and a secondary winding 420. The second half-bridge inverter arm 404 is configured to generate a second AC signal V22 across the secondary winding 420 of the second output transformer 418.

In one embodiment, the first half-bridge inverter arm 402 comprises a first N-channel MOSFET 422 and a second N-channel MOSFET 424 coupled in series across a supply voltage (i.e., V+ and V−). The first N-channel MOSFET 422 has a drain coupled to a first DC voltage signal V+(or positive supply) and a source coupled to a first switching node S1. The second N-channel MOSFET 424 has a drain coupled to the first switching node S1 and a source coupled to a second DC voltage signal V− (or negative supply). In some embodiments, the second DC voltage signal V− is a ground or common signal. The first switching node S1 is AC coupled to a first terminal of the primary winding 410 of the first output transformer 412 through a capacitor C1. A second terminal of the primary winding 410 is coupled to the second DC voltage signal V−.

The second half-bridge inverter arm 404 comprises a third N-channel MOSFET 426 and a fourth N-channel MOSFET 428 coupled in series across the supply voltage. The third N-channel MOSFET 426 has a drain coupled to the first DC voltage signal V+ and a source coupled to a second switching node S2. The fourth N-channel MOSFET 428 has a drain coupled to the second switching node S2 and a source coupled to the second DC voltage signal V−. The second switching node S2 is AC coupled to a first terminal of the primary winding 416 of the second output transformer 418 through a capacitor C2. A second terminal of the primary winding 416 is coupled to the second DC voltage signal V−.

The gates of the first MOSFET 422 and the second MOSFET 424 receive control signals VA, VB from a common controller (not shown) configured to drive the first half-bridge inverter arm 402 so as to generate the first AC signal V21. Similarly, the gates of the third MOSFET 426 and the fourth MOSFET 428 receive control signals VC, VD from the common controller configured to drive the second half-bridge inverter arm 404 so as to generate the second AC signal V22. While separate controllers can be used to drive the first half-bridge inverter arm 402 and the second half-bridge inverter arm 404, in a preferred embodiment the common controller drives both arms 402, 404. Thus, the size and cost of the split inverter system 400 are reduced and the characteristics of the first AC signal V21 and the second AC signal V22 can be symmetrically adjusted relative to each other.

FIG. 4A illustrates the lamp structure 406 according to an embodiment of the invention coupled between the secondary winding 414 of the first output transformer 412 and the secondary winding 420 of the second output transformer 418. The lamp structure 406 comprises a first CCFL 430 and a second CCFL 432 coupled between respective terminals of the secondary windings 414, 420 in a floating configuration wherein the terminals of the secondary windings 414, 420 are not grounded. The first AC signal V21 and the second AC signal V22 combine to generate a lamp voltage (Vlmp=Vlmp1+Vlmp2) across the first CCFL 430 and the second CCFL 432.

FIG. 4B illustrates the lamp structure 408 according to an embodiment of the invention coupled between the secondary winding 414 of the first output transformer 412 and the secondary winding 420 of the second output transformer 418. The lamp structure 408 comprises three CCFLs 434, 436, 438 coupled in parallel between respective first terminals of the secondary windings 414, 420. An artisan will recognize from the disclosure herein that the lamp structure 408 is not limited to the three CCFLs 434, 436, 438 and that any number of lamps can be used including, for example, a single CCFL 434. Respective second terminals of the secondary windings 414, 420 are coupled together. As shown in FIG. 4B, the second terminals of the secondary windings 414, 420 may be co