Abstract:
Methods and apparatus are disclosed for converting DC power to AC and for driving multiple discharge lamps and, more particularly, Cold Cathode Fluorescent Lamps (CCFLs), External Electrode Fluorescent Lamps (EEFLs), and Flat Fluorescent Lamps (FFLs). Disclosed methods, among other advantages, allow accurate current sharing among the lamps, minimization of the total number of power switches, and, in general, simplification of the complexity of the control system.

Description:
TECHNICAL FIELD  
       [0001]     The embodiments described below relate, generally, to fluorescent lamps and, particularly, to methods and apparatus for driving multiple discharge lamps such as Cold Cathode Fluorescent Lamps (CCFLs), External Electrode Fluorescent Lamps (EEFLs) and Flat Fluorescent Lamps (FFLs).  
       BACKGROUND  
       [0002]     In LCD televisions, a large number of discharge lamps are used to provide bright backlight and high quality images. The popular discharge lamps in LCD panel backlights include CCFL, EEFL and FFL. Usually, DC to AC switching inverters power these lamps with very high AC voltages.  
         [0003]     A common technique for converting a relatively low DC input voltage to a higher AC output voltage is to chop up the DC input signal with power switches, filter out the harmonic signals produced by the chopping, and output a sine-wave-like AC signal. The voltage of the AC signal is stepped up with a transformer to a relatively high voltage since the running voltage could be 500 volts over a range of 0.5 to 6 milliamps. CCFLs are usually driven by AC signals having frequencies that range from 50 to 100 kilohertz.  
         [0004]     To ensure uniform backlight brightness and to maximize the lamps lives, lamps need to carry substantially equal currents. Therefore, it is desirable to accurately regulate the lamp currents. While each inverter can drive a pair of lamps in series to achieve good current matching within the two lamps, the large size LCD display panels may require over 20 lamps and, therefore, more than 10 inverters. This significantly increases the cost and size of a display system.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  shows a prior-art circuit using multiple inverters for driving multiple lamps.  
         [0006]      FIG. 2  shows a simplified schematic diagram of a matrix inverter based on full-bridge inverter topology, in accordance with an embodiment of the invention.  
         [0007]      FIG. 3A  is a simplified circuit diagram for accurate control of individual lamp currents.  
         [0008]      FIG. 3B  depicts details of current, voltage, and phase relationships in the circuit shown in  FIG. 3A .  
         [0009]      FIG. 4  shows a simplified circuit diagram for realization of the control portion shown in  FIG. 3A .  
         [0010]      FIG. 5  shows an example for combining transformers. 
     
    
     DETAILED DESCRIPTION  
       [0011]     Various embodiments of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these embodiments. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description of the various embodiments.  
         [0012]     The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific embodiments of the invention. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section.  
         [0013]     The description of the embodiments of the invention and their applications as set forth herein is illustrative and is not intended to limit the scope of the invention. Variations and modifications of the embodiments are possible and practical alternatives to, or equivalents of the various elements of, the embodiments disclosed herein and are known to those of ordinary skill in the art. Such variations and modifications of the disclosed embodiments may be made without departing from the scope and spirit of the invention.  
         [0014]     The presented embodiments relate to circuits and methods for converting DC power to AC power and, specifically, for driving discharge lamps such as CCFLs, EEFLs and FFLs. The disclosed circuits and methods offer, among other advantages, nearly symmetrical voltage waveforms to drive multiple discharge lamps, accurate control of lamp currents to ensure good reliability, and good current matching. These embodiments disclose a matrix inverter which reduces the cost by more than 30% while maintaining the same current sharing accuracy. These inverters have lower component count, smaller size, and lower cost.  
         [0015]     In the following description, several specific details are presented to provide a thorough understanding of the embodiments of the invention. While the full-bridge inverter topology is used for the explanation, one skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or in combination with other components, or in other inverter topology, etc. In some instances, well-known implementations or operations are not shown or described in detail to avoid obscuring some aspects of various embodiments of the invention.  
         [0016]      FIG. 1  shows a prior-art circuit that uses multiple inverters for driving multiple lamps. If the lamp voltage is not very high, it is also common to drive two lamps in series in a floating configuration to achieve substantially identical currents through the two lamps. However, to ensure good current matching among 2N lamps, N inverters must be used in the prior art arrangements. Each inverter receives the lamp current feedback and regulates the lamp current based on a brightness command.  
         [0017]     To minimize the EMI interference, these inverters must be synchronized to a central clock. This may require a central control IC to manage the clock, and fault protection means. These requirements increase the complexity and the cost of the system. In addition, if the full-bridge inverter topology is employed, a total of 4N switches (preferably MOSFET) are required, along with a total of 4N MOSFET drivers.  
         [0018]      FIG. 2  shows an embodiment of the proposed matrix inverter, based on the full-bridge inverter topology. In this embodiment, for powering 2N lamps in floating configuration, the inverter only needs 2N+2 power switches—reducing the controller cost and complexity—wherein all switches are turned on and off at the same frequency or at the same time.  
         [0019]      FIG. 3A  illustrates a simple control scheme for realizing independent and accurate control of individual lamp currents. The example shown in  FIG. 3A  drives  4  lamps. To simplify the description, it is assumed that the top and the bottom switches in each totem-pole operate at 50% duty cycle; however, the duty cycle of each totem-pole can be varied to achieve higher degrees of regulation flexibility.  
         [0020]     In this example, the phase between adjacent pairs of totem-poles is controlled. If the phase of two adjacent totem-poles is 180 degrees, the transformer connected between these two totem-poles receives the maximum driving volt-second on the transformer primary side and, therefore, produces the maximum lamp current on the transformer secondary side. If the phase of the adjacent totem-poles is zero degrees, the transformer between these two totem-poles will produce zero lamp current. Therefore, the phases between the two adjacent totem-poles may be used to modulate the individual lamp currents.  
         [0021]      FIG. 3B  depicts details of current, voltage, and phase relationships in the circuit shown in  FIG. 3A . The phase φ 1  modulates lamp current LI 1 , and the phase φ 2  controls the lamp current LI 2 . Therefore, the currents of all 4 lamps can be accurately regulated to the same level. This scheme only requires 6 power switches in contrast with the prior art shown in  FIG. 1 , which requires 8 power switches. As also shown in  FIG. 3A , the middle totem-pole conducts primary winding currents of both adjacent transformers. Because of the phase shift, the RMS current stress of these switches is lower than the direct sum of the two primary winding currents, resulting in lower conduction loss in power switches.  
         [0022]      FIG. 4  shows a schematic diagram for realizing the control function depicted in  FIG. 3A . In this example also, the duty cycles of the totem-pole switches are fixed at about 50%. The oscillator block OSC generates the clock signal CLK 0  which is fed into the D-flipflop Q 0 . The output of Q 0  becomes PWM 0  which drives the first totem-pole of the MOSFET switches S 1  and S 2 . The output of the first lamp current feedback amplifier EA 1 , is compared, in CMP 1 , with a ramp (RAMP 1 ) derived from CLK 0  to generate the first clock signal CLK 1 . Clock signal CLK 1  and PWM 0  combine to drive Flip-flop Q 1  to generate PWM 1 , which in turn drives the second totem-pole switches S 3  and S 4 . Similarly, CLK 2  is derived from comparing the second error amplifier EA 2  output and RAMP 2 , in CMP 2 , where RAMP 2  is generated from CLK 1 . CLK 2  and PWM 1  combine to generate PWM 2  which drives the third totem-pole switches S 5  and S 6 .  
         [0023]      FIG. 5  shows an example in which transformers are combined. In this embodiment, by combining the matrix inverter scheme with the passive current sharing scheme, the matrix inverter will drive a greater number of lamps with good current sharing. By having the primary windings of two transformers in series, the matrix inverter can drive 4N lamps with only 2N+2 switches in a full-bridge inverter configuration.  
         [0024]     The configuration shown in  FIG. 5  also has other advantages, such as reliable lamp ignition. For example, if the lamps in the T 1A  secondary are ignited, the large current flow in the primary winding will be reflected to the secondary winding of T 1B . If those two lamps are not ignited, a large current will flow into the resonant cap and generate a high voltage to strike the lamps.  
       CONCLUSION  
       [0025]     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling of connection between the elements can be physical, logical, or a combination thereof.  
         [0026]     Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.  
         [0027]     The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
         [0028]     The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments.  
         [0029]     Changes can be made to the invention in light of the above Detailed Description. While the above description describes certain embodiments of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the compensation system described above may vary considerably in its implementation details, while still being encompassed by the invention disclosed herein.  
         [0030]     As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed embodiments, but also all equivalent ways of practicing or implementing the invention under the claims.  
         [0031]     While certain aspects of the invention are presented below in certain claim forms, the inventors contemplate the various aspects of the invention in any number of claim forms. Accordingly, the inventors reserve the right to add additional claims after filing the application to pursue such additional claim forms for other aspects of the invention.