Abstract:
A high power cold cathode gas discharge system employs two electrode structures, where each structure includes a plurality of sub-electrodes connected in parallel to a driver so that the current delivered by the system is spread over multiple sub-electrodes. Each sub-electrode is connected to the driver through a current limiting device such as a capacitor which limits the current delivered by each sub-electrode to be below a certain threshold. By spreading the current delivered by the system over multiple sub-electrodes, the useful life of the system will not be reduced because of sputtering, which results in a high power and long life fluorescent lamp and other gas discharge devices.

Description:
BACKGROUND OF INVENTION 
     This invention relates in general to cold cathode gas discharge devices, and in particular, to a high power cold cathode gas discharge system. 
     Hot cathode fluorescent lamps (HCFLs) have been used for illumination. While HCFLs are able to deliver high power, the useful life of HCFLs is typically in the range of several thousand hours. For many applications, it may be costly or inconvenient to replace HCFLs when they become defective after use. It is therefore desirable to provide illumination instruments with a longer useful life. The cold cathode fluorescent lamp (CCFL) is such a device with a useful life in the range of about 20,000 to 50,000 hours. 
     HCFL and CCFL employ entirely different mechanisms to generate electrons. The HCFL operates in the arc discharge region whereas the CCFL functions in the normal glow region. This is illustrated on page 339 from the book  Flat Panel Displays and CRTS,  edited by Lawrence E. Tannas, Jr., Von Nostrand Reinhold, New York, 1985, which is incorporated herein by reference. The HCFL functions in the arc discharge region. As shown in FIG. 10-5 on page 339 of this book, for the HCFL functioning in the arc discharge region, the current flow is of the order of 0.1 to 1 ampere. The CCFL functions in the normal glow region. Functioning in the normal glow region of the gas discharge, the current flow in the CCFL is of the order of 10 −3  ampere, according to FIG. 10-5 on page 339 of the above-referenced book. Thus, the current flow in the HCFL is about two orders of magnitude or more than that in the CCFL. 
     The HCFL typically employs a tungsten coil coated with an electron emission layer. For more details, see page 61 of  Applied Illumination Engineering, Second Edition , Jack L. Lindsey, 1997, published by The Fairmont Press, Inc. in Lilburn, Ga. 30247, which is incorporated herein by reference. A ½ watt or 1 watt of power is needed to heat the tungsten coil to about 1,000° C. At this temperature, the electrons can easily leave the electron emission layer and a small voltage of the order of about 10 volts will pull large currents into the discharge. The large current flow is in the form of a visible arc, so that the HCFL is also known as the arc lamp. The small voltage will also pull ions from the discharge which return to the tungsten coil, thereby ejecting secondary electrons. However, since the cathode-fall voltage (˜10 V) is small, the sputtering effect of such ions would be small. The lifetime of an HCFL is determined primarily by the evaporation of the electron emission layer at the high operating temperature of the HCFL. 
     The CCFL emit electrons by a mechanism that is entirely different from that of the HCFL. Instead of employing an electron emission layer and heating the cathode to a high temperature to make it easy for electrons to leave the cathode, the CCFL relies on a high cathode-fall voltage (˜150 V) to pull ions from the discharge. These ions eject secondary electrons from the cathode and the cathode- fall then accelerates the secondary electrons back into the discharge producing several electron-ion pairs. Ions from these pairs return to the cathode. Because of the high cathode-fall voltage (˜150 V), the ions are accelerated by the cathode-fall voltage from the discharge to the cathode, thereby causing sputtering. Different from the HCFL, no power is wasted to heat the CCFL to a high temperature. 
     The HCFL operates at a relatively low voltage (˜100 V) whereas the CCFL operates at high voltages (of the order of several hundred volts). The HCFL operates at a temperature of about 40° C. and above, with the cathode operating at a relatively high temperature of about 1,000° C., whereas the CCFL operates in a temperature range of about 30-75° C., with the cathode operating at a temperature of about 150-190° C. For further information concerning the differences between HCFL and a CCFL, please see the paper entitled “Efficiency Limits for Fluorescent Lamps and Application to LCD Backlighting,” by R. Y. Pai,  Journal of the SID, May  5, 1997, pp. 371-374, which is incorporated herein by reference. 
     CCFLs typically comprise an elongated tube and a pair of electrodes where the current between the electrodes in the CCFL is not more than about 5 milliamps and the power delivered by the CCFLs less than about 5 watts. In order to increase the power delivered by the CCFL, it is possible to increase either the length of (and consequently, the voltage across the CCFL) or the current in the CCFL. It may be difficult to manufacture CCFLs whose tubes are excessively long. Furthermore, when the tube length of the CCFL is excessive, they must be operated at high voltage so that this increases the cost and reduces the reliability of the CCFL drivers. Another way to increase the power output of the CCFL is to increase the current in the CCFL. However, as noted above, because of the high cathode-fall voltage which may be about 150 V, ions are accelerated from the discharge towards the cathode, thereby causing sputtering. This means that if a large current is flowing in the CCFL, the return of the ions to the cathode may cause excessive sputtering, which drastically reduces the useful life of the CCFL. 
     None of the above-described gas discharge devices are entirely satisfactory. It is, therefore, desirable to provide an improved gas discharge device where the above-described disadvantages are not present. 
     SUMMARY OF THE INVENTION 
     This invention is based on the observation that the above-described sputtering caused by the return of the ions to the cold cathode may be reduced by distributing or spreading the current over two or more sub-electrodes rather than a single electrode, so that each sub-electrode is not required to carry excessive current. In this manner, the sputtering that does occur will not be excessive and will not drastically reduce the useful life of a cold cathode gas discharge system. This enables the cold cathode gas discharge system to be capable of being operated at higher current, while at the same time, the useful life of the system will not be significantly reduced by the larger current flow. This enables the system to provide higher power without significantly compromising the useful life of the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a cold cathode gas discharge system to illustrate a conventional CCFL. 
     FIG. 2 is a schematic view of a cold cathode gas discharge system useful for illustrating an embodiment of the invention. 
     FIG. 3 is a cross-sectional view of a portion of the system of FIG. 2 to illustrate the system in more detail. 
     FIG. 4 is a schematic view of a cold cathode gas discharge system to illustrate an alternative embodiment to that in FIG.  2 . 
     FIG. 5 is a cross-sectional view of a cathode configuration to illustrate another embodiment of the invention. 
     FIG. 6 is a schematic view of a circuit which is the equivalent of the electrode configuration of FIG.  5 . 
     FIG. 7 is a cross-sectional view of an electrode configuration to illustrate yet another embodiment of the invention. 
     FIG. 8 is a schematic view of a circuit which is equivalent to the electrode configuration of FIG.  7 . 
     FIG. 9 is a schematic view of a circuit which is equivalent to the electrode configuration of FIG. 7, except that the series capacitor  25  of FIG. 7 has been omitted. 
     FIG. 10 is a schematic view of a circuit which may be arrived at by employing multiple electrode structures similar to that of FIG.  7 . 
    
    
     For simplicity in description, identical components are labeled by the same numerals in this application. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a schematic view of a conventional CCFL  100 . As shown in FIG. 1, CCFL  100  includes a vacuum sealed gas discharge tube  1 , which contains gas discharge material such as mercury, xenon and one or more inert gases such as argon, helium, neon or other inert gases. On the inner wall of tube  1  is a fluorescent layer  2 . Tube  1  also contains two electrodes  3 , one at each end of the tube. A lead  4  for each electrode connects corresponding electrode  3  and passes through one of the ends of tube  1  to outside the tube. One of the leads  4  connects its corresponding electrode  3  through capacitor  5  to a node  4   a , while the other lead  4  connects the remaining electrode  3  to lead  4   b . When an appropriate AC voltage is applied between anodes  4   a ,  4   b , such as an AC voltage at about 30 kHz, the gas discharge in tube  1  generates ultraviolet radiation which excites the phosphor layer  2  to generate visible light. Typically, the current flowing through electrodes  3  is controlled to be less than 5 milliamps because of the sputtering problems discussed above. If the current applied through electrodes  3  exceeds 5 milliamps, the useful life of the CCFL  100  is drastically reduced. From tests which have been conducted, the useful life of a conventional CCFL  100  varies inversely with the square of the current carried by the CCFL. For this reason, the conventional CCFL of the type shown in FIG. 1 is typically used to deliver low power, such as at below 5 watts. 
     FIG. 2 is a schematic view of a cold cathode gas discharge system useful for illustrating the invention. System  200  includes a vacuum sealed container  6  such as a tube, containing gas discharge material such as mercury, xenon and one or more inert gases such as argon, helium, neon or other inert gases. An optional phosphor layer  7  may be employed on the walls of tube  6 . Tube  6  contains two pairs of sub-electrodes: pair  8   a  and pair  8   b . As shown in FIG. 2, each sub-electrode is connected through a corresponding capacitor  10  to nodes  11   a ,  11   b . When a suitable AC voltage is applied across the nodes  11   a ,  11   b , such as at 10-100 kHz and 100 V to 50 kV, the current flow between the two pairs of sub-electrodes would cause gas discharge and generation of ultraviolet radiation in tube  6 . Where the optional phosphor layer  7  is present on the wall of tube  6 , the phosphor layer is caused to generate visible light in response to the ultraviolet radiation. A given color visible light source or a given wavelength ultraviolet light source can then be obtained. 
     Since the current flow between nodes  11   a ,  11   b  is now spread across two pairs of sub-electrodes  8   a ,  8   b , the current experienced by any individual sub-electrode is less than that passing between the two nodes, so that the sputtering effect on such sub-electrode is reduced as compared to a situation where the entire current passing between the nodes passes through such sub-electrode. Thus, if the two sub-electrodes in pair  8   a  each carries 5 milliamps of current, this enables a current of 10 milliamps to flow between nodes  11   a ,  11   b , so that the power delivered by system  200  would be twice that of the conventional CCFL  100  carrying 5 milliamps. While each electrode is embodied in a pair of sub-electrodes (e.g.  8   a ) for a total of two pairs ( 8   a ,  8   b ) of sub-electrodes as shown in FIG. 2, it will be understood that each electrode may comprise more than two sub-electrodes such as n sub-electrodes may be employed to deliver 5n milliamps of current between n pairs of sub-electrodes, so that the power delivered by such device will be 5n watts, where n is a positive integer greater than 1. 
     A lead  30  for each sub-electrode is connected to its corresponding electrode  8   a  or  8   b  and passes through one of the ends of tube  6  to outside the tube to a driver  35  and capacitor  37  through leads  38 . Driver  35  receives power from a power supply (not shown) such as a power outlet connected to a power utility company through leads  36 . Layer  7  is a phosphor layer deposited on the inner wall of the tube  6 . Where a DC voltage is used to operate the CCFL, the capacitor  37  may be omitted. 
     When a suitable DC voltage, or a suitable AC voltage, is applied across the sub-electrodes  8   a ,  8   b  by means of a power supply and driver  35 , the current flow between the two pairs of sub-electrodes would cause gas discharge and generation of ultraviolet radiation or visible light in tube  6 . 
     Since the useful life of the sub-electrodes in a cold cathode gas discharge system varies inversely with the square of the current carried by the sub-electrodes in the system, where the operating current carried by each of the sub-electrodes in pairs  8   a ,  8   b  is reduced to 2.5 milliamps from 5 milliamps, this means that the useful life of the cold cathode gas discharge system  200  can be increased by 4 times. 
     Each of the sub-electrodes can have a construction similar to cathodes in a normal cold cathode gas discharge system, and can be made of metal or metal with mercury alloy and getter. The installation method of the sub-electrode can be as shown in FIG. 2, each sub-electrode having a lead  30  that passes through one of the ends of the tube  6  to outside the tube. 
     The installation method of the sub-electrode can also be as shown in FIG.  3 . FIG. 3 is a cross sectional view of a pair of sub-electrodes, such as pair  8   a , or pair  8   b , in FIG. 2, of a cold cathode gas discharge system to illustrate a detailed construction of the system. As shown in FIG. 3, each of sub-electrodes in the pairs  8   a ,  8   b  comprises an electrode body  9   a , lead  4   c  and a glass tube  27 . As shown in FIG. 3, glass tube  27  surrounds most of its corresponding lead  4   c , thereby leaving a corresponding gap  28  between such lead  4   c  and the tube. The gaps  28  are narrow and deep and may be used to avoid shorting between adjacent sub-electrodes caused by electrode sputtering. The glass tubes  27  may be sealingly attached with leads  4   c  to tube  6  at sealing area  29 . 
     As shown in FIG. 2, current limiting devices  12  are employed to connect the sub-electrodes  8   a ,  8   b  to nodes  11   a ,  11   b  which are connected to a driver and an AC power supply (not shown). The function of the current limiting devices are to limit the amount of current that is delivered to the sub-electrodes. Preferably, each sub-electrode has a corresponding current limiting device that connects it to the driver and power supply, in the manner shown in FIG.  2 . Thus, each of the sub-electrodes in the two pairs  8   a ,  8   b  is connected through a corresponding capacitor to a corresponding node in order to limit the amount of current that is delivered to such sub-electrode. While capacitors may be advantageously employed for coupling an AC voltage to its corresponding sub-electrode, it will be understood that other electrical components may be used instead, such as a resistor, an inductor, or any combination of capacitor, inductor, resistor. Such and other variations are within the scope of the invention. 
     In FIG. 2, the current limiting devices  10  are shown as located outside tube  6 . This is not essential, and these devices may be placed either inside or outside tube  6 . Thus, in an alternative embodiment as shown in FIG. 4, the capacitors connecting the sub-electrode pair  8   a  to node  11   a  are placed inside the tube while the capacitors  12  connecting the pair  8   b  to node  11   b  are placed within the tube. Capacitors  12  and sub-electrode pair  8   b  may be constructed so that they form a unitary body, such as in the implementation illustrated in FIG.  5 . 
     As shown in FIG. 5, each of the electrode structure or configuration in the sub-electrodes may include an electrode body  13 , and lead  14 . Each of the capacitors for such sub-electrode may be implemented as two electrically conductive layers  16  and a dielectric layer  15  between the two conductive layers  16 . All of the conductive layers  16  are then connected to an electrically conductive shell  17 . Thus, on each side of lead  14  are two capacitors, each formed by two electrically conductive plates  16  and a dielectric layer  15  in between, so that the two sub-electrodes and their corresponding pairs of capacitors form a unitary body as illustrated in FIG.  5 . The circuit equivalent of the structure in FIG. 5 is illustrated in FIG. 6, where each of the four capacitors  18  is formed by a corresponding pair of electrically conductive plates  16  and a dielectric layer in between. 
     FIG. 7 illustrates another embodiment where a plurality of sub-electrodes and their corresponding capacitors are implemented as a unitary body. Shown in FIG. 7 is a cross-sectional view of such body. FIG. 8 is the circuit equivalent of the electrode structure in FIG.  7 . As shown in FIGS. 7 and 8, lead  19  forms the connector that may be connected to a driver and an AC power supply (not shown) for supplying power to the sub-electrodes. Lead  19  is connected through a capacitor  25  to six capacitors  24 , each capacitor being in the shape of a cylinder as shown in FIG.  7 . Capacitor  25  is formed by lead  19  and an electrically conductive layer  32  and a dielectric layer  33  between them. The electrically conductive layer  32  of capacitor  25  is in contact with corresponding electrically conductive layer  22  of six other capacitors  24 , where each capacitor  24  comprises an outer electrically conductive layer  22 , an inner electrically layer  20  that also serves as the electrode body and a dielectric layer  21  sandwiched in between layers  20  and  22  as shown in FIG.  7 . The entire assembly of the seven capacitors in FIG. 7 are then contained in the housing  23  whose inner cross-sectional dimensions are such that the outer layer  32  of capacitor  25  is in electrical contact with the outer layers  22  of the remaining six capacitors  24 . While a capacitor  25  is employed connected in series with capacitors  24  to electrode bodies  20 , it will be understood that capacitor  25  may be omitted, which will not affect the operation of the sub-electrodes in a cold cathode gas discharge system. This is illustrated in FIG.  9 . 
     Thus, in reference to FIGS. 7 and 8, when each of the six sub-electrodes carries 5 milliamps. of current, the six sub-electrodes together would carry  30  milliamps. The structure shown in FIG. 7 may be further extended to deliver an even higher current and therefore power in a gas discharge. Thus, if an electrical conductor  119  is electrically connected to six electrical conductors  19 ′, and each of the six electrical conductors  19 ′ is connected to 6 sub-electrodes in the same manner as electrical conductor  19  as shown in FIG. 7, then the total current delivered would be 6×6×5 or 180 milliamps. This is illustrated schematically in FIG.  10 . By using such a tree type sub-electrode configuration, a cold cathode gas discharge system employing such structure may deliver several 100 milliamps. or over 1 ampere of current, thereby delivering high power for illumination and other purposes. 
     Even though sub-electrode configurations described above may be used to deliver large currents, such currents are spread over a number of sub-cathodes so that the problems caused by sputtering described above would not affect the useful life of such sub-cathodes and of the cold cathode gas discharge systems using such sub-electrodes. As compared to existing HCFL and CCFL designs, the invention is advantageous in that it is a simple and compact in structure and may be used to deliver high power and yet has a long useful life. 
     While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their equivalents. All references mentioned herein are incorporated in their entirety.